Animal Physiology – Osmoregulation & Excretion Chapter 44

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

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

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)

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)

Nitrogenous Wastes Ammonia Urea Uric Acid 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

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

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

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

Nephron

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

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

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

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

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

Nephron Steps (Page 2) Secretion Reabsorption 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

Nephron (Page 3) Excretion 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

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

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

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

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

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