HOMEOSTASIS & HORMONES

“…it is all about a balancing act…”

OVERVIEW

A) HomeostasisB) Hormones: chemical nature and mode of

actionC) Hypothalamus and Pituitary glandD) Adrenal glandsE) Hormonal regulation of blood sugar levels

Homeostasis: Greek: ‘homoios’ = similar

‘stasis’ = standing still

Intracellular fluid (cytoplasm)

Homeostasis is the maintenance of a relatively stable internal environment

What is the ‘internal’

environment?

Are variables absolutely constant?

STEADY STATE

Low

Variable

High

NO

Examples of Physiological conditions requiring homeostasis:

O2 and CO2 levels in the bodyenergy requirementsglucose level in bloodwater / ion balancepHtemperature

fluid surrounding cells

where organism

lives

Living systems are seen to be open systems. What does this mean?

Require a continuous exchange of matter between the environment and themselves

Oxygen

Food

Carbon dioxide

Heat

UreaWater

An organism may be thought of as a complex system of chemical processes

these processes are: self-regulating

tend to maintain a steady state even though the external environment changes

Why is homeostasis is important?An organism functions more efficiently if

conditions are maintained within narrow limits close to optimum conditions

To prevent large fluctuations from the

optimum

What causes these fluctuations? Changes in external and internal environments

Internal conditions may be maintained constant within the body by:

developing a variety of mechanisms:

StructuralPhysiologicalBehavioural

Fat

Fur

Feathers

Structural

Which structure is missing here?

May 22, 2002

Sweat

Increase in heart rate Cardiac arrest

in frozen frog

Physiological

HOT: Seek shade

HOT: Seek shade

COLD: Bask in the sun

Behavioural

Three basic components of a control system:

1. Detector / Receptor / Sensor

2. Regulator / Control centre / Co-ordinator / Integrating centre

3. Effector

Stimulus(Input)

Response(Output)

What is the role of each component?

Detects deviation from set point

Produces response

Compares input with set point & determines response

Integrating Centre in mammals is:

An endocrine glandBrain or spinal

cord

What is Feedback?Feedback refers to responses made

after a change has been detected

Time

Positive feedbackTwo forms of feedback:

Negative feedback

Time

EquilibriumDivergence

Negative Feedback:refers to the mechanism by which a system

responds to a fluctuation in the opposite direction

Corrective mechanism: DECREASES body temperature

Body temperature: RISES

Corrective mechanism: INCREASES body temperature

Body temperature: DECREASES

Sensor + Regulator:

Effector:

Blood sugar regulation: Negative feedback

In negative feedback (feedback inhibition):

the initial stimulus provokes a response which tends to reduce the magnitude of the stimulus

as conditions return to their optimum, the corrective processes can be switched off

Negative feedback also applies to the regulation of a population size:

Death rate increases

Birth rate increases

Why is negative feedback very common in the body?

increases the stability of systems

Examples of Negative Feedback Control:

O2 and CO2 levels in the bodyhormone levels, e.g. thyroxinesex hormonesmetabolic levels e.g. glucosewater balancethe regulation of pHbody temperature

Control of thyroxine release as an example of negative feedback

TRH (thyroid releasing hormone / thyrotrophin releasing hormone)

TSH (thyroid stimulating hormone)

TRH

TSH

Positive feedback: a disturbance leads to events which increase

the disturbance even further

rare in biological systems WHY rare?

lead to: an unstable situation extreme states

Examples of Positive Feedback Control:

1. Blood clotting Activated platelet releases chemicals

More platelets are activated

A blood clot forms

Examples of Positive Feedback Control:

2. Child birth

Oxytocin stimulates muscular contractions of the uterus

More oxytocin is released

3. The control of ovulationOestrogen stimulates the secretion of LH (luteinising hormone)

4. Stimulation of a nerve fibresodium gates open and sodium ions rush into

the axon.

Cytoplasm

Extracellular fluid

Na+Na+

K+ K+

Na+Na+

Na+

Na+

Na+

Na+

K+

K+ K+

Na+

Na+

K+

K+

Does the disturbance ever stop? in most cases, once the purpose of the

feedback loop is completed, counter-signals are released which:

suppress or break the loop

Oxytocin level drops once baby

is born

Essay Title

Write an essay on feedback mechanisms and their role in control of biological systems.

[SEP, 2001]

OVERVIEW

A) HomeostasisB) Hormones: chemical

nature and mode of action

C) Hypothalamus and Pituitary gland

D) Adrenal glandsE) Hormonal regulation of

blood sugar levels

A chemical communication system is made up of at least two types of cell:one cell:produces & releases the hormone message released

second cell with receptor:receives message target cell activated

Receptors are SPECIFIC

Explain why insulin affects all body cells but ADH only the kidney.

Receptors for insulin occur on all body cells but those for ADH only in the kidney.

ADHTarget:Kidney cell

Receptor

Hormones are chemicals secreted by:

Endocrine cells

Nerve signals (arrows) cause release of neurohormones

Neurosecretory cells

Hormones are classified over distance they operate:

Histamine

[endocrine only is in syllabus]

Endocrine hormones : are released into the bloodstream and interact with distant target cells e.g. adrenaline

Where are the receptors located?

Position of receptor depends on the solubility

of the hormone

Lipid-soluble (lipophilic) hormones

intracellular receptors

Water-soluble hormones:

receptors on the cell surface

Water-soluble & lipid-soluble hormones act in different ways:

Water-soluble hormones:Activate enzymes within the cell

Lipid-soluble Hormones:

Induce transcription

and translation to form

enzymes

Explain why lipid-soluble hormones are slow acting.

Enzymes need to be formed

before a reaction occurs.

What can you say about the solubility of ‘growth factor’? Explain.

Water soluble. Receptor is in plasma membrane as it cannot pass through it.

Steroid Model vs. Protein Model

• Diffuse through cell membrane

• Receptors in cytoplasm• Response has slow onset• Long-lasting• Typically derived from

cholesterol• Examples: Testosterone,

Oestrogens, Progesterone

• Do not diffuse through cell membrane

• Receptors on surface of cell• Response has quick onset• Short-lasting• Typically derived from

amino acids• Examples: Insulin,

Glucagon

Steroid Model Protein Model

Question: End-of Year SEP, 2010

Use your knowledge of biology to describe the significance of the following.

Steroid hormones act by activating genes.(5 marks)

Enzymes within the cell are activated when a hormone binds to a receptor

in the membrane by the help of a:

second messenger e.g. cyclic AMP (cAMP)

cAMP forms from ATPThe enzyme that catalyzes the formation of cAMP from ATP is:located on cytoplasmic side of plasma membranecalled adenylyl cyclase or adenylate cyclase

cAMP forms from ATP

Formation of cAMP

Adenylyl cyclase

AB

Formation of cAMP

G protein receptor

signal

CEnzymes activated

cAMP is a common second messenger

What is a second messenger?

A second messenger is a substance released in the cytoplasm after the first messenger – the signal –

binds its receptor

GTP: guanosine triphosphate

What is the “first messenger”?

Hormone

Why is a “second messenger”

needed? Hormone cannot

enter the cell.

Cyclic AMP (cAMP) has many effects on the cell :

including glucose mobilisation for: energy muscle contraction

Let us study the role of cAMP in the release of glucose from the liver

A Cascade of Reactions (part 1) an enzyme cascade is a system for amplifying

a biological signal

A Cascade of Reactions (part 2)

cyclic adenosine monophosphatecAMP

bgaGTP

adrenaline

Gs

Inactive proteinkinase A

Active proteinkinase A

adenosine triphosphateATP

aGTPGDP

G-protein linked receptor adenylate cyclase

Why are only a few molecules of the

hormone needed to produce many molecules of

product?Each enzyme in the cascade can act on many molecules of the next molecule.

Amplification occurs through a cascade of

reactions.

End-of-Year 2012

Adrenaline affects a range of target tissues in the body. Suggest how the adrenaline molecule can cause different effects in different target tissues. Different tissues have different (types of adrenaline) receptors;(causing) cAMP concentration/ second messenger to increase or decrease;Second messenger (may be) different;activates different / other enzymes / enzyme reactions (in different target cells)

cAMP is not the only second messenger

e.g. nitric oxide (NO)

cAMP is not the only second messenger

nitric oxide (NO) causes relaxation of

blood vessels

Calcium can be a second messenger

calcium is widely occurring

an increase in cytoplasmic Ca2+ concentration will cause:

NOTE: the same second messenger is involved in two very different biological actions

A muscle cell to contract

A salivary gland cell to fluid secrete

12

Remember: The nature of the biological activity triggered by second messenger is dependent on the cell type

The same second messenger is involved in more than one biological action.

True of all second messenger pathways:

Protein hormones cannot enter the cell. Say why.

Question: End-of Year SEP, 2010Use your knowledge of biology to describe the significance of the following. (5 marks)

Protein hormones act by means of a second-messenger system.

A Gland is: a structure which secretes a specific chemical

substance or substances

Sweat gland

Salivary gland

Gastric gland

Thyroid gland

A Gland secretes chemicals into:

Some other part of the body

The bloodstream

Glands in the body are:

secrete a product into a duct

secrete hormones have no duct (ductless) have a rich blood supply

Exocrinegland

Endocrine gland

Why is the pancreas described as both an endocrine and an exocrine gland?

Produces both enzymes and hormones.

Why is the pancreas richly supplied with blood?

Blood is needed to carry hormones towards the target organs.

Four different chemical categories of hormones:

1. POLYPEPTIDES hormones composed of less than 100 amino

acids e.g.

Insulin Glucagon Oxytocin Growth hormone ADH

Growth hormone

2. GLYCOPROTEINS hormones composed of a:

polypeptide larger than 100 amino acids attached to a carbohydrate e.g. FSH, LH

Carbohydrate

Polypeptide

3. STEROIDS these hormones are lipids derived from

cholesterol e.g. testosterone, oestrogen,

progesterone, cortisol

Oestrogen

4. AMINES derived from the amino acids tyrosine

and tryptophan

include all hormones produced by the: adrenal medulla thyroid gland

Adrenal glands

Kidneys

SummaryA hormone is a chemical messenger with the following properties:

it travels in the blood;

it has an effect at a site different from the site where it is made, called the target;

Summaryit fits precisely into receptor molecules in the

target like a key in a lock – it is therefore specific for a particular target;

it is a small soluble organic molecule;

it is effective in low concentrations.

OVERVIEW

A) HomeostasisB) Hormones: chemical nature and mode of

actionC) Hypothalamus and Pituitary glandD) Adrenal glandsE) Hormonal regulation of blood sugar levels

The Hypothalamus & Pituitary Gland

1. Nervous 2. Endocrine systems

major centres in the body for the coordination of the:

Position of Hypothalamus & Pituitary Gland

Pituitary gland

The Pituitary Glandis a small red-grey glandweighs about 0.5g [size of a pea]hangs from the base of the brain

by a short stalk

is a link between the nervous system and many endocrine glands

Neurosecretory cells end at the Posterior Pituitary

The Posterior Pituitary Gland:

does not synthesise any hormones but stores and releases two hormones:

ADH (antidiuretic hormone or vasopressin)

Oxytocin

Mammary glands,uterine muscles

Hypothalamus

Kidney tubules

OxytocinHORMONE

TARGET

ADH

Posteriorpituitary

Neurosecretorycells of thehypothalamus Axon

Anteriorpituitary

Function of ADH: Increased permeability to water by:

Distal convoluted tubule

Collecting tubule

ADH causes formation of aquaporins in tubule walls

Functions of Oxytocin: contraction of the uterus during birth ejection of milk from the nipple

The Anterior Pituitary is connected to the hypothalamus by a:

Portal system:connects two

organs, neither of which is the heart

Relationship between secretory cells & blood vessels in hypothalamus & pituitary

The hypothalamus collects information from:

1. other regions of the brain 2. blood vessels passing

through it

What does the hypothalamus do with this information?

passes it to the pituitary gland

How does the pituitary gland respond?

By releasing hormones,

which directly or indirectly

regulate the activity of all other endocrine

glands

What parts of the body does the pituitary gland affect?

How does the hypothalamus control the anterior pituitary gland?

Hypothalamus produces:1. Releasing hormones2. Release-inhibiting hormones

The pituitary gland is under the influence of the hypothalamus in two ways. How?

1. The anterior pituitary produces hormones or stops producing them when the hypothalamus releases its own hormones.

2. The hormones released from the posterior pituitary are actually secreted by neurosecretory cells in the hypothalamus.

Hypothalamus hormone

Anterior pituitary hormones

Site of action

Prolactin-releasing hormone Prolactin release-inhibiting hormone

Prolactin

Inhibition of prolactin secretion

Ovary and mammary gland

Gonadotrophin-releasing hormone (GnRH)

FSHLH

Ovary and testis

Thyrotrophin-releasing hormone (TRH)

TSH Thyroid gland

Adrenocorticotrophin-releasing hormone (CRH)

ACTH Adrenal cortex

Hormones Released by the Anterior Pituitary:

TSH- Thyroid Stimulating Hormone ACTH - Adrenocorticotrophic hormone LH - Luteinising hormone FSH – Follicle Stimulating Hormone Prolactin

Functions of Anterior Pituitary Hormones:

Prolactin – stimulates milk production and secretion

FSH - stimulates growth and maturation of eggs in

females- stimulates sperm production in males

LH – stimulates secretion of sex hormones from ovaries and testes

Functions of Anterior Pituitary Hormones:

TSH – stimulates secretion of thyroxine from the thyroid gland

ACTH – stimulates release of glucocorticoids such as cortisol from adrenal cortex

Hormones Released by the Anterior Pituitary:

TSH ACTH LH FSH Prolactin

Tropins or Trophic hormones

What is a trophic

hormone?

A trophic hormone

stimulates other endocrine glands to release their

hormones

Anterior pituitary secretes

hormones that stimulate:

Gonad(testis)

Thyroid gland

Adrenalcortex

Other glands to secrete hormones

The release of the trophic hormones is

regulated by negative feedback of hormones from the target glandsacting on receptors in the: hypothalamus anterior

pituitary

The Pituitary Gland has often been called “the master gland” of the body

The Pituitary Gland has often been called “the master gland” of the body

such a term is justified since the anterior and posterior pituitary glands release a battery of hormones that collectively influence all cells and affect virtually all physiologic processes

the pituitary gland may be the king, but the

power behind the throne is clearly the hypothalamus

HOWEVER: the pituitary gland secretes some

hormones that are actually produced by neurones in the hypothalamus

under the influence of still other

brain hormones, it produces a number of its own hormones, which control the activities of various endocrine glands throughout the body

Essay Titles:

Describe the role of the anterior pituitary gland in the co ordination of hormonal activity. ‑

[SEP, 1996]The pituitary gland is referred to as a “master

gland” in the human body. Discuss. [MAY, 2004]

Give an account of the role of the hypothalamus and pituitary gland in hormonal control in the human body. [SEP, 2011]

OVERVIEW

A) HomeostasisB) Hormones: chemical nature and mode of

actionC) Hypothalamus and Pituitary glandD) Adrenal glandsE) Hormonal regulation of blood sugar levels

Where are the adrenal glands positioned?

Adrenal glands

Kidneys

Two regions in the adrenal glands:Cortex

Medulla

Two regions in the adrenal glands:

- richly supplied with nerves and blood vessels

- cells of the medulla are modified neurones of the sympathetic NS

Cortex- forms 80% of the gland- produces steroid hormones

Medulla

Steroid hormones of the Adrenal CortexMineralocorticoids mainly aldosterone

(stimulates kidney to conserve sodium and excrete potassium)

Glucocorticoids mainly cortisol: (under stress, raises blood glucose concentrations )

The Cortex is able to:

i) synthesise cholesterolii) take up cholesterol from the circulation

following absorption from the diet

Why does the cortex need a supply of cholesterol ?

To synthesise steroid hormones.

Hormones of the Adrenal Medulla

Adrenaline (epinephrine)Noradrenaline (norepinephrine) [not in syllabus]

Adrenaline

– Blood glucose increases– Blood glycerol and fatty

acids increase– Heart rate increases– Blood pressure rises– Breathing rate increases– Air passages dilate– Pupils dilate– Blood flow redistributes,

leading to increased alertness and decreased digestive and kidney activity

• Causes short term “fight or flight” or alarm stage:

Adrenal medulla is closely linked to the nervous system

OVERVIEW

A) HomeostasisB) Hormones: chemical nature and mode of

actionC) Hypothalamus and Pituitary glandD) Adrenal glandsE) Hormonal regulation of blood sugar levels

The Pancreas

The Pancreas produces:

Pancreatic duct

1. HORMONES2. PANCREATIC JUICE

Is it correct to state that the pancreas is both an endocrine & an exocrine gland?

YES. Endocrine: produces hormones

Exocrine: produces enzymes.

Hormones and Pancreatic juice do not mix

Produce hormones

Produce enzymes

Why is the islet of Langerhans highly vascularised?

Hormones produced here

must be transported to

their target organ via the

bloodstream.

Beta cells produce insulin

Which cell produces which hormone?

REMEMBER: BIN

sugar

Alpha cells produce glucagon

When are insulin and glucagon secreted?

Insulin

Glucagon

Islet of Langerhans

Beta cell

INSULIN

Cells become permeable to GLUCOSE

How is it possible for insulin to bring an effect inside body cells?

Insulin binds to its

receptors.

How does insulin make cells more permeable to glucose?

Glucose transporters are inserted within the plasma membrane.

Insulin is:a small protein composed of 51 amino acidsreleased: blood glucose level rises above

90 mg per 100 cm3 bloodcarried in the plasma bound to globulin

Insulin has an effect on:

every organ of the bodybut main effect is on the:

Liver Muscles

Glucagon is a protein composed of 29 amino acids

role:- increases blood glucose level target: liver effects:

1. Glycogenolysis2. Gluconeogenesis

glucagon has no effect on muscle glycogen

Glycogenolysis:Conversion of glycogen to glucose

Glycogenesis:Conversion of glucose to glycogen

Gluconeogenesis:Conversion of non carbohydrate sources (proteins, fats and lactic acid) to glucose

Question: MAY 2013

The following statements are linked to human physiology. Use your knowledge of biology to write short notes on each of the following.

Gluconeogenesis aids in maintaining adequate sugar levels in the blood during strenuous physical exercise. (5 marks)

By what type of feedback mechanism is blood sugar controlled?

Negative feedback

In blood sugar what is the DETECTOR, REGULATOR, EFFECTOR?

Muscles & Liver:

Effector

Pancreas:Detector ; Regulator

Maintaining the set-point

90 mg per 100 cm3 of blood

Diabetes Mellitus is a metabolic disease:

glucose is excreted in urine as it exceeds the maximum which the kidneys can reabsorb

Hypoglycaemia:blood glucose level

falls

Hyperglycaemia:blood glucose level

rises

Normal blood glucose level [70-100 mg/100 cm3 of blood]

Both lead to COMA

Two Types of diabetes:Type I Diabetes [Juvenile Onset]

Type II Diabetes [Maturity Onset]

Type I Diabetes – Juvenile Onset Decrease in the number

of beta cells insulin deficiency

Requires daily injections of insulin hormone

Type II Diabetes – Maturity OnsetInsulin is secreted

BUTBody cells lose ability to

use insulinWHY?

Membrane receptors in the target cells are insensitive to insulin

Type II Diabetes

Diabetes mellitus makes a person feel thirsty

Glucose in urine causes water to be

withdrawn from blood

This results in diuresis.

Diuresis is the condition when:

large volumes of dilute urine are produced

Diuresis dangerous as it leads to:

dehydration of the body

Why does a diabetic person lose weight ?

The breakdown of glycogen is not inhibited

The stores of glycogen are quickly used up

Body fats and proteins are then used as respiratory substrates

A rapid loss of body mass

Time

Glu

cose

C

on

cen

trati

on

Meal eaten

Insulin is produced and glucose levels fall to normal again.

Glucose levels rise after a meal.

Normal

Time

Glu

cose

C

on

cen

trati

on

Meal eaten

Insulin is not produced so glucose levels stay high

Glucose levels rise after a meal.

Diabetic

Cortisol is an anti-stress hormone: secreted by the adrenal cortex:

in a circadian rhythm [any biological process that displays an endogenous oscillation of about 24 hours]

1

2

Under stress

Some effects of cortisol :1. stimulates cells to:

decrease their use of blood glucose

shift instead to utilise fats and proteins for energy

2. raises blood sugar level

3. blocks the immune system reaction

Keep your balance!!