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

CO2 ◦ 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 + CO2

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

http://www.sumanasinc.com/webcontent/animations/content/kidney.html

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