Osmoregulation and Excretion

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Urea and NaCl form the osmotic gradient that enables the kidney to produce urine that is hyperosmotic to the blood.

Slide 50

Two Solute Model: How the kidney concentrates urine

Key

Active

transport

Passive

transport

INNER

MEDULLA

OUTER

MEDULLA

CORTEX

H2O

300

300

300

H2O

H2O

H2O

400

600

900

H2O

H2O

1,200

H2O

300

Osmolarity of

interstitial

fluid

(mOsm/L)

400

600

900

1,200

100

NaCl

100

NaCl

NaCl

NaCl

NaCl

NaCl

NaCl

200

400

700

1,200

300

400

600

H2O

H2O

H2O

H2O

H2O

H2O

H2O

NaCl

NaCl

Urea

Urea

Urea

Slide 51

Adaptations of the Vertebrate Kidney to Diverse Environments

The form and function of nephrons in various vertebrate classes are related to requirements for osmoregulation in the animal’s habitat.

Mammals

The juxtamedullary nephron contributes to water conservation in terrestrial animals.

Mammals that inhabit dry environments have long loops of Henle, while those in fresh water have relatively short loops.

Slide 52

Birds and Other Reptiles

Birds have shorter loops of Henle but conserve water by excreting uric acid instead of urea.

Other reptiles have only cortical nephrons but also excrete nitrogenous waste as uric acid.

Slide 53

Freshwater Fishes, Amphibians, Marine Bony Fishes

Freshwater fishes conserve salt in their distal tubules and excrete large volumes of dilute urine.

Kidney function in amphibians is similar to freshwater fishes. Amphibians conserve water on land by reabsorbing water from the urinary bladder.

Marine bony fishes are hypoosmotic compared with their environment and excrete very little urine.

Slide 54

Hormonal circuits link kidney function, water balance, and blood pressure

Mammals control the volume and osmolarity of urine by nervous and hormonal control of water and salt reabsorption in the kidneys.

Antidiuretic hormone = ADH increases water reabsorption in the distal tubules and collecting ducts of the kidney. An increase in osmolarity triggers the release of ADH, which helps to conserve water.

Mutation in ADH production causes severe dehydration and results in diabetes insipidus.

Alcohol is a diuretic - it inhibits the release of ADH.

Slide 55

Regulation of fluid retention by antidiuretic hormone = ADH