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Homeostasis
Biology 2: Form and Function
MOHD ASIF KANTH
M.SC ZOOLOGY ,B.ED9070262786
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 [email protected]/
• 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
• 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 [email protected]/[email protected]
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 [email protected]/
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
