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 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

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, H2O 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

• 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