Animal Physiology – Osmoregulation & Excretion

(Lecture Version) Chapter 44

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Osmoregulation Def – MGT of body’s water & solute concentration

Osmoregulation depends on the environment the organisms lives in

Marine Vertebrates (Fish) – face dehydration pressures from the sea (salt H2O) Hypertonic solution = H2O exits fish cells, then Water loss through gills or skin To prevent desiccation:

Decreased urine production & consumption of large amounts of sea H2O

Active transport of salts out of body

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Osmoregulation (Page 2)

Freshwater vertebrates (fish) Environment is hypotonic, so need to counter osmotic

pressure Active transport of salts into body Excrete substantial amounts of dilute urine

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Excretion

Def – removal of metabolic wastes Wastes include:

CO2 & H2O (Respiration wastes) Nitrogenous waste (from protein metabolism)

Ammonia, urea, or uric acid

Excretion Organs (Humans) Skin, lungs, kidneys, & liver (site of urea production)

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Nitrogenous Wastes

Ammonia Highly toxic, but water soluble Generally excreted by waterborne organisms

Urea Not as toxic as ammonia Humans & Earthworms excrete In mammals, ammonia is decomposed into urea in liver

Uric Acid Pastelike substance that you have seen on the outside of your car NOT water soluble; Least toxic form Deposited by birds (and reptiles) – minimum of H2O loss

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Urine Production in 4 Easy Steps

1. Filtration Pressure-filtering of body fluids Removal of water & solutes Cells, proteins, & large particles remain

2. Reabsorption Reclaims valuable substances from the filtrate Glucose, vitamins, & hormones

3. Secretion Adds other substances (toxins & excess ions)

to the filtrate

4. Excretion Altered filtrate leaves the body

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Human Kidney

Humans need to conserve water, but also remove toxins

Kidney adjusts volume & concentration of urine due to animal’s intake of water & salt Fluid intake is high & salt intake low = dilute (hyposmotic) urine Fluid intake low & salt intake high = concentrated (hyperosmotic)

urine

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Nephron

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The Nephron

Functional unit of the kidney

Each kidney contains ~1 million nephrons

Blood: Renal artery (afferent & efferent) aterioles peritubular

capillaries renal vein

Filtrate (urine): Glomerulus OR tubule OR Loop of Henle ureters Bladder

urethra

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Nephron Videos / Animations

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6 Main Steps in the Nephron

1. Filtration from glomerulus in Bowman’s Capsule

2. Proximal (near) tubule – secretion & reabsorption Changes the volume & composition of the filtrate

3. Descending Loop of Henle – reabsorption of H2O continues

4. Ascending Loop of Henle – Reabsorption of salt (NaCl) w/o giving up H2O = dilution of urine

5. Distal (far) tubule – K+ and NaCl levels are regulated

6. Collecting Duct – filtrate becomes more concentrated as more water is reabsorbed

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Nephron Steps Filtration

Passive (diffusion) & nonselective Blood pressure forces fluid from the glomerulus into the Bowman’s Capsule Bowman’s Capsule contains specialized cells which increase the rate of filtration Anything small enough to filter out does so

Glucose, salts, vitamins, wastes such as urea, other small molecules

Secretion Active & Selective Uptake of molecules that did not get filtered into Bowman’s Capsule Occurs in Proximal tubule

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Nephron Steps (Page 2)

Reabsorption Water & solutes (glucose, amino acids, & vitamins) that entered the tubule

during filtration are returned to peritubular capillaries then to the body Proximal tubule, Loop of Henle, and to collecting tubule

Excretion Removal of metabolic wastes (nitrogenous wastes) Everything that passed into the collecting tubule is excreted from

the body

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Nephron (Page 3)

Loop of Henle – Acts as a countercurrent exchange mechanism Maintains a steep salt gradient surrounding the loop This gradient ensures that water will continue flowing out of

collecting tubule of the nephron Creates hypertonic urine Conserves water Longer Loop of Henle = More water reabsorption

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Hormonal Control of Kidneys

Under the control of Nervous & Endocrine systems

Hence, kidney can quickly respond to the changing requirements of the body

ADH (Anti-Diuretic Hormone) Produced by the hypothalamus Stored in the Posterior Pituitary Targets the collecting tubule of the nephron Hypothalamus has osmoreceptor cells that monitor blood

concentrations of salts On a feedback loop to maintain homeostasis of fluid concentration

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ADH @ Work

When body’s salt concentration is too HIGH, ADH is released into the blood ADH increases permeability of the collecting tubule

So more water is collected from the urine, and urine volume is decreased

When body’s salt concentration is too LOW (dilute), due to water intake being too high or salt intake too low, ADH is reduced = more urine production

EtOH = ADH inhibition = excessive urine production May lead to dehydration

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