Homeostasis

1. Homeostasis Biology 2: Form and Function

2. Overview • Homeostasis = maintenance of constant internal environment • Physiological controls • Negative feedback loops • Positive feedback loops • Behavioral controls

11. 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

12. 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

14. 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

15. 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 NH3 • Annelida • Metanephridia organized on a per segment basis collect waste from coelom via the nephrostome, counters water uptake by epidermis, excretes NH3 • Insecta • Malpigian tubules collect nitrogenous wastes from haemocoel, excretes Uric Acid

20. 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

29. 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

32. 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, H2O follows

34. 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

35. The Loop of Henle • Descending limb is permeable to water but not NaCl • H2O 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