Animal Physiology – Osmoregulation & Excretion

1. Animal Physiology – Osmoregulation & Excretion Chapter 44

2. 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) • So they are in a hypertonic solution • What happens to H2O inside the fish? – Exits • Fish lose great amounts of water via gills & skin • To combat this loss, fish produce little urine & consume large amounts of sea H2O combined with active transport of salts

3. 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

4. Excretory Systems • Terrestrial Organisms • Evolved mechanisms that expel wastes into the environment & retain water as well • Different organisms = different mechanisms • Protista = Contractile vacuole • Platyhelminths = Protonephridia/Flame-bulb system • Annelids (Earthworm) = Metanephridia • Insects & Terrestrial Arthropods = Malpighian Tubules • Humans = Nephrons (Kidneys)

5. Excretion (Page 2) • 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)

6. 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

8. 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

9. Human Kidney • Dual functionality: Osmoregulation & Excretion • Renal = kidney • Renal vein, renal artery, renal nerve, renal failure • Kidneys are the body’s filters • Kidneys filter 1,000 – 2,000 L blood per day • Produce 1.5 L urine per day

10. Human Kidney (Page 2) • 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

12. Nephron

13. Pathways • Blood: • Renal artery  (afferent & efferent) aterioles  peritubular capillaries  renal vein • Filtrate (urine): • Glomerulus OR tubule OR Loop of Henle  ureters  Bladder  urethra

14. The Nephron • Functional unit of the kidney • Each kidney contains ~1 million nephrons • Vocab: • 1. Tubule (Proximal & Distal) • 2. Glomerulus – Ball of capillaries • 3. Bowman’s capsule – End of tubule that surrounds the glomerulus • 4. Loop of Henle – Descending & Ascending limbs

15. Nephron Particulars • Renal Cortex • Glomerulus – tightly packed ball of capillaries • Bowman’s Capsule – encapsulates the glomerulus • Actual site of blood filtration • Proximal Tubule – site of substantial secretion & absorption • Distal Tubule – another important site of secretion & absorption

16. 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

17. 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 • From Bowman’s Capsule, the filtrate travels to the proximal tubule, the loop of Henle, distal tubule, then to the collecting duct or tubule • From the collecting tubule, filtrate trickles into the ureter & finally the urinary bladder (temp storage)  Urethra  out

18. Nephron Steps (Page 2) • Secretion • Active & Selective • Uptake of molecules that did not get filtered into Bowman’s Capsule • Occurs in Proximal tubule • 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

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

20. Kidney Dialysis -- When the kidney(s) fail, patients must undergo dialysis (artificial mechanical filtration)

21. Nephron Particulars (Page 2) • Renal Medulla • Loop of Henle • Descending limb – Impermeable to Salt, but permeable to water • Filtrate becomes increasingly concentrated • Ascending limb – Impermeable to water, but permeable to Salt • NaCl diffuses out of the lower part, increasing salt concentration of the surrounding tissue

23. Nephron Particulars (Page 3) • Collecting Duct • Carries the remaining filtrate through tissue that has high osmolarity (salt concentration) • ADH (Anti-Duretic Hormone) – Determines whether water is removed here or not • If ADH is present, collecting ducts become permeable to water & filtrate = hypertonic urine • If ADH is NOT present, collecting ducts’ walls remain impermeable to water = hypotonic urine

24. 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

25. 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