• Cell plasma membrane maintains ionic, but not osmotic difference between intracellular and extracellular fluids.

• Epithelium surrounding the body often maintains both ionic and osmotic difference between animal and their environments.

• Gills, salty gland and kidney are primary organs of osmoregulation in vertebrates

• Appropriate solute concentrations and water are maintained by osomregulation

Exchange of water and salts depends on

• The size of gradient

• Surface area of the animal

• Permeability of the animal’s surface

The surface-to-volume ratio is greater for small animals than large animals. A small animal will dehydrate or hydrate more rapidly than a larger animal

•Amphibians have moist, highly permeable skins

To avoid desiccation, stay in cool, damp microenvironment, stay close to water

water and slats are stored in a large-volume lymphatic system and an oversized urinary bladder.

•Insect’s waxy cuticle

•Burning fat to produce water in seal

Various Strategies for Preserving Body Water

Seals became fat when eating fish but get thin eating marine invertebrate

The respiratory loss of water is minimized by temporal countercurrent system

Water loss via respiration depends on

• Difference between body temperature and air temperature

• Humidity of inhaled air

Osomoregulatory in Various Classes of Animals

Euryhaline aquatic animal can tolerate a wide range of salinities

Stenohaline animals can tolerate only narrow osmotic range

Freshwater animals face two kinds of osmoregulatory problems:

•gain of water

•loss of salt

To prevent the net gain of water and net loss of salts, freshwater animals

•Drink no water

•Produce a dilute urine

•Replace lost salts from ingested food

•Active transport salt from external environment

Marine invertebrates and hagfish (vertebrate) are iso-osmotic to seawater, and have similar osmolarity and ionic concentrations to seawater

Elasmobranch (e.g. shark, rays and skates, Latimeria) is iso-osmotic to seawater by maintaining low concentration of electrolytes and high concentration of urea and TMAO (trimethylamine oxide)

Marine teleost, bird and mammals are hypo-osmotic to seawater

Marine teleosts face two kinds of osmoregulatory problems:

•loss of water

•gain of salt

To prevent the net loss of water and net gain of salts, marine teleosts

•Drink water

•Active transport Na+, Cl- and K+ from gill to seawater.

•Secretion of divalent salts (Ca2+, Mg2+, SO42-) by kidney to urine

Marine reptiles and marine birds

• Drink seawater

• Kidney is unable to excrete the salts

• Salt gland (near eyes, nose and in the tongue) secrete concentrated salt solution

Most mammals lack salt gland and will become dehydrated if they drink seawater

Desert animal faces double jeopardy

•Excess heat

•Absence of free freshwater

Camel strategies:

•Change body temperature

•Produce dry feces & concentrated urine

•Store high levels of urea

Camel do not sweat and has large body mass and thick fur

Marine mammals:

Drink no water

Produce hypertonic urea

Absorb water from metabolic activity and ingested food

Terrestrial arthropods

Create high concentrated solutions in the rectum to absorb water from the air

Salivary glands secrete highly concentrated KCl

Structure of the Kidney

• 2 distinct regions:

• Outer cortex:– Many capillaries.

• Medulla:– Renal pyramids separated

by renal columns.

Nephron is functional unit of the kidney

Kidney Functions• Primarily on regulation of ECF through formation of urine. • Regulate volume of blood plasma and BP.• Regulate concentration of waste products in the blood.• Regulate concentration of electrolytes as Na+, K+, and HC03

-.• Regulate pH. • Secrete erythropoietin.

Nephron

• Functional unit of the kidney.

• Consists of:• Blood vessels

– vasa recta – peritubular capillaries

• Urinary tubules– Proximal tube– Loop of Henle– Distal tube– Collecting tube

Three main processes for urine production

1. Filtration

2. Reabsorption

3. Secretion

Glomerular filteration

Ultrafiltration in glomerulus depends on

1. pressure difference

2. Membrane permeability

Fig. 12-9, p.534

Juxtaglomerular Apparatus• Region in each nephron where the

afferent arteriole comes in contact the the thick ascending limb of the loop. Two types of cells

• Macula densa:– Monitor the osmolarity and flow– Inhibit renin secretion when

blood [Na+] in blood increases• Granular cells:

– Secrete renin.– Converts angiotensinogen to

angiotensin I.– Initiates the renin-angiotensin-

aldosterone system.

Stimulation of macula densa cellsto release vasoactive chemicals

Chemicals released that induceafferent arteriolar vasoconstriction

Arterial blood pressure

Driving pressure into glomerulus

Glomerular capillary pressure

GFR

Rate of fluid flowthrough tubules

Blood flow into glomerulus

Glomerular capillary pressureto normal

GFR to normal Fig. 12-13, p.538

Detection by aortic arch andcarotid sinus baroreceptors

Sympathetic activity

Generalizedarteriolar vasoconstriction

Afferent arteriolarvasoconstriction

GFR

Glomerular capillaryblood pressure

Cardiacoutput

Totalperipheralresistance

Short-termadjustment for

Arterialblood pressure

Long-termadjustment for

Urine volume

Conservation offluid and salt

Arterial blood pressure

Arterial blood pressure

Fig. 12-14, p.539

Tubular re-absorption

• Return of most of the filtered solutes and H20 from the urine filtrate back into the peritubular capillaries.

• About 180 L/day of ultrafiltrate produced, however only 1 – 2 L of urine excreted (>99%).

• Minimum of 400 ml/day urine necessary to excrete metabolic wastes (obligatory water loss).

• Filtered glucose and amino acids are normally reabsorbed by the nephrons.– Carrier mediated

transport:• Saturation.• Exhibit Tm. (320 mg

min-1, 3mgml-1)

Glucose re-absorption

Solute concentrations in the interstitial fluid increase from the cortex to the depths of the medulla.

Urea increases most in the inner medulla. NaCl increases most in outer medulla

Proximal tube 70% Na+, Cl- and H20 reabsorbed across the PT into the

blood. 90% K+ reabsorbed.• Fluid reduced to ¼ original volume but still iso-osmatic 300

mOsm/L Na+/K+ ATPase pump located in basal and lateral sides of

cell membrane creates gradient for diffusion of Na+ across the apical membrane.

Na+/K+ ATPase pump extrudes Na+. Cl- follows electrical gradient into the interstitial fluid. H20 follows by osmosis. Reabsorption is constant, not subject to hormonal

regulation.

Fig. 12-16, p.541

Fig. 12-17, p.541

Descending Limb Loop of Henle

• Deeper regions of medulla reach 1200 mOsm/L.

• Impermeable to passive diffusion of NaCl & urea

• Permeable to H20.

• Hypertonic interstitial fluid causes H20 movement out of the descending limb via osmosis.

• Fluid volume decreases in tubule, causing higher [Na+] in the ascending limb.

Ascending Limb Loop of Henle• Na+ actively

transported across the basolateral membrane by Na+ / K+ ATPase pump.

• Cl- passively follows Na+ down electrical gradient.

• K+ passively diffuses back into filtrate.

• Walls are impermeable to H20.

Distal tubule

• Transport K+, H+, and NH3 into the lumen

• Reabsorption of Na+, Cl-, and HCO3

-

• H20 follows passively subject to hormonal

regulation.

Collecting Duct• Medullary area impermeable to high [NaCl] that surrounds

it.

• The walls of the CD are permeable to H20.

• H20 is drawn out of the CD by osmosis.

• Rate of osmotic movement is determined by the # of aquaporins in the cell membrane.

• Permeable to H20 depends upon the presence of ADH.

• ADH binds to its membrane receptors on CD, incorporating water channels into cell membrane.

Kidney Secretion• Secretion of substances

from the blood to the urine.

• Allows the kidneys to rapidly eliminate certain potential toxins.

• Substances (foreign & normal metabolites) conjugated with glucuronic acid or its sulfate, removed by organic anionic and cationic transport system

Renin-angiotensin system

Cells of macula densa senses blood pressure decrease, they stimulate releasing of renin from the granular cells, leads to an increase in angiotensin II and aldosterone, promotes Na+ and water reabsorption

Role of Aldosterone• 90% K+ reabsorbed in early part of the nephron.• When aldosterone is absent, no K+ is excreted in the

urine.• Final [K+] controlled in distal tube by aldosterone.• High [K+] or low [Na+] stimulates the secretion of

aldosterone.• Only means by which K+ is secreted.• Control of plasma of K+ important in proper function of

cardiac and skeletal muscles

Na+ Reabsorption

• In the absence of aldosterone, 80% remaining Na+ is reabsorbed.

– 2% is excreted (30 g/day).

• Final [Na+] controlled in distal tube by aldosterone.

Control of Na+ important in regulation of blood volume and pressure.

(Antidiuretic hormone) ADH increase number of water channels (aquaporins) and thereby promotes water reabsorption

Atrial natriuretic peptide (ANP)

• Produced by atria due to stretching of walls.

• Antagonist to aldosterone.

• Inhibits release ADH, renin and aldosterone

• Increases [Na+] excretion and urine production

Two factors control pH in mammals:• Excretion of CO2 via the lung (short term)• Excretion of acid via kidney (mainly)

Renal Acid-Base Regulation

• Kidneys help regulate blood pH by excreting H+ and reabsorbing HC03

-.• Most of the H+ secretion occurs across the walls of

the proximal tube in exchange for Na+.– Antiport mechanism.

• Normal urine normally is slightly acidic because the kidneys reabsorb almost all HC03

- and excrete H+.

• Returns blood pH back to normal range.

A type cell: acid secreting cell

B type cell: base secreting cell

Urinary Buffers

• Nephron cannot produce a urine pH < 4.5.• IN order to excrete more H+, the acid must

be buffered. • H+ secreted into the urine tubule and

combines with HPO4-2 or NH3.

• HPO4-2 + H+ H2PO4

- • NH3 + H+ NH4

+

Buffering of the renal filtrate by H2PO4- and NH4- permits greater secretion of protons.

Kidney in vertebrates

Hagfish: possess glomeruli, no tubules, excrete divalent ions (Ca2+, Mg2+ , and SO4

2- ), carry out little osmoregulation,

Freshwater teleost: larger glomeruli, produce dilute urine

Marine teleost: produce little urine, excrete NH3 from gills

Amphibians and reptiles: lack of loop of Henle, can’t produce concentrated urine

Mammals and birds: produce concentrated urine

Salty gland: marine birds, reptiles, elasmobranchChloride cells in gills: marine teleostRectal gland: Elasmobranch

Extrarenal osmoregulatory oragns

Uptake of salt in freshwater fish