Kidneys + Excretion

Close to Bowman’s capsule - Ion + water reabsorption - Na+ actively reabsorbing Cl- passively follows - Water passively reabsorbed (goes where party is) - Urine composition does not change

Complex / twisted

Farther away from Bowman’s capsule - Alters urine composition - Active NaCl reabsorption (some water follows) - Extra “free” water that needs to be excreted = U/P <1 - Altered by anti-diuretic hormone = alters water permeability

Diuresis = dilute urine (U/P<1) - Poorly permeable to water

Anti-diuresis = concentrated urine (U/P >1) - Highly permeable to water

Cortex (Proximal convoluted tubule) –> deep in the medulla (Loop of Henle) –> Cortex (Distal convoluted tubule) –> deep in medulla (Collecting duct)

Sets up NaCl concentration in interstitial fluid

• Descending + ascending limb –> urine flowing in opposite directions • Ascending limb = NaCl actively transported out, water cannot follow • Descending limb = permeable to water, water moves out due to NaCl actively transported out the ascending limb

Concentrated urine - Dilute urine coming in distal convoluted tubule –> water moves out –> urine starts to concentrate - Urine lows down collecting duct it encounters higher and higher NaCl in interstitial fluid –> water moves out –> concentrate urine - High ADH

Dilute urine - DCT + collecting duct become poorly permeable to water - Water remains in nephron = dilute urine

Marine invertebrates, freshwater & marine fishes, tadpoles - Highly toxic -Highly soluble - Cheap - no ATP

Amphibians, turtles, mammals - Less toxic - Highly soluble but requires less water to excrete - More costly (4-5 ATP)

Birds, lizards, snakes, terrestrial invertebrates, spiders, scorpions - Lowest toxicity - Lowest solubility requires lowest water to excrete - Most costly