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