Animal Physiology

9 min read

Animal physiology examines the mechanical, physical and biochemical functions of living animals. Mammalian physiology is the typical reference for higher zoology.

Homeostasis

The maintenance of a stable internal environment despite external change. Coined by Walter B. Cannon (1929), building on Claude Bernard's concept of the "milieu intérieur". Key regulated parameters: temperature, pH, osmolarity, blood glucose.

Circulatory system

Open vs closed

Open circulation — haemolymph bathes tissues; arthropods, most molluscs.
Closed circulation — blood confined to vessels; annelids, cephalopods, all vertebrates.

Heart structures

Fish — 2-chambered (1 atrium, 1 ventricle).
Amphibians & most reptiles — 3-chambered.
Crocodiles, birds, mammals — 4-chambered.

Mammalian heart

Four chambers: right atrium, right ventricle, left atrium, left ventricle.
Pulmonary circuit (right side) and systemic circuit (left side) — described first by William Harvey in De Motu Cordis (1628).
Cardiac cycle ~0.8 s at rest; stroke volume ~70 mL; cardiac output ~5 L/min.
SA node is the pacemaker.

Blood

Plasma (55%) + formed elements (45%).
RBCs — ~5 million/μL; carry haemoglobin (each Hb binds 4 O₂).
WBCs — neutrophils, lymphocytes, monocytes, eosinophils, basophils.
Platelets — clotting.
Human blood groups: ABO (Landsteiner 1900) + Rh.

Respiratory system

Cutaneous — amphibians.
Gills — fish; counter-current exchange.
Tracheal system — insects (direct delivery to cells).
Lungs — reptiles, birds, mammals; birds have parabronchi and air sacs (most efficient).

Mammalian gas exchange occurs in alveoli — millions of thin-walled sacs giving ~70 m² surface area in humans.

Hb-O₂ dissociation curve is sigmoidal; right-shifted by high CO₂, low pH (Bohr effect), high temperature.

Digestive system

Stages: ingestion, digestion, absorption, assimilation, egestion.

Mammalian digestion:

Mouth — mechanical + salivary amylase.
Stomach — pepsin + HCl.
Small intestine — pancreatic enzymes (trypsin, chymotrypsin, lipase, amylase), bile from liver.
Large intestine — water reabsorption, microbial fermentation.

Ruminants (cattle, buffalo, sheep) have a 4-chambered stomach: rumen, reticulum, omasum, abomasum.

Excretory system

Function: remove nitrogenous waste and maintain water/ion balance.

Nitrogenous wastes vary by water availability:

Ammonotelic — aquatic animals; ammonia (very toxic, needs lots of water).
Ureotelic — mammals, amphibians; urea (moderate toxicity).
Uricotelic — birds, reptiles, insects; uric acid (low water needed).

Mammalian kidney

~1 million nephrons per kidney.
Nephron parts: glomerulus, Bowman's capsule, proximal tubule, loop of Henle, distal tubule, collecting duct.
GFR: ~125 mL/min.
Hormonal control: ADH/vasopressin (water retention), aldosterone (Na+ retention), renin-angiotensin system.

Nervous system

Two divisions:

Central: brain + spinal cord.
Peripheral: somatic + autonomic (sympathetic + parasympathetic).

Neurons

Resting potential: ~ –70 mV.
Action potential: depolarisation (Na+ in), repolarisation (K+ out), refractory period.
Synaptic transmission — chemical (neurotransmitters) or electrical (gap junctions).
Key neurotransmitters: acetylcholine, dopamine, serotonin, glutamate, GABA, noradrenaline.

Mammalian brain regions

Cerebrum — cognition, sensation, voluntary movement.
Cerebellum — coordination, balance.
Brainstem (midbrain, pons, medulla) — vital reflexes.
Hypothalamus — homeostasis, autonomic.
Thalamus — sensory relay.
Limbic system — emotion, memory.

Key Points

William Harvey (1628) described the closed mammalian circulation.
Bird respiration with air sacs is the most efficient among vertebrates.
Ammonotelic, ureotelic, uricotelic correlate with water availability.
ADH is released from the posterior pituitary; aldosterone from the adrenal cortex.
Resting potential ~–70 mV; action potential peaks at ~+30 mV.

Endocrine system

Major glands and hormones:

Gland	Hormone	Function
Hypothalamus	Releasing/inhibiting hormones	Master regulator
Anterior pituitary	GH, TSH, ACTH, FSH, LH, PRL	Tropic hormones
Posterior pituitary	ADH, oxytocin	Water balance, parturition
Thyroid	T3, T4, calcitonin	Metabolic rate, Ca
Parathyroid	PTH	Raises plasma Ca
Adrenal cortex	Cortisol, aldosterone	Stress, Na balance
Adrenal medulla	Adrenaline, noradrenaline	Fight-or-flight
Pancreas	Insulin, glucagon	Blood glucose
Gonads	Testosterone / oestrogen, progesterone	Reproduction
Pineal	Melatonin	Circadian rhythm

Discoveries to know:

Insulin — Banting and Best, 1921 (Nobel 1923).
Thyroxine — Edward Kendall, 1914.

Reproductive physiology

Asexual — budding (Hydra), fission (planaria), regeneration.
Sexual — gametes; external (fish) or internal fertilisation.
Mammalian reproduction: ovarian and uterine cycles; placentation in eutherians; lactation under prolactin.

Immune system

Innate — skin, phagocytes, complement, inflammation.
Adaptive — B cells (antibodies, humoral); T cells (cellular immunity).
Vaccination — Edward Jenner (1796, smallpox).

For mammalian physiology questions, anchor your answers with key constants — resting potential –70 mV, GFR 125 mL/min, cardiac output 5 L/min, body fluid pH 7.35–7.45. Discoveries with dates and discoverers (Harvey 1628, Banting/Best 1921, Jenner 1796) signal command of the subject.

Special senses

Eye — vertebrate camera-type with cornea, lens, retina (rods and cones).
Ear — outer, middle, inner; cochlea for hearing, vestibular system for balance.
Olfaction — chemoreceptors in nasal epithelium.
Taste — papillae on tongue; five basic tastes including umami.
Mechanoreception — touch, pressure, vibration.
Lateral line system — in fish; detects water displacement.
Echolocation — bats, dolphins.