 Most metabolic reactions take place in water  Maintenance necessary for homeostasis ◦ Volume ◦ Concentration of solutes  Terrestrial animals have mechanisms to control water loss

 Osmoregulation ◦ Active regulation of osmotic pressure ◦ Too dilute or too concentrated equally bad  Excretion ◦ Getting rid of metabolic wastes

 CO 2 ◦ respiration  Water ◦ Surfaces ◦ Excretory organs  Nitrogenous wastes ◦ Deamination  ammonia ◦ Break down of nucleotides ◦ Excretory organs

 Ammonia ◦ Excrete into aquatic surroundings ◦ Convert to less toxic waste  Uric Acid ◦ Conversion of ammonia or break down of nucleotides ◦ Crystalline paste  Urea ◦ Amphibians & mammals ◦ Produced during urea cycle: ammonia + CO 2

 In Invertebrates: ◦ Osmoconformers ◦ Osmoregulators  Marine Vertebrates ◦ Replace fluid; excrete salt  Terrestrial Vertebrates ◦ Conservation of water

 Kidneys ◦ Cortex ◦ Medulla  Urinary Bladder  Associated ducts ◦ Ureters ◦ Urethra

 Filters blood; produces urine  2 types: ◦ Cortical ◦ Juxtamedullary nephrons

 Filtration: pressure ◦ Plasma leaves capillaries & enters Bownman’s capsule ◦ Filtration membrane of capsule controls what enters/is excluded  Reabsorption: active transport & diffusion ◦ Wastes, excess salts, etc. excreted ◦ Water, glucose, amino acids, some salts reabsorbed

 Secretion ◦ Substances move opposite reabsorption ◦ pH regulation ◦ Hormonal control

 Filtrate passes through regions of renal tubule, salt concentration gradient established  Proximal tubule reabsorbs water and salt ◦ Na: active transport; Cl: passive ◦ Water follows: osmosis

 Loop of Henle ◦ Passes through medulla – highly hypertonic ◦ Descending: permeable to water, not Na, urea  Water passes out due to osmosis ◦ Ascending: permeable to salt, not water  Distal convoluted tubule ◦ Impermeable to water ◦ Actively transports salts out  Collecting Duct ◦ Permeable to urea ◦ Water passes out - osmosis

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 ADH (antidiuretic hormone) ◦ Response to dehydration  Hypothalamus senses change in osmotic pressure  Posterior pituitary secretes ADH ◦ Regulates permeability of collecting ducts  More permeable = more water reabsorption

 Aldosterone ◦ Secreted by cortex of adrenal glands ◦ Stimulated by decrease in blood pressure ◦ Stimulates reabsorption of Na  Angiotensin II ◦ Raises bp ◦ Stimulates aldosterone  ANP (atrial natriuretic peptide) ◦ Produced by heart ◦ Increases Na excretion, decreases bp