Animal Physiology and Homeostasis
This domain tests physiological control: how animals maintain internal stability while exchanging gases, ions, water and heat. Expect questions on nerve and muscle physiology, respiration, osmoregulation and endocrine feedback.
Core rules
- Homeostasis uses a receptor, a control centre and an effector. Negative feedback reverses a deviation; examples include blood glucose, body temperature and plasma osmolarity. Positive feedback amplifies a change and is limited to events such as labour contractions.
- Nerve impulse: the resting potential (about -70 mV) depends on the Na+/K+ ATPase (3 Na+ out, 2 K+ in) and selective permeability. A threshold stimulus opens voltage-gated Na+ channels, causing depolarisation; K+ efflux repolarises the membrane. Myelinated axons conduct saltatorily.
- Synaptic transmission: Ca2+ entry triggers neurotransmitter release; acetylcholine is broken down by cholinesterase.
- Muscle contraction: sliding filament mechanism; Ca2+ binds troponin, exposing actin sites; myosin heads use ATP to cycle.
- Gas transport: haemoglobin binds O2 in lungs and releases it in tissues; the Bohr effect lowers affinity at high CO2 or low pH. CO2 travels mainly as bicarbonate.
- Osmoregulation: freshwater fish gain water and lose ions; marine teleosts lose water and gain ions; elasmobranchs retain urea. ADH increases water reabsorption in the kidney.
Worked example
A person lost at sea drinks seawater. Seawater is hypertonic to body fluids, so water moves out of cells and blood osmolarity rises. Osmoreceptors trigger ADH release, producing small volumes of concentrated urine. Reasoning: ADH increases collecting-duct water permeability, but the ingested salt load increases the osmotic deficit, so dehydration worsens.
Traps
The Na+/K+ pump uses ATP; it is not passive. Action potentials are all-or-none. ADH increases water reabsorption, not urine volume. Positive feedback does not usually maintain homeostasis.