Homeostasis Biology 2: Form and Function

Overview Homeostasis = maintenance of constant internal environment Physiological controls –Negative feedback loops –Positive feedback loops Behavioral controls

Osmoregulation Water is vital to the chemistry of life Therefore, must attain a water balance within the body Water balance systems are based on three processes: –Diffusion –Osmosis –Active transport Osmoregulation processes often tied to excretion

Diffusion –The spread of molecules along a concentration gradient by brownian motion, towards a state of entropy Osmosis (the diffusion of water) –Water moves from a low solute concentration (hypotonic) to a high solute concentration (hypertonic) Active transport –The movement of molecules across a membrane, usually against a gradient, involving the expenditure of energy

Osmoregulation in invertebrates Since most invertebrate phyla evolved in water, no shortage However, differences in concentration between the cell and the solution surrounding it may cause problems –e.g., Amoeba in freshwater: hypertonic cell in hypotonic solution –Result: movement of water into cell –Defense: Contractile vacuole pumps out water

Multicellular organisms use transport epithelia to control water loss and excretion –Platyhelminthes Protonephridia (flame cells) collect excess water in addition to nitrogenous wastes, empty into nephridiopore, excretes NH 3 –Annelida Metanephridia organized on a per segment basis collect waste from coelom via the nephrostome, counters water uptake by epidermis, excretes NH 3 –Insecta Malpigian tubules collect nitrogenous wastes from haemocoel, excretes Uric Acid

Osmoregulation in fish Depends on environment –Freshwater Cells are hypertonic to environment, must defend against water uptake –Excretion of dilute urine –Mucous covering of epidermis –Marine Cells are hypotonic to environment, must defend against water loss –Water gain through food uptake and drinking –Concentration of urine

Filtration Occurs in Bowmans capule –Afferent arteriole from renal artery enters glomerulus, exits via efferent arteriole –Blood filtered by capsule: all non-cellular products pushed into nephron (proximal tubule) –Filtrate includes products that must be retained: blood sugars, salts and vitamins

Secretion Occurs in proximal and distal tubule Secretion is initially active, although certain molecular transport occurs passively as a result e.g., NaCl actively pumped out, H 2 O follows

Reabsorption Materials that must be retained are brought back by active transport or passive diffusion Result of absorption/secretion in Loop of Henle is highly concentrated urine Nephron tubule is lined by transport epithelia Amount of water retained is controlled by hormones that control activity of transport epithelia

The Loop of Henle Descending limb is permeable to water but not NaCl H 2 O moves by osmosis to high salt concentration in interstitial fluid Thin segment of ascending limb is permeable to NaCl which moves passively by diffusion to equalize gradient Thick segment of ascending limb actively transports NaCl