Regulating The Internal Environment Ch. 44

The Excretory System Osmoregulation: management of the body’s water content & solute composition Controlled by movements of solutes between internal fluids and the environment Examples: Saltwater fish: Hypoosmotic to seawater, so it loses water by osmosis. Balances water loss by constantly drinking water & rarely urinating. Gills/skin give off salt. Freshwater fish: Hyperosmotic to its environment, so constantly gains water. Balances water gain by rarely drinking & excreting lots of urine. Lost salt is replenished by food and salt intake in gills.

Overview Filtration: Water & solutes forced by blood pressure across selectively permeable membranes of a capillary cluster into excretory tubule Reabsorption: epithelium reclaims valuable substances from filtrate Secretion: other substances like toxins and excess ions are extracted from body fluids and added to excretory tubule

Examples Excretory systems help maintain HOMEOSTASIS!!!!! by regulating water balance & removing waste Different organisms have various excretory mechanisms Contractile vacuoles: Found in protists. The vacuoles obtain water, merge with the plasma membrane, release water to the environment Flame bulbs: Found in flatworms (planaria). Flame bulbs found along branching tube system in flatworm. Body fluid filters across membrane, cilia sends fluid through tube system. Wastes excreted from nephridiopores.

Metanephridia: tubular excretory system found in most annelids, including earthworms. Fluid enters the nephrostome, passes through the collecting tubule (which includes a bladder that opens to outside by nephridiopore) Wastes remain in the fluid and are excreted Certain salts are pumped back into the blood Earthworm’s urine is very dilute, balancing the osmotic uptake of water through the skin

Malpighian tubules: Found in insects and other arthropods. Outfoldings of the digestive tract collect body fluids from hemolymph that bathes the cells Fluid enters the midgut and passes down to the hindgut with digested food Valuable materials are passed back out through the walls of the digestive tract and are reabsorbed Waste continues down and exits through the anus This excretory system is highly effective in conserving water and is a key adaptation for survival on land

The Mammalian Kidney Mammals have a pair of kidneys Supplied with blood by renal artery & renal vein Urine exits each kidney through a duct called the ureter The ureters drain into the urinary bladder and is expelled through the urethra during urination Kidney: consists of about a million nephrons Typically functions in both osmoregulation & excretion Nephron: individual filtering tubes found in kidney The tube travels from the cortex down into the medulla, back up to the cortex, then back down through the medulla, into the renal pelvis.

Filtration Glomerulus: ball of capillaries where filtration begins Bowman’s capsule: cup-shaped swelling that surrounds the glomerulus. Filtration starts as blood pressure forces fluid from the blood in the glomerulus into the Bowman’s capsule. The filtrate then passes through 3 regions Proximal tubule Loop of Henle (has a descending & ascending limb) Distal tubule The distal tubule empties into collecting duct, which empties into the renal pelvis, which is drained by ureter

Hormones Antidiuretic Hormone (ADH) Important in regulation water balance Increases reabsorption of water in the body Increases concentration of salts in urine Increases permeability of collecting duct to water More water diffuses out of the collecting duct Aldosterone Acts on nephrons’ distal tubules, causing more reabsorption of sodium & water Increases permeability of distal tubules to sodium More sodium diffuses out, water passively follows (why?)

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Adaptations Marine animals expel ammonia (NH 3 ) directly into water Birds, insects, reptiles convert urea  uric acid Uric acid is nearly insoluble in water Allows water conservation since nitrogen waste is given off as solid Mammals convert ammonia  urea Conversion occurs in liver Urea is less toxic than ammonia This makes it require less water to expel in urine Thus water conservation