HOMEOSTASIS

Teacher's Guide

The programs are broadcast by TVOntario, the televisionservice of The Ontario Educational CommunicationsAuthority. For broadcast dates consult the TVOntarioschedule in School Broadcasts, which is published inSeptember and distributed to all teachers in Ontario. Theprograms are available on videotape. Ordering information forvideotapes and this publication appears on page 26.

Canadian Cataloguing in Publication DataLang, Harold Murray ,1922 -

Homeostasis

To be used with the television program, Homeostasis.Bibliography: p.ISBN 0-88944-049-2

1. Homeostasis (Television program) 2. Homeostasis.1. TVOntario. II. Title.

QP90.4131984 574.1'88 84-093010-0

@Copyright 1984 by The Ontario Educational Communica-tions Authority

All rights reserved.

Printed in Canada

Note to Teachers

The six programs in this series are arranged in order ofincreasing complexity, and should therefore be shown incorrect sequence. The series focuses on the principle ofhomeostasis at the level of the individual organism - theoriginal application of the term. Extensions of this principleto all levels of life, from cell to biosphere, are mentioned inthe last section of the guide, Program 6: Hormonal Control.The guide suggests teaming activities to reinforce andcomplement each program, extending the teaming processto make a complete teaching unit for senior biology.

The Series

Producer/Director: David ChamberlainWriter: Alan RitchieNarrator: Susan CopelandConsultant: H. Murray LangAnimation: Groupe Imagination

The Guide

Project Leader. David ChamberlainWriter' H. Murray LongEditor: Elaine AboudDesigner: Susan Mark

ContentsProgram 1: COPING WITH CHANGE

(An Introduction to Homeostasis). .................. 1

Program 2: THE SEA WITHIN(Regulating the Body Fluids). ...................... 5

Program 3: OSMOREGULATION(Fine-tuning the Control of the Body Fluids) ............ 9

Program 4: THE FEEDBACK CYCLE(A Controlling Principle for Homeostasis). ............ 15

Program 5: BIOCHEMICAL BALANCES(Regulation &f Body Chemistry). .................... 19

Program 6: HORMONAL CONTROL(The Coordination of Homeostasis). ................. 22

Ordering Information ............................... 26

Coping With Change An Introduction to Homeostasis

1. I dentify the need for organisms to control their internal environment.2. Cite examples of controlled substances and conditions in organisms.3. Recognize that a steady state is maintained by organisms within

narrow limits through controlled fluctuations.4. Identify Claude Bernard as the originator of the concept of a steady

state and a controlled internal environment (le milieu interieur), andexplain that he advanced this idea at the same time as Charles Darwinput forth a theory for natural selection in 1859.

5. Define homeostasis, and identify Walter Cannon as the originator ofthe term.

6. Outline a model control system, and explain the interrelationships ofits components: receptor, control centre, effector, feedback loop.

Program DescriptionSince life began, every organism has had to struggle to maintain its lifeagainst the fluctuation of environmental conditions. How do organisms copewith extremes that may be life threatening? Homeostasis is about coping,about the ways that organisms adjust their internal environments to com-pensate for changes in the external environment. A sleeping dog is appar-ently oblivious to its surroundings, yet internal receptors are constantlymonitoring conditions and making adjustments. If the temperature drops,the muscles of the skin raise the hair to create more insulating air spaces.Shivering generates heat in the muscles below the skin. If the dog becomestoo hot, panting cools its body through evaporation of water from the lungsand tongue. These automatic mechanisms help to regulate and maintainbody temperatures within narrow limits.

Besides temperature, organisms regulate many other factors within nar-row limits. Consider blood sugar, for example. When a lion or tiger makes akill, it will gorge itself until filled. Once the prey has been consumed, it maynot make another kill for several days. Even humans are not always regulari n their feeding habits: we may overeat on some occasion and skip meals atother times. Yet all organisms maintain the concentration of blood glucoseat much the same level at all times.

The regulation of blood glucose and the regulation of body temperatureare just two examples of the principle of homeostasis. Claude Bernard, areknowned French physiologist, first suggested in 1859 the idea of a steadystate: the maintenance of constant conditions within organisms. The same

year, Charles Darwin published On the Origin of Species by Means ofNatural Selection. Both of these principles have been equally productive forscientific work in their respective fields. Bernard showed that every organ-i sm exists in two environments: an external one that is always changing, andan internal one, "le milieu interieur," that is carefully controlled to remainnearly the same.

Bernard: All the vital mechanisms, varied as they are, have only oneobject, that ofpreserving constant the conditions of life in the internalenvironment. The independence conferred on an organism by its stableinternal environment sets it free to achieve its fullest development.The principle of the regulation of the internal environment within narrow

limits was nameless until about 1930, when an American physiologist,Walter B. Cannon, coined the term homeostasis. The word has Greek roots:homoio meaning "the same," and stasis meaning "standing still:' Thushomeostasis may be defined as the preservation of constancy in the internalenvironment of an organism. It is often stated as the maintenance of asteady state within an organism.

But the mechanisms of homeostasis do not maintain absolute and un-changeable set-points. Usually there is a fluctuation above and below a set-point. For example, if you record your body temperature every two minutes youmight produce a graph as in Figure 1.

Bodytemperature/OC\

Figure 1

Thus, when human body temperature is stated as 37°C, this figure repre-sents the average, or mean, of many fluctuations. When 37*C is called

*From J. Olmstead and E.E. Olmstead, Claude Bernard and the Experimental Methodin Medicine (New York: Abelard Schuman Company Ltd., 1952), p. 224.

ObjectivesStudents should be able to:

1

"normal," it denotes a further averaging of many individual temperatures;yours may normally be above or below this mean. At different times of theday, and during different activities, human body temperature will differ Whenwe have a fever, and during ovulation, the set-point will be higher Theanalogy of the control of room temperatures by a thermostat may assist inunderstanding the fluctuations.

Different kinds of organisms maintain different set-points for temperaturecontrol. Birds, for example, maintain their bodies about five degrees warmerthan mammals do due to increased metabolic rate. Birds and mammalsmaintain a nearly constant temperature, and an called homeothermic. Thegraph of the body temperature of these organisms as the temperature of thesurroundings (Figure 2) is raised shows a flat part of the curve wherehomeostatic mechanisms can control temperature within narrow limits.

Beyond certain points the mechanisms can no longer protect the organismfrom extreme temperatures.

Cold-blooded creatures show quite a different graph when the tempera-ture of their surroundings is raised (Figure 3). They are called poikilothermic

because their temperature varies with that of their surroundings.

receptor must do something to communicate what it has detected. Usuallyit sends a message in the form of a chemical or nervous signal. Thismessage reaches a control centre, which then selects an appropriateresponse, and sends the right message to activate this response. Thismessage is transmitted to an effector, usually a gland or a muscle, or cilia orflagella. I n an effective homeostatic mechanism, the effector, or itsresponse, must initiate a signal that will turn off the receptor or reset it forfurther action. This step is called the feedback loop (Figure 4). In theprogram sych a control system is illustrated at the single cell level using theprotist Euglena (Figure Q. Similar homeostatic mechanisms are effective for

Stress--*

Figure 4

ReceptorControlcentre

Feedback loop

Effector ---* Response

Figure 5. Euglena, a single-celled protoctist (protist)

Figure u Figure 3

Organisms copewith change in a wide

range of factors in their surround-ings. If we plot graphs of these controlled entities against the changes inthe environment, in every case the curve will resemble that of the homeo-thermic organism in a rising surrounding temperature. Thus, homeostatic

mechanisms control the total water content of the body, the levels ofglucose, sodium, calcium, hydrogen, and potassium ions in the blood, andthe arterial blood pressure. How do such mechanisms work?

For each control system, there must be a receptor that can detect thechange in or stress on the organism. It might also be called a sensor. The

organisms that function at the tissue level, such as fungi, sponges, andplants. Higher animals have more highly developed responses, based onnervous systems, and these will be illustrated in later programs.

From single cells to large complex organisms with trillions of cells,homeostatic mechanisms are at work to maintain life.

Before ViewingProgram 1 can stand as its own introduction to the principle of homeostasis.However, students will be in a better position to understand the conceptsinvolved if they have previously studied the physiology of several vertebrate(or mammalian or human) systems. It would be advisable to complete

Activities 1, 2, and 3 before viewing, in order to better prepare the studentsfor the program. If students have not studied Euglena in earlier grades, itwould be a good idea to teach its structure, and to show how it can functionas either an autotroph or a heterotroph.

The thermostat analogy (Activity 3) reveals the whole basis of homeo-stasis. It should be discussed prior to viewing Program 1.

After ViewingHave students discuss the concepts presented in order to consolidate theirlearning. Then discuss the review questions (Activity 4).

Activities

1. After sitting still for at least 5 min, record yourtemperature every 2 min for a period of 10 min.For accuracy, standardize your method: shakethe thermometer down each time until the fluidreaches the same low point on the scale; then

similar to Figure 6.

Questions:

place the bulb of the thermometer under yourtongue in exactly the same position for exactly60 s; then wait exactly 60 s before inserting thethermometer again. Plot the results on a graph(Figure 6).Exercise moderately for 2 min by jogging "onthe spot:' Record your temperature immedi-ately after you stop exercising, and again after5 and 10 min. Plot these temperatures on agraph similar to Figure 6.Exercise vigorously for 2 min by running upand down stairs. Record your temperaturei mmediately after you stop, and again after 5and 10 min. Plot these temperatures on a graph

What does the graph show about your restingbody temperature? What hypothesis can yousuggest to explain your observations? Do youhave enough data to calculate your "normal"temperature?What are the immediate and long-term resultsof moderate and vigorous exercise? Are theseresults consistent with those of othermembers of the class? Suggest what ishappening in your body that would explainyour observations.

Activity 2: Phototaxic Behavior ofGreen Flagellates

Materials:2 culture jars of living Euglena (or Chlamydomonas)black paper, scissorscompound microscopes, well slides, cover slipsdroppers, methyl cellulose

Method:1. Several hours before class cover one of the

culture jars with black paper except for a holethat you cut in the shape of an initial or a circle1 cm in diameter. Leave -the other cultureuncovered, as a control. Place both cultures ina fairly bright location.

2. When the class is assembled remove the blackpaper. Have students compare the distributionof the organisms in the jars. Ask forsuggestions of hypotheses to account for theobservations.

3. Students should examine some of theorganisms through a microscope by placing asmall drop of the culture in a well slide. Havethem make a sketch of a typical organism.Encourage them to observe the organisms forseveral minutes to find out how they move. Ifthe organisms move too quickly for study, add

3

Time (s)Figure 6. Graph of changes in body temperature

Activity 1: The Fluctuations inHuman Body Temperature 2.

Materials:1 clinical thermometer per teamclock or watch that shows time in s

Method: 3.

a small drop of methyl cellulose solution toslow them down. Can students detect anyother changes in the organisms?

Questions:

1. How do the organisms respond to light?2. What advantage would this behavior give the

organism?3. What structures do the organisms have that

might detect light?4. What structures do the organisms have that

would effect a response to light? How do thesestructures function?

Room temperature rises (Electrical signals) Room temperature dropsabove set-point Furnace turns off to set-point

Figure 7. Chalkboard discussion of thermostat model

Thermostat setat 20°C

5. What function might be performed by thecontractile vacuole?

Activity 3: Thermostat Analogy

• How does a thermostat work in controlling roomtemperature?

• In what ways might the operation of the thermostatbe analogous to temperature control in the body(Figure 7)? Conduct a class discussion in the

pattern set by Joseph Schwab in "Invitation toI nquiry No. 38" in Biology Teacher's Handbook (seeMayer, Further Reading).

Activity 4: Review Questions

1. Why do organisms need to control their internalenvironment?

2. What entities (conditions and substances) areregulated by organisms? (State one condition, twomolecules, and two ions that are controlled.)

3. Define homeostasis.4. Who first suggested the idea of a controlled internal

environment? What advantage did he see for theorganism if it controlled its internal environment?How was his idea a useful principle for physiology?

5. Outline a model control system and explain howeach part of it would function in maintaininghomeostasis for an organism.

Further ReadingBerry, Gordon S. Biology of Ourselves - A Study of

Human Biology. Toronto: John Wiley and Sons,Canada, Ltd., 1982.

Mayer, Wm V., ed. Biology Teacher's Handbook. 3rd ed.New York: John Wiley and Sons, Ltd., 1978.

Nelson, G.E.; Robinson, G.G.; and Boolootian, R.A.Fundamental Concepts of Biology. 2nd ed.New York: John Wiley and Sons, Ltd., 1970.

Olmstead, J., and Olmstead, E.E. Claude Bernard andthe Experimental Method in Medicine. New York:Abelard Schuman Company Ltd., 1952.

Roddie, l. Physiology for Practioners. Edinburgh:Churchill Livingston Company, 1975.

Sherman, I.W, and Sherman, VG. Biology - A HumanApproach. New York: Oxford University Press, 1979.

Weisz, Paul B. The Science of Biology. 4th ed.New York: McGraw-Hill Book Company, 1971.

4

The Sea Within Regulating the Body FluidsObjectivesStudents should be able to:

1. Identify the importance of the control of water within an organism, andthe "compartments" in which water occurs: the extracellutar fluids(ECF) and the intracellular fluids (ICF).

2. Identify MacCallum's hypothesis: the similarity in composition of thebody fluids of diverse organisms suggests that life must haveoriginated in the sea, and has maintained fluids resembling that seaever since.

3. I dentify three ions and three molecules in the body fluids that aremaintained homeostatically within narrow limits.

4. Explain the processess of osmosis, diffusion, exocytosis, and endocy-tosis as homeostatic mechanisms at the cellular level.

5. Explain the hypothesis of the mechanisms ("pumps") that maintain theconcentrations of sodium and potassium in the ICE

Program DescriptionProgram 2 deals with Claude Bernard's "le milieu infrieur," the internalmedium within the organism. Bernard's idea was that the organismmaintained a steady state within itself as protection against extremes ofchange in the external environment. Water forms a large proportion of everyorganism, but its maintenance within controlled limits poses problems.Consider, for example, the single-celled amoeba that lives in freshwaterponds. Within its cell are a number of solutes, including sugars and the ionsof several salts. By osmosis, water is constantly entering through the cellmembrane from outside, where there are more water molecules per unitvolume of solution. To keep from becoming too dilute,or from bursting (asred blood cells do in pure water), the amoeba has developed a pump thatgets rid of the excess water. This is the organelle - the contractile vacuole.It gradually fills with water, then contracts to expel its contents. Then itrepeats the process. Diffusion, the process by which substances move bymolecular action from where the concentration is high to where it is lower,accounts for the movement of oxygen and carbon dioxide across the cellmembrane of the amoeba Ions needed for life, such as potassium andcalcium, will be pumped across the cell membrane into the cytoplasm of theamoeba by molecular pumps that use energy to accomplish this activetransport. Thus, at the level of the single cell, the amoeba regulates its cellcontent to maintain life.

In contrast to the single cell, multi-cellular organisms maintain twodifferent internal environments:

• the fluid within cells - the intracellular fluid (IGF); and• the fluid that surrounds and bathes the cells - the extracellular fluid

(ECF).

Figure 1. A comparison of the concentrations of ions in sea water and in the bodyfluids (ECF) of several organisms, using sodium as a standard (100 units)

5

Thus the ECF is analogous to the pond in which the amoeba lives in that itprovides a watery medium for every cell, bringing the substances needed forlife and carrying away the wastes.

A comparison of the chemical compositions of the ECF of quite differentorganisms reveals the surprising fact that the ions are very similar in kindand in concentration (see Figure 1). In fact, their concentrations are quitesimilar to those in sea water. This prompted a Canadian physiologist, A.B.MacCallum, to suggest about 60 years ago that these similarities areevidence that life orginated in the sea Did homeostatic mechanisms evolvpso that organisms could maintain their internal environments much like thatof the early seas? After about three billion years of evolution, do we stillcarry around within us an environment resembling that ancient sea? Todaythe sea has changed somewhat; all the rivers have been carrying salts andi ons from the land, which make the ocean saltier that when life began.

On average, about two-thirds of the human body is water, although this ishigher in newborn infants, and lower in women, because more fats arestored. Water is the solvent and vehicle of the body fluids. The water in thebody is distributed in three main "compartments" or regions. Two-thirds ofthe water is found in the ICE, that is, inside the body cells (Figure 2a). Of theremaining one-third, one-quarter makes up the blood plasma, while the restis in the fluids surrounding cells-, mostly as lymph (Figure 2b). There is adynamic equilibrium between the three compartments; by osmosis andother mechanical processes, water flows readily from one region to another(Figure 2c).

Figure 2a Figure 2b

receptors: dilution receptors, volume receptors, and pressure receptors.Homeostatic controls are set in motion, and within three hours most of theexcess load is removed, excreted as urine (Figure 2e).

A comparison of the concentrations of the various ions in the body fluids(see Activity 2) reveals that there is little change between the plasma and thefluid between the cells. Apparently, fluids pass readily from the bloodvessels to the body spaces and vice versa because of the porous nature ofcapillary walls. There is, however, a marked difference between the ECF andthe ICE The cell membranes evidently exert an active control on what getsi nto or out of cells. Cell membranes must use energy in the form of ATP topump the needed ions in, and the undesirable ions out. `

Potassium is a very important ion within cells. It helps to maintain theintercellular osmotic pressure and to regulate pH. It promotes reactionsnecessary in carbohydrate and protein metabolism and plays a vital role inmembrane polarization involved in the conduction of nerve impulses and thecontraction of muscle fibres. Ninety-eight percent of the body's potassium isi n the cells, only two percent is outside. Most of the potassium is bound toproteins in the cells; the rest of it is attracted to inorganic phosphate ions:

Figure 2d Figure 2e

Figure 2c

" When we drink a large quantity of some beverage, we readily absorbmuch of the water through the linings of our stomach and intestines. For atime, the volume of blood increases and our body fluids become more dilute(Figure 2d). The extra stress on our system is detected by three kinds of

The average human adult consumes from 2 to 3.5 g of potassium per dayi n food. About the same amount is excreted every day. This amounts to adaily turnover of from 1.5 to five percent of the body's total potassium. Thel osses are greater when we suffer disease, trauma, or surgery. The body

6

ECF I CF

1/3 2/3

Figure 3

does not store potassium, but requires a daily intake to supply its needs. Inorder to keep 30 times as much potassium inside the cells as in the ECF, it

i s hypothesized that there are active transport pumps through the cellmembrane. Perhaps the pump works by expelling sodium ions as it takes inpotassium ions: each of these ions is about 30 times more concentrated onthe opposite side of the membrane. Figure 3 shows a possible mechanismto explain the action of the pump.

Before ViewingStudents should do Activities 1 and 3 in order to understand the difficultiesthat cells face in a watery environment and how unicellular organisms copewith them. Decide also whether it would be better to have students considerthe data in Activity 2: the relative concentrations of different ions in bodyfluids. The data will go by so quickly on the screen that advanceconsideration may be useful. The units "milliequivalents" may need to beexplained (see footnote to Activity 2).

After ViewingStudents should complete the activities and discuss the points raised in theprogram. They may write notes based on the concepts by answering thereview questions.

Activities Activity 2: A Comparison of the Ion Concentrations in the Fluids of theHuman Body

Activity 1: The Effects of the Method:Environment on Cells 1. Examine the following table of ion concentra-

tions in the three "compartments" of the bodyThis activity, described in Inquiry 18, The FunctioningAnimal, p.43 (see Further Reading), asks students toi nvestigate the effects of different surrounding fluidson red blood cells. Students find that the red bloodcells, if they are to remain normal, must be surround-ed by plasma that is close to a 0.9 percent solution ofsodium chloride in molarity. In pure water, the cellsabsorb too much water by osmosis and burst, a condi-tion called hemolysis. On the other hand, cellssurrounded by a hypertonic solution, such as tenpercent sodium chloride, become plasmolyzed andshrink, a condition called crenation.

CONCENTRATIONS OF IONS IN THE FLUIDS OF THE HUMAN BODY

fluids. Compare and contrast the concentrationof each ion to determine what mechanisms areat work to cause the differences.

7

(expressedin relative units, see P. a-)

EXTRACELLULAR FLUID (ECF)

IONS Blood Plasma Between Cells

INTRACELLULAR FLUID (ICF)

Within Cells

Sodium 142 145 5Potassium 5 4 150Calcium 5 3 1Magnesium 3 2 40Chloride 104 116 5Bicarbonate 27 27 10Hydrogen Phosphate 2 3 110

Questions:1. Why is there a close similarity between the

figures in the first and second columns?2. What must the cell membranes be doing to

maintain the low concentration of sodium ionswithin cells?

3. Are there any other ions that show a patternsimilar to that shown by the sodium ions?What relationship might there be between'these and the sodium ions?

4. Contrast the concentrations of the potassiumions with those of the sodium ions. What mustthe cell membranes do to maintain the concentration of potassium ions within the cells?What other ions are similarly concentrated withcells?

'The units are not important here; the concept is the relativeconcentration. The measurements given are expressed inmilliequivalents (mEq.). These are calculated by multiplyingthe mass of ions (mg) per L by the valence, and dividing bythe molar mass. This gives a measure of the number ofionic charges per L of solution.

Activity 3: Examination of Amoebato Observe Exocytosis andEndocytosis

Materials:culture of living Amoebaculture of living Colpidium or Parameciumcompound microscopesmicroscope slides (or well slides), cover slips

droppersMethod:

1. Exocytosis: the expulsion of wastes from thecell.Examine the amoeba. Locate the contractilevacuole, and observe it steadily for severalminutes. How long does it take for the contrac-tile vacuole to fill? How long does it take toempty? How soon does it begin to fill again?

2. Endocytosis: the engulfing of food into a cell.Add a drop of a smaller organism, such asColpidium or Paramecium, to the slide containing amoeba Observe carefully for severalminutes, watching the behavior of the amoebaWhen the smaller organism touches orapproaches the amoeba, how does the cellmembrane react? What behavior of the innerprotoplasm follows the stimulus to themembrane? How quickly are pseudopodsformed? How do they form a food vacuole?What happens to the organisms inside thefood vacuole?

Note: If living cultures cannot be obtained, show ashort film of the functioning of the amoeba Severalfilms are available: Amoeba (Gaumont-British); TheAmoeba (Educational Pictures); Response in a SimpleAnimal (BFA Educational Media); The Biology of theAmoeba (Statens).

Activity 4: Review Questions

1. Why do organisms need to control the amount ofwater they contain? (What are the consequences oftoo much or too little water?)

2. In a multicellular organism, where is water located?Name the various "compartments" in which wateroccurs.

3. What ions are similar in their concentrations indifferent organisms and in the seas? What hypo-thesis is suggested by these similarities?

4. For each of the following terms (i) define the term,(ii) give one example of a substance that wouldenter or leave a Gel! by the process and (iii) explainhow the process contributes to homeostasis:

(a) osmosis(b) diffusion(c) exocytosis(d) endocytosis

5. What ions does a cell membrane:(a) concentrate within the cell?(b) pump out of a cell?

6. Using the fluid mosaic model of a cell membrane,explain how potassium and sodium ions might bemaintained in suitable concentrations by pumps inthe membrane.

Further ReadingLang, H.M; Palfery, E.G.; and Van Nieuwenhove, E.L.R.

The Functioning Animal, Toronto: Gage Educational

Publishing, 1978. McElroy, W13.; Swanson, C.P; and Macey, R.I. Biologyand Man. Englewood Cliffs, N.J.: Prentice-Hall Inc.,1975.

Mikal, Stanley. Homeostasis in Man: Fluids, Electro-lytes, Proteins, Vitamins and Minerals in ClinicalCare. Boston: Little Brown and Co., 1967.

Wallace, Robert A. Biology, The World of Life. 2nd ed.(Chapter 9) Santa Monica, Calif.: Goodyear Pub. Co.1978.

Weisz, Paul B. The Science of Biology. 4th ed. (Part 6:Steady States) New York: McGraw-Hill, 1971.

Osmoregulation Fine-tuning the Control of the Body FluidsObjectivesStudents should be able to:

nephrons flows together on its way to be excreted. In turn, the convolutedtubule has three parts: the proximal tubule, nearest to the capsule; the loopof Henle, and the distal tubule that connects to the collecting region.

1. Define osmoregulation and identify the parts of the body involved inthe process and its control.

2. Describe the structure, location, and function of each part of thekidney and each part of its unit of structure - the nephron.

3. Explain the role of osmosis in the reabsorption of useful fluids.4. Explain the role of hormones in the control of reabsorption.5. Explain the adaptations of the excretory systems of animals that live in

quite different environments: marine invertebrates, marine fish, fresh-water fish, and mammals.

Program DescriptionAn important aspect of homeostasis is the careful regulation of thecomposition of the body fluids, a process called osmoregulation because itinvolves the movement of water across membranes. Osmosis is determinedby the concentration of solutes on both sides of a membrane, and is apassive process. This means that osmosis is a movement of water basedentirely on the behavior of molecules. This is quite different from activetransport, which depends on the expenditure of energy, in the form of ATPBoth osmosis and active transport are involved in osmoregulation.

I n vertebrates, the kidneys are the main organs of osmoregulation. Fifteenpercent of the blood being pumped out of the heart is directed into thekidneys. Here the pressure of the blood forces 20 percent of the plasma tol eave the blood capillaries. This creates a filtering action, because the bloodcells are kept in the capillaries when some of the plasma leaves. The fluidthat leaves the blood amounts to 180 L per day. This would amount to a

-t because the bodyneeds: water, many kinds of ions, glucose, and amino acids. However, as thefiltrate enters the kidney tubules, the processes of reabsorption begin, andby the time it has passed completely through the nephron, 99 percent of thefluid will have been returned to the blood. The remaining one percent will beexcreted as urine. This process of reabsorption by the cells lining thenephrons is selective; it regulates the concentration of the extracellular fluid.

Each human kidney contains about a million nephrons. A nephron (Figure1) consists of three main regions: Bowman's capsule where the filtrateleaves the knot of capillaries; the convoluted tubule where selective reab-sorption occurs; and the collecting region where the urine from many

Figure 1. A nephron

I n the proximal tubule, about 80 percent of the filtrate is reabsorbed: theglucose, amino acids, hormones, vitamins, and ions. By active transport, the

cells lining e u e move these materials into the cells and return them tothe blood in the capillaries surrounding the tubule. In addition, water reab-sorption by osmosis occurs in response to the reabsorption of the solutesby active transport. Next, in the loop of Henle, the lining cells pump sodiumi ons into the tubule by active transport. This occurs because of the highconcentration of sodium ions in the medulla region of the kidney. Thischanges the osmotic relationship between the filtrate and the ECF surround-i ng the tubules and capillaries, making it possible to keep reabsorbing waterfrom the filtrate. The action is called the counter-current model, since thetwo arms of Henle's loop run counter to one another, and work in opposite

.ways (Figure 2). In the first part (the descending arm), water diffuses out of

9

10

Figure 2. Reabsorption in the uriniferous tubule of the nephron

I n the last or distal part of the tubule, the action that began in the ascend-i ng loop of Henle continues. Here, in addition to the continued reabsoptionof water, the same hormone - aldosterone - regulates the balance ofseveral ions: hydrogen, potassium, ammonium, and magnesium. Thus thefunction of aldosterone is to protect the volume and composition of thebody fluid. The net result of all the reabsorption is that the urine becomesincreasingly concentrated in solutes (hypertonic) as water is reclaimed.

`Although aldosterone is not mentioned in the program because of time limitations,we discuss it briefly here in order to provide further background information.

Another hormone, ADH (anti-diuretic hormone), produced in the pituitarygland, acts on the membranes of the cells in the distal tubule to maintainthe osmotic pressure of the ECF If the body fluids become too dilute, thenthe secretion of ADH is inhibited, and more water is excreted in the form ofdilute urine. If, on the other hand, the ECF becomes too concentrated, thenthe pituitary releases more ADH into the blood. When the ADH reaches thecells of the distal tubule, it increases the reabsorption of water, reducing thevolume of urine. Perhaps you have experienced the action of caffeine or

' alcohol: both inhibit the release of ADH, resulting in diuresis, the productionof large quantities of dilute urine.

The hormones that control the action of the nephron are triggered byseveral different receptors. There are osmoreceptors in certain centres ofthe hypothalamus of the brain and in the wall of the carotid artery; bloodvolume receptors next to the glomerulus within the Bowman's capsules ofthe nephrons; and blood pressure receptors in the atria of the heart. InProgram 6 of this series, you will learn how such receptors interact withinthe endocrine system to maintain homeostatic control. It appears as ifseveral alternate systems have evolved to protect life through fine-tuningosmoregulation. Figure 3 summarizes the interactions of the differentcontrols and responses.

If we compare the process of osmoregulation in several organisms fromdifferent environments (Figure 4), we see evidence of the evolution ofincreasing adaptations. Such minor adjustments of structure and functionhave made possible the exploitation of a wide variety of difficult environ-ments. In many marine invertebrates, such as jellyfish and sea cucumbers,the body fluids tend to match the sea water in composition and osmoticproperties. In this isotonic state, they can maintain osmoregulation withoutmuch expenditure of energy. They neither gain nor lose water and salts byosmosis or diffusion.

Many marine invertebrates have adapted to life in estuaries or rivermouths where fresh water mingles with salt water. Because these organismscontain more salts in their body fluids, fresh water poses a problem for thembecause by osmosis they constantly take in water from their surroundings.To cope with this problem many different adaptations have evolved. For example, barnacles and mussels close up tightly when th ey are surrounded

by fresh water, and open up only when salt water returns. Some crabs havedeveloped special organs at the bases of their antennae for excreting theexcess water.

You might expect marine fish to resemble marine invertebrates in havingi sotonic body fluids, but this is not the case. Actually, marine fish have bodyfluids with ion concentrations more like those of the freshwater fish andl and vertebrates. Evolutionists interpret this as evidence that the ancestorsof marine fish must have evolved in fresh water and later adapted to life in

the tubule, and sodium is pumped into it. The changes make the filtrateincreasingly hypertonic, or more concentrated with respect to the ECF Inthe second part (the ascending arm) of the loop, sodium ions are activelypumped out of the filtrate. Now, hormones from the adrenal cortex, chieflyaldosterone, affect the membranes of the cells lining the tubule, makingthem more permeable to absorb water from the filtrate."

Figure 3. Controls and processes involved in osmoregulation

the sea This adaptation required them to solve the problem of the continual Land-dwelling vertebrates face quite a different problem: how to retainloss of water to the sea by osmosis. They would also tend to absorb salts enough water in their ECF to keep from drying up. They lose water bytoo readily from the sea To cope with the problem of hypotonic fluids, evaporation from the body surfaces, from the breathing organs, and throughmarine fish have to drink a lot of water and excrete excess salt through their the elimination of wastes. They must also maintain the balance of salts in

gills. In their kidneys, he nephrons lack the glomerull (knotsof capilaries) the within the proper limits for life . The adaptations of the humanfor filtering, but have many capillaries surrounding the tubules for reabsorp- osmoregulation system apply to most land vertebrates, although there aretion of water. Marine fish excrete isotonic urine, scanty and concentrated in i nteresting modifications for life in desert animals, such as the camel, thewastes and salts. kangaroo rat, and the gerbil.

Freshwater fish have body fluids that are hypertonic to water, that is, there You may wonder how changes in the structure and functioning of aare higher levels of solutes in their ECF than in their surroundings. Their nephron might come about. It is hypothesized that particular hormones,problem is that water will constantly be entering their ECF by osmosis. They such as ADH, affect only target cells that have specific protein molecules inrarely drink water. Their body coverings of scales limits the entry of water to their membranes. In this way, the hormone affects only the permeability ofthe membranes of their mouths and gills. They have developed efficient the target cells, and has no apparent effect on other cells in the body. ADHkidneys with large glomeruli for excreting the excess water. They excrete causes a change in permeability of the target cells, making them morelarge quantities of dilute (hypotonic) urine. Their gill membranes take in the permeable to reabsorb water and specific ions. This change is reversible,ions of salts from the water by active transport. adjusting for changing conditions of the ECF The change in permeability

1 1

SEA CUCUMBER

Scanty, concentrated urine

MAMMAL

Figure 4. Osmoregulation in different environments

1. FRESHWATER FISH

2 MARINE FISH

3_ AMPHIBIANS, REPTILES

4. MAMMALS

Figure 5

Activities

may be due to the action of a specific enzyme, succinic dehydrogenase, anenzyme that removes hydrogen from succinic acid. This action operates thesodium pump, moving sodium ions across the cell membrane. Russianphysiologists have suggested that this hormone-enzyme-permeabilitysystem may account for the evolution of the function of reabsorption fordifferent environments. In freshwater fish the enzyme is only active in thedistal part of the uriniferous tubule, reabsorbing very little water (Figure 5). Inmarine fish the distal tubules are reduced, and the activity of the enzyme isvery low. Here sodium is reabsorbed only when the filtrate is isotonic withthe blood. In amphibians and reptiles the main activity of the enzyme is stillin the distal tubules, but there is some activity in the proximal tubules aswell, enhancing the retention of water for a land environment. In birds andmammals there is increased activity of the enzyme in the proximal tubules,but the strongest action is in the ascending part of the loop of Henle, and inthe distal tubules. Thus we see how adaptation for life out of the water hasbeen made possible by minor changes in molecular activity that increasedthe efficiency of the control of the water and salts in the body fluids.

Thus as organisms adapted to a wide range of changing environments,they continued to achieve homeostasis by fine-tuning of systems that werealready present. The net result was an even greater freedom to exploit

Activity 1: A Study of the Structureof the KidneyMany laboratory manuals explain how to dissect freshkidneys (pork or lamb are most like humans') from aslaughterhouse or meat counter. Some teachers usepreserved kidneys from a biological supply company.Some sources of laboratory instruction are:

Benson et al., Investigations in Biology, I nvestiga-tion 37.Crawford, Patterns in Biology, p. 389, LaboratoryActivities 2 and 3.Galbraith, Lab Manual for Biological Science(revised), Investigation 32.(See Further Reading, Program 3)Lang et al., The Functioning Animal, I nquiry 19.(See Further Reading, Program 2)

Activity 2: Active Transport inLiving CellsI n Benson et al., Investigations in Biology, I nvestiga-tion 13 asks students to investigate the uptake ofcongo red dye by the membranes of yeast cells, bothalive and dead.

Activity 3: The Composition ofUrine

Before Viewing

After Viewing

Almost identical instructions are given in the followingbooks, but the first tests for two additional ions:

Benson et al., Investigations in Biology, I nvestiga-tion 38.Crawford, Patterns in Biology, p. 397, "The Physicaland Chemical Analysis of Urine."

increasingly hostile environments. The remaining programs will examine inmore detail some of the ways that homeostatic mechanisms becamefine-tuned through biochemical controls.

Students should do the first two activities before viewing Program 3, in orderto gain a thorough understanding of the details of kidney structure andfunctioning.

There are a number of good activities related to osmoregulation; two aredescribed in Activities 3 and 4. In addition, you might enjoy using the inquiryfilmloop of the Biological Sciences Curriculum Study: The Kidney andHomeostasis (available from Gage Educational Publishing or LongmanCanada Limited). A similar form of inquiry in print is the Nuffield BiologyProgram, Book 2: Living Things in Action, Chapter 15 (see Further Reading).

Activity 4: A Comparison ofAdaptations in Kidneys ofDifferent OrganismsAspects of the evolution of kidneys for life in differentenvironments are well described in Galbraith andWilson, Biological Science: Principles and Patterns ofLife, pp. 317-19 and 449-51 (see Further Reading).

Some of the same ideas may emerge in Crawford,Patterns in Biology, p. 387, Inquiry Investigation: "IsHomeostasis -Common to All Vertebrates?" - and p. 390,I nquiry Activity: "Variation in Kidney Tubules inVertebrates." There is also an enrichment column onthe camel. See also the textbooks by Volpe and byWallace in the reading list.

Activity 5: Review Questions1. Define osmoregulation. In what organs of the

human body are the processes involved inosmoregulation and its control carried on?

13

14

2. Make a labelled diagram to represent the structureof the human kidney as seen in longitudinal section,and showing its connections to the circulatorysystem and to the urinary bladder. Beside each labelstate the function of that part.

3. What is a nephron? Make a labelled diagram toshow the parts of a nephron and their relationshipsto the circulatory system.

4. Describe in detail the functioning of each part of thenephron.

5. Explain the role of osmosis in the reabsorption ofwater in the nephron.

6. What is the role of hormones in the control ofosmoregulation?

7. Explain the adaptations of excretory systems of thefollowing organisms that permit them to live in quitedifferent surroundings:(a) marine invertebrates(b) marine fish(c) freshwater fish(d) land-dwelling vertebrates

Further ReadingBenson, Garth D., et al. Investigations in Biology. Don

Mills, Ontario: Addison-Wesley Publishers, 1977.Borow, Maxwell. Fundamentals of Homeostasis. 2nd

ed. (Chapter 3) Flushing, N.Y: Medical ExaminationPublishing Company, 1977.

Crawford, Ian. Patterns in Biology. Toronto: McGraw-Hill Ryerson Ltd., 1983.

Galbraith, Donald I. Lab Manual - Biological SciencePrinciples and Patterns of Life. Revised ed. Toronto:Holt, Rinehart and Winston of Canada, Ltd., 1976.

Galbraith, D.I., and Wilson, D.G. Biological SciencePrinciples and Patterns of Life. 3rd ed. Toronto:Holt, Rinehart and Winston of Canada, Ltd., 1978.

Ginetzinsky, H. `The Role of Hyaluronic Structures inthe Evolution of Water Excretion:' In TheDevelopment of Homeostasis. London: AcademicPress,1960.

Nuffield Foundation. Living Things in Action. Reviseded. London: Longman,1974.

Volpe, E. Peter. Man, Nature and Society. AnIntroduction to Biology. 2nd ed. Dubuque, Iowa:Wm. C. Brown, 1979.

Wallace, Robert A. Biology, The World of Life. 2nd ed.Santa Monica, Calif.: Goodyear Publishing Co., 1978.

The Feedback Cycle A Controlling Principle for HomeostasisObjectivesStudents should be able to:

1. Distinguish between positive and negative feedback, and identify theconsequences of each for an organism.

2. Explain the role of negative feedback in maintaining homeostasis.3. I dentify the location of temperature receptors in the body.4. Describe five responses initiated by the hypothalamus when body

temperatures rise or fall appreciably.5. Identify the advantages that a nervous system gives animals for coping

with change.

Program DescriptionThe first program in this series showed a model of a basic control system,consisting of a receptor, a control centre, and an effector To complete thesystem, a feedback message was needed to inform the receptor that theresponse had been completed and to stop sending its message. In thisprogram feedback mechanisms are examined in more detail.

There are two kinds of feedback: positive and negative. Positive feedbackenhances the original signal, making a stronger response. It is like the howlyou hear in a sound amplifying system when a microphone picks up thesound from its own speaker and magnifies it out of control. In a system thatfluctuates, positive feedback causes the increasing swings, leading to insta-bility (Figure 1). In a living system, positive feedback leads to death, sincethe organism can no longer control its environment within narrow limits. Anexample of positive feedback in an organism is severe shock.

Figure 1

Negative feedback, on the other hand, maintains stability. It does this bycausing the product of the system to shut the system off. I n a fluctuatingsystem negative feedback succeeds in keeping the swings within properlimits (Figure 2). Even if the environment adversely affects an organism withan original wide fluctuation from normal conditions, negative feedback willrestore equilibrium. Thus negative feedback is the adaptive mechanism thatmaintains life i n every organism (Figure 3).

Figure 3

To illustrate the feedback principle this program reviews the homeostasisof temperature control which was discussed in Program 1. The receptors forheat and cold are sensory cells in the skin and in the hypothalamus. Thehypothalamus is located deep in the head at the base ofthe brain (see.Figure 4). This i s the region where nerve fibres cross over from the spinalcord to the brain, and from the eyes to the opposite sides of the brain. Thusthe hypothalamus is at a very important junction of many nerve fibres. Itcontains 16 clusters of nerve cells that are concerned with many aspects ofhomeostasis: controlling hunger, thirst, and the response to fright, as well asregulating the temperature and composition of the blood. These centresdetect misalignment of the different components of the steady state andrespond by producing nerve impulses and hormones. Thus the hypothala-mus contains many control centres, and allows the master control functionto shift from one centre to another as the need arises.

Figure 2

15

16

Figure 4. Longitudinal median section of the human brain, showing the location of thehypothalamus and pituitary

The pituitary is quite close to the hypothalamus and intimately connectedto it. The pituitary is suspended from the base of the brain by a short stalkthat contains both nerve fibres and blood capillaries.

The program then examines temperature control by looking first at theresponse to a cool environment. Cold detectors in the skin send a nervei mpulse to the hypothalamus and to the cerebral cortex of the brain (Figure5). The initial response of the nerves is to dilate the blood vessels in theskin, causing rosy cheeks. One of the nerve clusters in the hypothalamus isalso monitoring the temperature of the blood, ready to respond when thetemperature deep in the body drops too low. Nerve impulses travel from thehypothalamus to three different effectors. One of these is the set of musclesthat control the size of the arterioles carrying blood to the skin. When theskin becomes too cold, these muscles constrict the arterioles so that lessblood will flow. This has the effect of preventing further heat loss to theenvironment - it stops further cooling of the blood. But the disadvantagenow is that the skin will become still colder. In making this response thebody chooses to sacrifice some external cells in order to maintain the interi-or environment. The result of this decision may be frostbite.

The second effector of the response against cold is the set of muscles inthe skin that make the hair stand erect. In most mammals and birds this res-ponse fluffs up the body covering, trapping more air and making it a betterinsulating layer. In humans, however, we have so little hair that the method is

not effective. We notice the gooseflesh, the little bumps in the skin made bythe erector muscles at the base of each hair. This response might be calleda vestige of an evolutionary change, because it suggests that we likely aredescended from ancestors that had more hairy bodies. Both of the first tworesponses are aimed at conserving the body's supply of heat by restrictingfurther heat loss.

The third effector of response is in the deeper muscles of the skin. Nervei mpulses make these muscles contract in waves that we call shivering.The contraction of the muscles releases stored energy and thus generatesheat. Shivering helps warm the body because blood flowing through thesemuscles will soon be on its way to the rest of the body.

There is still another response - a fourth way to keep warm. A cluster ofcells in the hypothalamus produces a releasing hormone. This enters theblood capillaries and is carried the short distance down the stalk to theanterior lobe of the pituitary gland. Here the releasing hormone stimulatescertain cells to produce another hormone: thyroid stimulating hormone (orTSH). TSH enters the blood and is carried to the thyroid gland in the neck.Here TSH stimulates the cells to produce yet another hormone, thyroxin.This, too, is carried by the blood. Eventually it reaches every cell in the body.One of its effects is to raise the basic metabolic rate, making every cellgenerate more heat as it uses up stored materials. In effect, it sets up our"thermostat" and we get warm all over.

The fifth response is initiated by the brain. In the cortex of the cerebrum athought forms: "I feel cold." The brain decides that we had better movearound. We stamp our feet and swing our arms about or put on warmerclothes. This action of the voluntary muscles (the muscles that are con-trolled by thought and that move the skeleton) generates still more heat, andwarms up our blood.

Feedback to the hypothalamus comes in two ways. First, the rise intemperature of the blood stops the hypothalamus from emitting its nerveimpulses. Second, when the thyroxin in the blood reaches the cells in thehypothalamus, it turns off the releasing hormone.

What happens when your body temperature rises? You may have noticedthat your skin becomes redder when you become hot. This is caused by theincreased flow of blood to the skin, the body's heat radiator. Sensor cells inthe hypothalamus, responding to an increase in the temperature of theblood, have inhibited the message to the muscles that control the arteriolesin the skin. These muscles have relaxed, letting the arterioles dilate to allowa much larger flow of blood to the skin; this cools the skin by transferringheat to the environment.

At the same time the change in the impulses from the hypothalamus hassignalled to the sweat glands in the skin to release perspiration. Evaporationof water requires heat for the change of state. Vaporization of the sweatcarries heat away from the skin, cooling the skin and the blood flowingthrough it. In very hot weather perspiration may get rid of as much as 1.5 L

RECEPTORS

Muscles closearterioles, reducingblood flow to skin

Start exercising:muscles get warm

Figure 5. The homeostasis of temperature control

of water in an hour If we lose too much water this way we can suffer heatshock. Under normal conditions, however, the combination of sweating andi ncreased blood flow to the skin will result in enough cooling so that thehypothalamus will shut off the emergency cooling measures.

The regulation of body temperature has illustrated how intricate some ofthe mechanisms for homeostasis are. You can see that there are severalways of increasing and decreasing body temperature. The same kinds of

pattems are evident when we look at the regulation of the many differententities that organisms regulate. The more altemate effectors an organismcan use to maintain homeostasis, the better are its chances of survival.

Organisms that developed nervous systems are better able to controlthe stability of their internal environments because nerves make possible aquicker response to a stimulus, triggering rapid reflex regulatorymechanisms.

17

18

Activities

Before ViewingAsk students to do Activity 1 and to prepare a diagram (similar to Figure 4)of the median longitudinal section through the brain, so that they willunderstand the relationships between the hypothalamus, the brain stem, thepituitary, and the cerebral hemispheres. Make copies of Figure 5 to give outafter the viewing.

Activity 1: The Structure of theMammalian Brain

Several laboratory manuals provide good instructionsfor students to explore the structure of the brain of asheep. Preserved material is firmer and more suitablethan fresh material for this activity. Some usefulsources are:

Benson et al., Investigations in Biology, I nvestiga-tion 4.Cooley and Vanderwolf, The Sheep Brain: A BasicGuide.Crawford, Patterns in Biology, p. 415.(See Further Reading)

Activity 2: The Effect of Tempera-ture on the Heartbeat of Daphnia

Here is a manageable investigation, using a smallinvertebrate:

Brown and Creedy, Experimental Biology Manual,p. 204.(See Further Reading)

Activity 3: Review Questions

1. What is positive feedback? What is the effect ofpositive feedback in an organism?

2. What is negative feedback? What is the effect ofnegative feedback in an organism?

3. Make a diagram to represent a simple model of a

homeostatic system, showing the relationship ofnegative feedback to the other components.

4. Where are the temperature receptors located in thehuman body?

5. Describe five responses initiated by the hypothala-mus when body temperature rises or fallsappreciably.

6. How do plants, fungi, protoctists (protists), andmonerans deal with changes in their surroundings,since they lack nervous systems?

7. What advantages do animals have in coping withchanges in their surroundings in comparison withorganisms that lack nervous systems?

Further Reading

After Viewing

Benson, Garth D., et al. Investigations in Biology. DonMills, Ontario: Addison-Wesley Publishers, 1977

Brown, G.D., and Creedy J. Experimental BiologyManual. London: Heineman Educational Books,Limited, 1970. '

Cooley, Richard K., and Vanderwolf, C.H. The SheepBrain. A Basic Guide. London, Ontario: A.J. KirbyCompany, 1979.

Crawford, Ian. Patterns in Biology. Toronto: McGraw-Hill Ryerson Co., 1983.

Kirk, David, et al. Biology Today. 2nd ed. Del Mar,Calif.: Random House, 1975.

Sherman, I.W., and Sherman, V.G. Biology. A HumanApproach. New York: Oxford University Press, 1979.

Wallace, Robert A. Biology, The World of Life. 2nd ed.Santa Monica, Calif.: Goodyear Publishing Co., 1978.

Weisz, Paul B. The Science of Biology. 4th ed.New York: McGraw-Hill Co., 1971.

Distribute the copies of Figure 5, and discuss the intricacies of thehomeostatic and feedback mechansims. Have students prepare notes basedon the review questions (Activity 3).

Biochemical Balances Regulation of Body ChemistryObjectivesStudents should be able to:

1. State the Law of Mass Action, and give an example of a reactioncontrolled by this principle.

2. Explain the interaction of carbon dioxide and oxygen in the tissues andi n the lungs in terms of mass action.

3. Write a word equation to represent the action of hemoglobin in thepresence of surplus oxygen or surplus hydrogen ions.

4. Construct a diagram to explain how the level of carbon dioxide in theblood is controlled homeostatically.

5. Define competitive inhibition, and give an example of the principle.6. Explain how competitive inhibition affects homeostasis at the cellular

l evel.

Program DescriptionThe series, thus far, has shown several examples of homeostasis - theways that particular entities within the body fluids are regulated. The regu-l ation of the composition and temperature of body fluids is the first step inensuring that chemical changes in the body are under control. This programexamines some phases of homeostasis that involve chemical reactions.

A first principle of chemical reactions is called the Law of Mass Action.This law states that at a given temperature the rate of a chemical process isdirectly proportional to the masses of the reacting substances. That is, if wehave more of a particular substance that takes part in a reaction, then thereaction will go faster to use up that substance, provided it is in a closedsystem. An example is the reaction of carbon dioxide with water to producecarbonic acid:

This reaction can proceed in either direction; if there is a lot of carbonicacid (as in a freshly opened bottle of carbonated beverage), then the reactionwill go to the left, producing carbon dioxide and water. We can show thiswith arrows in both directions:

Now the Law of Mass Action says that if we have a large mass of carbondioxide present, the reaction will go more rapidly to the right. On the otherhand, if there is little carbon dioxide present, but a larger mass of carbonicacid, the reaction will proceed mainly to the left.

Carbon dioxide is produced in the cells of an organism when cellularrespiration makes energy available. By diffusion, carbon dioxide moleculeswill move out of the cells into the surrounding extracellular fluid (ECF) andinto the blood. Here, it will react with water to form carbonic acid. Anenzyme in the blood, carbonic anhydrase, speeds up this reaction 13 000times. While five percent of the carbon dioxide is carried in simple solution,just dissolved in the water, 12.5 times as much is carried in chemical combi-nation. But carbonic acid is not very stable in the presence of the many ionscarried in the plasma and the tissue fluids. So as fast as carbonic acidforms, it breaks up into ions:

Notice those reversible reactions; all of these substances are in a dyna-mic equilibrium which can shift in either direction to absorb an additionalload of materials from the cells or the environment. If you added up thepercentages, you may have wondered why they only totalled 80 percent. Theother 20 percent of carbon dioxide is carried in the blood bound to proteins,i ncluding the hemoglobin of the red blood cells. Now the hydrogen ion thati s set free in the above reaction can combine with hemoglobin as well.When it does this it changes the shape of the hemoglobin molecule so thatoxygen, carried by the hemoglobin, is set free to diffuse into the cells. Thereit will be used in cellular respiration:

19

In this way, hydrogen ions can be tied up with bicarbonate, or withhemoglobin, or with other proteins and ions, to prevent the blood frombecoming too acid. This action to control the pH of the blood is calledbuffering. It is one of the aspects of homeostasis.

When the blood reaches the capillaries of the lungs, the Law of MassAction goes to work again to reverse these reactions. In the air within thelungs there is a greater concentration of oxygen than in the blood. Thusoxygen diffuses from the air into the blood. As it combines with the reducedhemoglobin the molecule changes shape again, releasing the hydrogen ions:

At the same time, the carbon dioxide bound to a few of the hemoglobinmolecules will be set free. The hydrogen ions will then combine withbicarbonate ions:

This reaction ultimately sets free more carbon dioxide. By diffusion,carbon dioxide will move from the blood, where its concentration is high,into the air of the lung, where its concentration is low. When we next exhale,some of the carbon dioxide will be excreted from the body (see Figure 1).

So far the program has dealt with the effects of mass action. A secondkind of chemical control of the rate of a reaction is one where the endproduct combines with the enzyme that caused the change, thus competingwith the substance that started the reaction. For example, suppose aparticular enzyme converts substance X into substance Y: X enzyme . -Y.Now as soon as Y is formed it combines with the enzyme. This means thatthere will be very little of the enzyme left to continue changing substance X.This kind of action is called competitive inhibition because the competitionof both X and Y for the enzyme limits or inhibits the reaction. It is anexample of negative feedback:

Figure 1. Summary of reactions involved in gas exchange in tissues and lungs

An example of competitive inhibition in a living system occurs in animalcells during the production of one of the amino acids, isoleucine:

Thus isoleucine competes with the starting substance, threonine, to inhibitthe rate of its own production in the cell. If no isoleucine is present, the cellwill make some. As soon as any is present, production stops. Thus we havea homeostatic system where the product controls its own rate of production.

20

Activities

Before ViewingA review of cellular respiration and the ways that gases are carried in theblood would be a valuable introduction to this program. Inquiry-orientedteachers may wish to have students investigate the first two activities beforeviewing the program. Others will prefer to leave them until after viewing.

Activity 1: To Study Gas Exchangein Humans

Refer to Benson et al., Investigation in Biology,I nvestigations 33 and 34 (see Further Reading). Thefirst investigation demonstrates the effects of carbondioxide on the rate of breathing: normally after exer-cising, during hyperventilation, and while breathingi nto a plastic bag. The second involves the chemistryof exhaled gas. An alternate version of the latter isfound in Galbraith, Lab Manual - Biological Science,Investigation 23 (see Further Reading).

Activity 2: The Mechanism ofStomatal Movement

Mass action determines many of the responses ofplants to changing conditions, and is thus importanti n maintaining homeostasis. This laboratory activity,described fully in Brown and Creedy, ExperimentalBiology Manual, p.177 (see Further Reading), uses thespiderwort Tradescanda, a hardy plant for theclassroom. It is motivating for students because of itspurple epidermal cells, setting off the green guardcells. The leaves are placed in four different solutions,i n darkness and in light, and after 15 minutes studentscount 25 stomata and record how many of them areopen and how many are closed. From the data theydeduce the conditions that determine stomatalopening.

After Viewing

Activity 3: The Use of an Enzyme Further ReadingInhibitor to And Out the Individual Baker, J.J.W, and Allen, G.E. The Study of Biology. 4thSteps in the Pathway of a Reaction ed. Reading, Mass.: Addison-Wesley Publishing

Company, 1982.Benson, Garth D., et al. Investigations in Biology. Don

Mills, Ontario: Addison-Wesley Publishers, 1977.Borow, Maxwell. Fundamentals of Homeostasis. 2nd

ed. (Chapter 6) Flushing, N.Y: Medical ExaminationPublishing Company, 1977.

Brown, G.D., and Creedy, J. Experimental BiologyManual. London: Heinemann Educational Books,Limited, 1970.

Galbraith, Donald I. Lab Manual - Biological SciencePrinciples and Patterns of Life. Revised ed. Toronto:Holt, Rinehart and Winston of Canada, 1976.

Mikal, Stanley. Homeostasis in Man - Fluids,Electrolytes, Proteins, Vitamins and Minerals inClinical Care. Boston: Little, Brown and Co., 1967.

Nelson, G.E.; Robinson, G.G.; Boolootian, R.A.Fundamental Concepts of Biology. 2nd ed.New York: John Wiley and Sons Ltd., 1970.

This laboratory activity makes use of a four-steppathway from the Krebs Cycle. It focusses on theconversion of succinic acid to fumaric acid, usinggerminating bean seeds or the kidney of a freshlykilled rat or mouse. Refer to Brown and Creedy,Experimental Biology Manual, p.133 (see FurtherReading).

Activity 4: Review Questions

1. What is the Law of Mass Action? State an exampleof a chemical reaction that is controlled by this law.

2. How does the Law of Mass Action relate to theinterchange of oxygen and carbon dioxide:(a) between a tissue cell and the blood in a

capillary?(b) between the blood in a capillary and the

alveolus?3. Write a word equation to show how hemoglobin

reacts:(a) in the presence of excess hydrogen ions(b) in the presence of excess oxygen.

4. Make a diagram to represent the homeostaticcontrol of the level of carbon dioxide in the humanbody.

5. Define competitive inhibition, and give an exampleof the principle.

6. Explain how competitive inhibition affects homeo-stasis in a cell.

Discuss the principles of mass action and competitive inhibition. Completethe remaining activities. Ask students to make notes, based on the reviewquestions (Activity 4).

21

Hormonal Control The Coordination of Homeostasis

1. Define hormone and endocrine gland.2. Identify common hormones (such as thyroxin, ADH, insulin, and

glucagon) and state where they are produced and where they producetheir effects.

3. Describe the location and relationship between the hypothalamus andthe pituitary gland.

4. Describe the role of the hypothalamus and the pituitary gland in thecontrol of the reabsorption of water. They should also be able toexplain how feedback operates in this system.

5. Outline the two-hormone system for the control of blood glucose.6. Name the different kinds of cells in the pancreas and identify the

secretions produced by each.7. Outline the hormonal control system that determines the level of

thyroxin in the body.

Program DescriptionMany of the homeostatic mechanisms that have been illustrated in earlierprograms have been controlled by hormones. The glands of the body may bedivided into two classes: the exocrine glands that pour their secretionsdirectly into ducts that deliver them where they are needed; and the endo-crine glands, whose secretions are called hormones, which are delivered bythe blood stream to all parts of the body. Hormones are secreted in smallamounts, and they produce their effects at a distance from their source.Specific target cells are affected by hormones, while other cells areapparently. unaffected.

What might account for the difference in action upon the cells? Onehypothesis is that target cells must have specific receptor modules, prob-ably proteins, located in their cell membranes. When the right hormonereaches these receptors, the protein might produce a secondary messengermolecule inside the cell, one that would activate a particular enzyme andinitiate a series of reactions (Figure 1). For example, in Program 3 it wasexplained that the anti-diuretic hormone (ADH) from the pituitary glandcontrolled the permeability of the cell membranes in the distal portion of theuriniferous tubules in the kidney, making them reabsorb more water andspecific ions. This change in permeability is reversible, adjusting for variedconditions of the body fluid (ECF). The change in permeability may be

caused by the action of a particular enzyme, succinic dehydrogenase, thatremoves hydrogen from succinic acid.

Figure 1. Hypothetical model to account for the action of a hormone only on specifictarget cells

I n the example above, the need for the hormone ADH stimulates its pro-duction. When it is no longer needed the production of the hormone ceases.This is the simplest of the hormonal control mechanisms. In this programwe see examples of two more complex mechanisms. In the first example,two hormones are produced that have opposite actions. Thus they are calledantagonistic. Their combined effect is to control the level of blood glucoseto within the very narrow limits of 0.1 percent. The second example is a morecomplex mechanism involving four hormones and a very finely tuned controlof metabolism.

The program uses the control of blood glucose as an example of a home-ostatic control in which two hormones act antagonistically. Program 1mentioned that when we eat at rather odd intervals, or even if we fast, it isimportant that the level of glucose in the blood, be regulated. Glucose mustreach all of our cells at a fairly constant rate to supply energy for theirvarious activities. It is especially important for the brain cells to be nour-ished, as they are the first to malfunction when the level of glucosebecomes too low. How is the homeostasis of glucose accomplished?Everyone knows of the disease sugar diabetes (diabetes mellitus), involvinga misalignment of this control system. Diabetics must consciously regulatetheir glucose level by controlling their intake of sugars and hormone.The rest of us are lucky in that our homeostatic controls are automatic.

ObjectivesStudents should be able to:

22

Sugar diabetes is caused by the failure of the body to produce sufficienti nsulin. Sir Frederick Banting was a Canadian doctor who established, in1922, the relationship of insulin from the pancreas to the control of bloodsugar. In the pancreas there are about a million small clusters of cells calledthe islets of Langerhans. They are distinct from other cells of the pancreasthat secrete digestive juices. The islets pour their secretions directly into theblood rather than into the pancreatic duct. There are two kinds of cells in the

I nsulin is a small protein consisting of 51 amino acids. Its action is toreduce the level of glucose in the blood. It does this by converting glucoseto glycogen in muscle and liver cells, and by converting glucose to fats andproteins in other body cells. These actions effectively store glucose in largermolecules for future use. Insulin also accelerates the breakdown of glucosein most cells, releasing energy. There are two hypotheses about how insulinaccomplishes these actions. First, as discussed in Program 4, insulin mayaffect cell membranes, making the membranes facilitate the movement ofglucose into the cells. Or, second, insulin may accelerate the phosphoryla-tion of glucose to make it more chemically active so that it can react faster.

The second hormone, glucagon, is also a very small protein, consisting ofonly 29 amino acids. It is produced in the alpha cells of the islets ofLangerhans when the level of glucose in the blood falls below 0.1 percent.Glucagon acts on the liver to take glucose out of storage, convertingglycogen to glucose-1 phosphate:

In other tissues, glucagon accelerates the breakdown of glucose, producingenergy. In the kidneys, glucagon changes the rates of excretion of sodium,potassium, and phosphate ions. We can now complete our homeostasisdiagram:

REGULATED RECEPTOR MESSAGE EFFECTORS RESULTSENTITY

Figure 3

Thus the homeostatic diagram for the action of insulin in the control ofglucose level looks like this:

In the antagonistic pair, the second hormone succeeds in turning off theaction of the first, before large swings can occur in the levels of theregulated entities. If we consider only the first hormone, insulin, a graph ofthe levels of the regulated entity would look like this:

REGULATED RECEPTOR MESSAGE EFFECTORS RESULTSENTITY

Figure 4

23

insulin Ti ne alpha ceps produce anotner hormone, glucagon, that actsantagonistically to insulin.

islets of Langerhans. designated aloha The beta cells produce

24

The second hormone refines the control so that the regulated entity iskept within even narrower limits:

Figure 6

Thus, homeostasis, the regulation of the internal environment withinnarrow limits, is fine-tuned by the interaction of antagonistic pairs ofhormones.

A third kind of homeostatic control is the interaction of sets of hormones,where one hormone stimulates the production of a second that suppressesthe first. This kind of interaction maintains a steady level of hormone in theblood. One example of such a system is the production of thyroxin, whichcontrols the rate of metabolism in the body. Extra thyroxin makes each cellrelease more energy, using up more oxygen and producing more heat.Thyroxin is produced in the thyroid gland in the neck. Its production isstimulated by TSH (thyroid stimulating hormone) from the anterior lobe ofthe pituitary gland at the base of the brain. When the level of thyroxin fallstoo low, the pituitary causes more TSH to flow. This in turn stimulates thethyroid to produce more thyroxin. As blood containing more thyroxin flowsthrough the brain, the level of TSH is suppressed. So we have a feedbackl oop (Figure 6) where two hormones produced by two glands turn each otheron and off, regulating the level of thyroxin in the blood, and thus controllingour rate of metabolism.

Figure 6. The interaction of two glands and two hormones

But it is not quite that simple. The detectors of the concentration of thyroxinare not in the pituitary, but in the hypothalamus above it, in the brain. Hereare located the control centres for many of the regulated entities. A group ofchemoreceptors with a good blood supply is constantly monitoring the com-position of the blood. When these cells detect an insufficiency of thyroxin,they secrete a short polypeptide, called thyrotropin releasing factor (TRF),i nto the blood. There is a special portal system of veins that carry thishormone directly to the anterior lobe of the pituitary, where it causes thecells to release TSH. When thyroxin levels rise, another group of controlcells, also in the hypothalamus, secretes another short polypeptide, somato-tropin release inhibiting factor (SRIF). SRIF inhibits the production of severalhormones from the pituitary: TSF, human growth hormone, insulin, andglucagon. Thus, our diagram of the interactions (Figure 7) involves threeglands, four hormones, and a very fine-tuned homeostatic control.

Figure 7. Homeostatic control involves three glands and four hormones

Further StudyThere has not been time in this series to show all the aspects of homeo-stasis. Students can extend this study by researching other fascinatingaspects of hormonal control, such as the antagonistic pair of hormones thatcontrols the level of calcium ions in the blood (parathormone and calcitonin),and the interacting feedback control mechanisms that determine themenstrual cycle of the human female. How many hormones are involved,where are they produced, and where do they exert their effects? How doesan understanding of this cycle enable women to plan their pregnancies?

The interrelationship between genetics and homeostasis has not beendiscussed in this series, again, because of time limitations. How is theability to develop homeostasis inherited? How can the same hormones, FSHand LH, have such different results in the two sexes? How do genes affectthe functioning of hormones and vice versa? Do certain substances blockthe flow of genetic information?

Some authors extend the meaning of homeostasis beyond the limits ofthe individual organism. Certainly, feedback systems appear to control thelife and functioning of larger biological units: the species, the population,the community, the ecosystem, the biome, indeed the whole biosphere. Canthe stability of such large systems be considered homeostasis? How does aspecies cope with variations in the environment, yet maintain itself genera-tion after generation? How do homeostatic mechanisms reduce errors anddamage in genes and repair DNA molecules to ensure genetic stability?How does the Hardy-Weinberg Law relate to the stability of genes in apopulation? In what way is evolution a kind of homeostatic adaptation ofspecies? Dynamic equilibrium, the establishment of controls that keepfluctuations within limits, seems to be as much a characteristic of the largerbiological entities as of the individual organism. Can we use our model of

Activities

Activity 1: Invitations to Inquiry

If you like the teacher-centred inquiry mode of teach-ing, you may wish to adapt some of the six Invitationsthat Joseph J. Schwab has provided on this topic inthe Biology Teacher's Handbook, pp. 462-482 (seeMayer, Further Reading). They are:

Invitation 39: Control of Blood SugarI nvitation 40: Blood Sugar and the InternalEnvironmentI nvitation 41: Blood Sugar and InsulinInvitation 42: Blood Sugar and HungerInvitation 43: Basal Metabolic RateInvitation 44: The Stress Reaction: Adrenaline

Activity 2: Effects of a Hormone onthe Heart Rate of Daphnia

This laboratory-oriented inquiry makes use ofephinephrine (adrenalin) to stimulate the heart rate of

the small crustacean. Refer to Lang et al., TheFunctioning Animal, I nquiry 21 (see Further Reading).

Activity 3: An Investigation of theEffect of Adrenalin and Acetylcho-line on the Heart of a Frog

Not every teacher will want to pith a live frog for thisactivity, but those who do can refer to Brown andCreedy, Experimental Biology Manual, page 254 (seeFurther Reading).

Activity 4: The Importance ofHomeostasis

homeostatic control to account for the balance in predator-prey relation-ships, or the co-evolution of insects and flowering plants, or the mainte-nance of the level of the elements involved in the bio-geo-chemical cycles?Self-regulation within narrow limits is thus a characteristic of life at all levels,from the individual cell to the entire biosphere.

Before ViewingThe students could view Program 6 without any prior introduction, however,Program 3 introduces the concept more simply. Some of the activitiesrelated to this program could be used to prepare students for viewing.

After Viewing

This is a series of questions dealing with osmoticpressure and the level of calcium ions - a "dry" lab,or cerebrational inquiry. It can be found in Benson etal, Investigations in Biology, I nvestigation 10 (seeFurther Reading).

Students should do several of the related activities. It would be useful toteach the additional examples that are discussed in Further Study: thecontrol of calcium ion concentration and the regulation of the menstrualcycle of the human female. Complete the discussion by asking students toprepare notes based on the review questions (Activity 5).

Activity 5: Review Questions1. (a) Define an endocrine gland and tell how it differs

from an exocrine gland.(b) Define hormone and explain the hypothesis of

how a hormone affects certain target cellswithout having an effect on most cells of thebody.

2. Construct a table with the following headings:HORMONE WHERE PRODUCED WHERE IT ACTSNATURE OF THE ACTIONComplete the table for the following hormones:thyroxine, ADH, insulin, glucagon, parathormone,calcitonin, estrogen, progesterone.

3. Describe the location of the hypothalamus and ofthe pituitary gland. What is the nature of theconnection between them? How does the hypothal-amus control the functioning of the pituitary?

4. Describe the control of the reabsorption of water inthe human body. How does feedback work in thissystem?

5. Make a diagram to explain the homeostatic controlof the level of the glucose in the blood.

25

6. Describe the structure of the pancreas and stateexactly where each of its secretions are produced.

7. Outline the hormonal control of the level of thyroxinin the body.

8. In what ways is homeostasis a characteristic of lifeat all levels? In your answer give examples of theapplication of control systems at the population,species, community, biome, and biosphere levels.

Further Reading

OrderingInformationTo order this publication or videotapes of theprograms in the series Homeostasis, or for additionalinformation, please contact the following:

Baker, J.J.W., and Allen, G.E. The Study of Biology.4th ed. Reading, Mass.: Addison-Wesley PublicationCo.,1982.

Benson, Garth D., et al. Investigations in Biology. DonMills, Ontario: Addison-Wesley Publishing Co.,1977.

Brown, G.D., and Creedy, J. Experimental BiologyManual. London: Heineman Educational Publishing,1970.

Emmel, Thomas Co. Worlds Within Worlds. New York:Harcourt Brace Jovanovich, Inc., 1977.

Jones, Kenneth C., and Gaudin, A.J. IntroductoryBiology. New York: John Wiley and Sons, Ltd., 1977.

Kimball, John W Biology. 5th ed. Reading, Mass.:Addison-Wesley Publishing Co., 19B3.

Lang, H.M.; Palfery, E.G.; and Van Nieuwenhove, E.L.R.The Functioning Animal. Toronto: Gage EducationalPublishing, 1978.

Mayer, Wm. V., ed. Biology Teacher's Handbook.3rd ed. New York: John Wiley and Sons, Ltd., 1978.

McElroy, WD.; Swanson, C.P; and Macey, R .I. Biologyand Man. Englewood Cliffs, N.J.: Prentice Hall Inc.,1975.

Volpe, E. Peter. Man, Nature, and Society. AnIntroduction to Biology. 2nd ed. Dubuque, Iowa:Wm. C. Brown, Ltd., 1979.

OntarioTVOntario Sales and LicensingBox 200, Station QToronto, Ontario M4T 2T1(416) 484-2613

Videotapes BPNProgram 1: Coping with Change 226401Program 2: The Sea Within 226402Program 3: Osmoregulation 226403Program 4: The Feedback Cycle 226404Program 5: Biochemical Balances 226405Program 6: Honnonal Control 226406

26