Veterinary · Pre-clinical and foundation · also MSVE VPB-121, MSVE VPB-221
Digestion in monogastrics, birds and ruminants, muscle and nerve, hormones, reproduction and lactation, growth, and the animal's response to climate and work.
In the exams: Same exams as Physiology I: ICAR JRF 08, AIEEA-PG 14, PRC, NLEV, China, AVE
Veterinary Physiology II continues from body fluids, blood, circulation, respiration and excretion into the systems that make an animal productive: how it digests its food, moves, senses and controls itself, reproduces, feeds its young, grows, copes with climate and work, and behaves. It is taught in the second year of most veterinary degrees, for example as VPB-121 and VPB-221 under India's Minimum Standards of Veterinary Education, and under similar names in every veterinary school in the world.
The subject is always comparative. A dog, a cow, a hen and a horse solve the same problems in different ways: the cow ferments cellulose in a rumen, the hen grinds grain in a gizzard, the horse ferments in its hindgut. Endocrine and reproductive control differ in ways that decide how a practitioner synchronises oestrus, diagnoses pregnancy or manages milk fever. Heat stress, work and handling link physiology directly to production and welfare. Reference values in this manual are given only as patterns or ranges, because normal values differ by species, breed, age, method and laboratory; always use the reference interval supplied with the result.
Physiology is examined everywhere. In India it is a major subject of ICAR AIEEA (PG) Animal Sciences and ICAR AICE-JRF/SRF core group 08, and of state veterinary officer papers. It is a fixed share of the Philippine licensure examination, Nepal's NLEV, China's national practising veterinarian examination, and the Australasian Veterinary Examination, and it underlies the applied questions of the BCSE, NAVLE and RCVS Statutory Membership Examination. Recall of species differences, mechanisms and short calculations carries marks in all of them.
A monogastric animal has a single, simple stomach. The dog, cat, pig, horse, rabbit and human are the usual examples, although the horse and rabbit add a large fermenting hindgut. The gut wall has the same four layers along its length: mucosa, submucosa, muscularis externa (inner circular and outer longitudinal smooth muscle) and serosa. The submucosal (Meissner's) plexus mainly controls secretion and the myenteric (Auerbach's) plexus mainly controls motility; together they form the enteric nervous system, which can run peristalsis and reflex secretion even when the extrinsic nerves are cut. Extrinsic control is parasympathetic, largely through the vagus and pelvic nerves, which generally increases motility and secretion, and sympathetic, which generally slows motility, contracts sphincters and reduces blood flow to the gut.
Smooth muscle of the gut shows slow waves, rhythmic oscillations of membrane potential set by the interstitial cells of Cajal, the pacemaker cells. Slow waves do not themselves cause contraction; contraction follows when spike potentials fire on top of a slow wave that reaches threshold. Slow-wave frequency therefore sets the maximum rate of contraction in each segment. Two patterns of movement matter. Peristalsis is a propulsive wave, contraction behind the bolus and relaxation in front, driven by the local stretch reflex. Segmentation is alternating ring contraction that mixes contents without moving them far.
Swallowing (deglutition) has a voluntary oral phase and reflex pharyngeal and oesophageal phases coordinated by the swallowing centre in the medulla. The soft palate closes the nasopharynx and the epiglottis covers the larynx. The oesophagus is striated muscle along its whole length in the dog and ruminants, while in the cat, horse and primates the distal part is smooth muscle; this is why species differ in how oesophageal disorders present.
The monogastric stomach has regions that differ by species. The pig and horse have a large non-glandular (oesophageal or squamous) region; the dog and cat have almost entirely glandular mucosa. In the glandular fundus, parietal (oxyntic) cells secrete hydrochloric acid and, in most species, intrinsic factor; chief cells secrete pepsinogen, which acid converts to pepsin. Parietal cells pump hydrogen ions with the H+/K+ ATPase, the proton pump. Acid secretion is driven by three signals: acetylcholine from vagal nerves, gastrin from G cells of the antrum, and histamine from enterochromaffin-like cells acting on H2 receptors. Somatostatin from D cells inhibits it. Control is usually taught in three phases: cephalic (sight, smell, chewing; vagal), gastric (distension and peptides in the stomach; gastrin) and intestinal (mainly inhibitory, as acid and fat in the duodenum release secretin and cholecystokinin and slow gastric emptying).
The exocrine pancreas secretes two fluids. Duct cells secrete a bicarbonate-rich fluid in response to secretin, which neutralises acid entering the duodenum. Acinar cells secrete enzymes in response to cholecystokinin (CCK) and vagal stimulation: trypsinogen, chymotrypsinogen, procarboxypeptidases, amylase, lipase with colipase, and nucleases. Enteropeptidase (enterokinase) on the duodenal brush border activates trypsinogen to trypsin, which then activates the other zymogens. Keeping these enzymes inactive until they reach the gut protects the pancreas; loss of acinar tissue causes exocrine pancreatic insufficiency with maldigestion, classically in German Shepherd dogs with acinar atrophy.
Bile from the liver emulsifies fat. Bile acids form micelles that carry monoglycerides and fatty acids to the enterocyte. CCK contracts the gall bladder; the horse, rat and some deer have no gall bladder and secrete bile continuously. About 95% of bile acids are reabsorbed in the ileum and returned to the liver, the enterohepatic circulation. Carbohydrate digestion ends at the brush border, where disaccharidases produce monosaccharides; glucose and galactose enter by sodium-glucose cotransporter 1 (SGLT1) and fructose by GLUT5. Lactase activity is high in the suckling animal and falls after weaning in most mammals. In the horse, the caecum and colon ferment fibre to volatile fatty acids that supply a large share of energy, and in the rabbit caecotrophy (eating soft caecal pellets) recycles microbial protein and B vitamins.
Birds have no teeth, a light skull and a digestive tract designed to process food quickly. Food is taken by the beak and swallowed with little chewing; salivary glands are present, but amylase activity in saliva is low and varies by species. The oesophagus in many species, including the domestic fowl and pigeon, has an outpouching called the crop (ingluvies), which stores food and softens it with moisture; little digestion happens there, although some microbial fermentation and salivary amylase activity occur. In pigeons and doves the crop of both parents produces crop milk, a secretion of desquamated epithelial cells rich in fat and protein, under the influence of prolactin.
The avian stomach has two parts. The proventriculus is the glandular stomach, whose oxynticopeptic cells secrete both hydrochloric acid and pepsinogen, unlike the separate parietal and chief cells of mammals. Food passes through it quickly. The ventriculus, or gizzard, is a thick muscular organ lined by a tough koilin layer secreted by its glands. Its paired thick and thin muscles contract in a rhythmic cycle and grind food, often helped by grit. Contents move back and forth between the gizzard, proventriculus and duodenum (gastroduodenal reflux), so gastric enzymes act on food repeatedly. In carnivorous and fish-eating birds the gizzard is thinner and more like a simple stomach.
Gastric pH in the fowl is strongly acid in the proventriculus and gizzard. The exact values reported vary with diet, feeding state and method, so textbooks give ranges rather than a single number.
The small intestine of the fowl comprises the duodenal loop, which encloses the pancreas, then the jejunum and ileum, separated by the remnant of the yolk stalk (Meckel's diverticulum). The pancreas secretes amylase, lipase and proteases, and bile enters the duodenum by bile ducts; the pigeon has no gall bladder, whereas the fowl has one. Carbohydrate, protein and fat digestion and absorption follow mammalian principles.
At the junction of the small and large intestine the fowl has a pair of large caeca. They fill with fine particles and fluid, ferment some fibre and uric acid with microbes, produce volatile fatty acids, and absorb water. The caeca empty several times a day, producing characteristic pasty, dark droppings. The short large intestine (colon-rectum) leads to the cloaca, which has three compartments: the coprodeum receives faeces, the urodeum receives the ureters and the genital ducts, and the proctodeum opens to the vent. The bursa of Fabricius, the site of B-lymphocyte maturation, lies dorsal to the proctodeum. Urine from the kidneys enters the urodeum and can be moved back into the coprodeum and colon by retrograde peristalsis, where water and sodium are reabsorbed; this is one reason birds conserve water well.
Passage of food through the fowl's tract is fast compared with mammals, measured in hours rather than days, and varies with feed form and intake. Nutrient requirements for poultry are therefore expressed per unit of feed, and feed form, particle size and grit strongly influence gizzard development and digestive efficiency.
Cattle, buffalo, sheep, goats and deer are ruminants. Their stomach has four compartments: the reticulum, rumen and omasum, together called the forestomach and lined by non-glandular stratified squamous epithelium, and the abomasum, the true glandular stomach. Camelids are not true ruminants; they have a three-compartment stomach with glandular areas in the forestomach, although they also ruminate. The rumen is a continuous-culture fermentation vat kept warm, moist, anaerobic and buffered. Its population includes bacteria, ciliate protozoa, anaerobic fungi and methanogenic archaea. Fibre-digesting bacteria break down cellulose and hemicellulose; amylolytic bacteria ferment starch and sugar.
Microbes ferment carbohydrate to pyruvate and then to the volatile fatty acids (VFA) acetate, propionate and butyrate, with carbon dioxide and methane as gases. VFA are absorbed across the rumen wall and supply most of the ruminant's energy. Acetate is the main substrate for fat synthesis, including milk fat; propionate is the main precursor of glucose by hepatic gluconeogenesis; butyrate is largely converted to beta-hydroxybutyrate in the rumen epithelium, which it also stimulates to grow. Forage diets give a high acetate-to-propionate ratio; starch-rich concentrate diets raise propionate. Fermentation releases reducing equivalents (hydrogen). Formation of acetate and butyrate releases hydrogen, whereas propionate formation uses it; methanogens dispose of most of the rest by reducing carbon dioxide to methane. This is why a shift towards propionate lowers methane per unit of fermented feed, and it is expressed in the stoichiometric relation methane = 0.5 acetate + 0.5 butyrate minus 0.25 propionate (all in moles).
Saliva is essential. Ruminants secrete large volumes, on the order of 100 litres or more a day in adult cattle, rich in bicarbonate and phosphate, which buffer rumen acid. Chewing roughage and rumination increase saliva flow. When a ration is abruptly changed to rapidly fermentable starch, lactate-producing bacteria multiply, pH falls, and ruminal acidosis follows; subacute forms reduce fibre digestion and milk fat.
Forestomach motility is coordinated by the gastric centre in the medulla through the vagus. The primary (mixing) cycle is a sequence of reticular and ruminal contractions that mix contents and move fine particles towards the reticulo-omasal orifice. The secondary (eructation) cycle involves ruminal contractions that move gas forward to the cardia for eructation; failure of eructation causes bloat. Frothy bloat traps gas in a stable foam, typically on legume pasture or fine grain diets; free-gas bloat follows an obstruction or failure of motility. Rumination is regurgitation, remastication, insalivation and reswallowing of the cud; cattle commonly spend many hours a day ruminating, and the time falls when effective fibre is low or the animal is ill.
Microbes use ammonia, peptides and amino acids to build microbial protein. Dietary protein is partly degraded in the rumen (rumen-degradable protein) and partly escapes to the abomasum (undegradable or bypass protein). Urea from the liver returns to the rumen in saliva and across the rumen wall, so ruminants recycle nitrogen, which explains how they survive on poor forage and why non-protein nitrogen can partly replace true protein. Microbial protein washed out to the abomasum and small intestine is digested like any other protein and is of good quality.
The omasum absorbs water, VFA and electrolytes. The abomasum secretes acid and pepsin; in the calf it also secretes chymosin (rennin), which clots milk casein. In the suckling calf, lamb and kid, the oesophageal (reticular) groove closes reflexly when the young animal sucks milk, stimulated by suckling and by milk components, directing milk from the oesophagus to the abomasum and bypassing the rumen. The rumen develops from a small, non-functional organ at birth as solid feed is eaten; VFA, especially butyrate, stimulate papilla growth. Protein degradability is often estimated in the nylon-bag technique by the exponential model degradation = a + b(1 - e to the minus ct), where a is the soluble fraction, b the slowly degradable fraction and c the rate constant; effective degradability also accounts for the rate of outflow.
A skeletal muscle fibre is a long multinucleate cell packed with myofibrils. Each myofibril is a chain of sarcomeres, the contractile units, bounded by Z lines. Thin filaments (actin with tropomyosin and troponin) are anchored at the Z line; thick filaments (myosin) occupy the central A band. The I band contains only thin filaments and shortens during contraction, as does the H zone; the A band keeps its length. This is the sliding filament theory: filaments slide past each other, they do not shorten.
Excitation-contraction coupling runs as follows. Acetylcholine released at the neuromuscular junction binds nicotinic receptors on the motor end-plate and produces an end-plate potential, which normally always triggers a muscle action potential. The action potential spreads along the sarcolemma and down the transverse (T) tubules. In the T tubule membrane, dihydropyridine receptors act as voltage sensors mechanically linked to ryanodine receptors in the sarcoplasmic reticulum (SR), which release calcium. Calcium binds troponin C, troponin moves tropomyosin off the myosin-binding sites on actin, and cross-bridges form. Each cross-bridge cycle uses one ATP: ATP binding detaches myosin from actin, ATP hydrolysis re-cocks the head, and release of phosphate produces the power stroke. After death, ATP falls and cross-bridges cannot detach, producing rigor mortis. Acetylcholinesterase in the synaptic cleft ends transmission; organophosphate and carbamate compounds inhibit it.
Force is graded by recruiting more motor units (a motor neuron and all the fibres it supplies) and by increasing firing frequency, which summates twitches into tetanus. The length-tension relation shows maximum active force at an intermediate sarcomere length where overlap of thick and thin filaments is optimal. Fibre types differ: type I fibres are slow, oxidative and fatigue-resistant; type IIA are fast and oxidative-glycolytic; type IIX (IIB in some species) are fast, glycolytic and fatigable. The proportions vary by muscle, species, breed and training, and they matter for meat quality and athletic performance.
Smooth muscle lines the gut, airways, blood vessels, uterus and bladder. It has no sarcomeres; actin and myosin are anchored to dense bodies and the cells shorten in a corkscrew fashion. Single-unit (visceral) smooth muscle, as in the gut and uterus, is coupled by gap junctions so cells contract together and often show spontaneous activity; multi-unit smooth muscle, as in the iris and large arteries, is controlled fibre by fibre by nerves. Calcium enters from outside through voltage-gated and receptor-operated channels and is released from the SR. In smooth muscle calcium binds calmodulin, not troponin; the calcium-calmodulin complex activates myosin light-chain kinase, which phosphorylates the regulatory light chain of myosin and allows cross-bridge cycling. Myosin light-chain phosphatase reverses this. Cross-bridges can stay attached with little ATP use (the latch state), so smooth muscle maintains tone economically. Smooth muscle also tolerates large stretch and adjusts its tension, the stress-relaxation response, which lets hollow organs fill without pressure rising much.
Hormones and drugs act on smooth muscle through receptors: oxytocin contracts the myometrium, beta-2 adrenergic agonists relax airway and uterine smooth muscle, and alpha-1 agonists contract vascular smooth muscle. Nitric oxide relaxes vascular smooth muscle by raising cyclic GMP.
Cardiac muscle is striated like skeletal muscle but its cells are branched, joined by intercalated discs with gap junctions, and form a functional syncytium. Calcium entering through L-type channels during the plateau triggers further calcium release from the SR, called calcium-induced calcium release. Its long refractory period prevents tetanus. Clinically, disturbed calcium handling in skeletal muscle explains malignant hyperthermia (porcine stress syndrome) in pigs carrying a ryanodine receptor mutation, in which triggers such as halothane or stress cause uncontrolled calcium release, rigidity and high body temperature, with pale, soft, exudative meat after slaughter.
The neuron receives input on its dendrites and cell body and sends output along its axon. The resting membrane potential, typically around minus 60 to minus 90 millivolts depending on cell type, arises mainly because the membrane at rest is far more permeable to potassium than to sodium, and potassium diffuses out down its concentration gradient. The sodium-potassium pump maintains the gradients. The equilibrium potential of any single ion is given by the Nernst equation; at mammalian body temperature it simplifies to about 61.5 millivolts times the log of the outside over inside concentration for a monovalent cation. With typical concentrations, potassium's equilibrium potential lies near minus 90 to minus 95 millivolts and sodium's near plus 60 millivolts.
An action potential is an all-or-none event. Depolarisation to threshold opens voltage-gated sodium channels; sodium entry drives the membrane positive. Sodium channels then inactivate and voltage-gated potassium channels open, repolarising the membrane, often with a brief after-hyperpolarisation. During the absolute refractory period no stimulus can fire another action potential; during the relative refractory period a stronger stimulus can. Conduction velocity rises with axon diameter and with myelination; in myelinated fibres the impulse jumps between nodes of Ranvier (saltatory conduction). Conduction velocity can be measured by stimulating a nerve at two points and dividing the distance between them by the difference in latency.
At a chemical synapse the arriving action potential opens voltage-gated calcium channels, calcium triggers exocytosis of transmitter, and the transmitter acts on postsynaptic receptors. Excitatory postsynaptic potentials (EPSPs) depolarise; inhibitory postsynaptic potentials (IPSPs) hyperpolarise or clamp the membrane. The neuron sums them in space and time at the axon hillock. Major transmitters are acetylcholine, noradrenaline, dopamine, serotonin, glutamate (the main excitatory transmitter of the brain), gamma-aminobutyric acid (GABA; the main inhibitory transmitter of the brain) and glycine (inhibitory, mainly in the spinal cord and brainstem). Strychnine blocks glycine receptors and tetanus toxin blocks glycine and GABA release, both causing spastic convulsions; botulinum toxin blocks acetylcholine release, causing flaccid paralysis.
The reflex arc has a receptor, an afferent neuron, an integrating centre, an efferent neuron and an effector. The stretch (myotatic) reflex is monosynaptic: muscle spindles detect stretch and excite alpha motor neurons of the same muscle, as tested in the patellar reflex. Golgi tendon organs detect tension and inhibit the muscle. Withdrawal (flexor) reflexes are polysynaptic and are paired with the crossed extensor reflex. In veterinary neurology, lesions are localised by combining reflexes with posture: an upper motor neuron lesion gives preserved or exaggerated reflexes and increased tone caudal to the lesion, while a lower motor neuron lesion gives reduced or absent reflexes, reduced tone and rapid neurogenic muscle atrophy. The Schiff-Sherrington posture, with thoracic limb extensor rigidity after an acute thoracolumbar spinal cord lesion, reflects loss of ascending inhibition from border cells in the lumbar cord.
The cerebral cortex plans voluntary movement; the basal nuclei and cerebellum refine it. The cerebellum compares intended and actual movement; its damage causes ataxia, dysmetria (often hypermetria) and intention tremor without weakness.
The autonomic nervous system has two main efferent divisions. Sympathetic preganglionic neurons leave the thoracolumbar cord; parasympathetic preganglionic neurons leave the brainstem (cranial nerves III, VII, IX and X) and sacral cord. All preganglionic neurons release acetylcholine onto nicotinic receptors. Parasympathetic postganglionic neurons release acetylcholine onto muscarinic receptors; most sympathetic postganglionic neurons release noradrenaline onto alpha and beta adrenergic receptors, with exceptions such as sweat glands in some species. The adrenal medulla is a modified sympathetic ganglion that releases adrenaline and noradrenaline into blood. The hypothalamus integrates autonomic, endocrine and behavioural responses, including temperature, thirst and hunger.
Special senses vary with the species' ecology. Domestic carnivores have a tapetum lucidum, which reflects light back through the retina and improves vision in dim light; the pig largely lacks one. Dogs and cats have rod-rich retinas and dichromatic colour vision. Olfaction is highly developed in dogs and pigs, and many species have a vomeronasal organ that detects pheromones, used in the flehmen response.
A hormone is a chemical messenger carried by the blood to target cells that carry specific receptors. Chemically, hormones are peptides and proteins (insulin, growth hormone, oxytocin), amines derived from tyrosine (catecholamines, thyroid hormones) or tryptophan (melatonin), and steroids derived from cholesterol (cortisol, aldosterone, oestradiol, progesterone, testosterone, and the active form of vitamin D). Water-soluble peptides and catecholamines act on cell-surface receptors and use second messengers such as cyclic AMP, inositol trisphosphate with diacylglycerol, and calcium; insulin and growth hormone use receptor-associated tyrosine kinase pathways. Steroids and thyroid hormones are lipid-soluble, travel largely bound to plasma proteins, and act mainly on intracellular receptors that regulate gene transcription, so their effects start more slowly and last longer. Most endocrine systems are regulated by negative feedback; positive feedback is the exception, as in the LH surge before ovulation and oxytocin release during parturition.
The hypothalamus controls the anterior pituitary (adenohypophysis) through releasing and inhibiting hormones carried in the hypophyseal portal system: thyrotropin-releasing hormone (TRH), corticotropin-releasing hormone (CRH), gonadotropin-releasing hormone (GnRH), growth hormone-releasing hormone (GHRH), somatostatin (inhibits growth hormone) and dopamine (inhibits prolactin). The anterior pituitary secretes growth hormone, prolactin, thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH) and luteinising hormone (LH). The posterior pituitary (neurohypophysis) stores and releases oxytocin and antidiuretic hormone (ADH, vasopressin), which are made in the supraoptic and paraventricular nuclei of the hypothalamus and carried down axons.
A hormone's plasma half-life determines how quickly its concentration falls once secretion stops: after one half-life half remains, after three about one-eighth. Peptide hormones usually have short half-lives of minutes; protein-bound thyroxine and steroids last much longer.
The thyroid follicle traps iodide, oxidises it and attaches it to tyrosine residues on thyroglobulin, then couples iodotyrosines to form thyroxine (T4) and triiodothyronine (T3). T4 is the main secreted product; peripheral deiodinases convert it to the more active T3. Thyroid hormones raise basal metabolic rate and heat production and are essential for growth and nervous development. Hypothyroidism is common in dogs; hyperthyroidism is common in older cats.
The adrenal cortex has three zones: zona glomerulosa (aldosterone, regulated mainly by angiotensin II and plasma potassium), zona fasciculata (glucocorticoids, regulated by ACTH) and zona reticularis (sex steroids). Cortisol is the main glucocorticoid in most domestic mammals and corticosterone in birds and rodents. Glucocorticoids promote gluconeogenesis, protein catabolism and lipolysis, suppress inflammation and immunity, and are needed for the response to stress. Excess causes hyperadrenocorticism (Cushing's syndrome), common in dogs; deficiency causes hypoadrenocorticism (Addison's disease), with hyponatraemia and hyperkalaemia when aldosterone is also lost.
The endocrine pancreas has alpha cells (glucagon), beta cells (insulin), delta cells (somatostatin) and PP cells (pancreatic polypeptide). Insulin promotes uptake and storage of glucose, amino acids and fat; glucagon promotes glycogenolysis and gluconeogenesis. Ruminants absorb little glucose and rely on hepatic gluconeogenesis, largely from propionate, so their blood glucose concentrations are normally lower than those of monogastrics; reference ranges vary by species and laboratory. Dogs develop mainly insulin-deficient diabetes, whereas cats develop a form resembling human type 2 diabetes, with insulin resistance and amyloid deposition.
Calcium is controlled by parathyroid hormone (PTH), which raises plasma calcium by bone resorption, renal calcium reabsorption and activation of vitamin D; calcitriol (1,25-dihydroxycholecalciferol), formed in the kidney, raises intestinal calcium absorption; calcitonin from thyroid C cells lowers calcium. At calving the dairy cow's sudden calcium demand for colostrum can exceed her ability to mobilise it, causing parturient paresis (milk fever).
The testis has two functions under two pituitary hormones. Leydig (interstitial) cells respond to LH by making testosterone. Sertoli cells, which line the seminiferous tubules and form the blood-testis barrier, respond to FSH and testosterone, nurse developing germ cells, secrete androgen-binding protein and produce inhibin, which feeds back to suppress FSH. Testosterone suppresses GnRH and LH. Spermatogenesis proceeds from spermatogonia through primary and secondary spermatocytes (meiosis) to spermatids, which remodel into spermatozoa (spermiogenesis). The whole process takes several weeks in domestic species, followed by transit and maturation in the epididymis, where sperm gain motility and fertilising ability.
Spermatogenesis requires a testicular temperature a few degrees below core body temperature in scrotal mammals. The scrotum, the pampiniform plexus acting as a countercurrent heat exchanger, the cremaster muscle and the tunica dartos maintain it. Cryptorchid testes retained in the abdomen do not produce sperm but still produce testosterone, so a unilateral cryptorchid animal behaves as a male.
Semen consists of spermatozoa from the testis and epididymis and seminal plasma from the accessory glands, which differ by species: the boar has large vesicular and bulbourethral glands and ejaculates a large volume with a gel fraction; the dog has only a prostate among the accessory glands; ruminants ejaculate a small, concentrated volume. Semen evaluation measures volume, concentration, motility and morphology. The number of insemination doses from an ejaculate is the total number of sperm meeting the criterion (for example, progressively motile sperm) divided by the number required per dose, which differs by species, site of deposition and whether semen is fresh or frozen.
The oestrous cycle has a follicular phase (pro-oestrus and oestrus) dominated by oestradiol from growing follicles, and a luteal phase (metoestrus and dioestrus) dominated by progesterone from the corpus luteum (CL). Rising oestradiol without progesterone causes behavioural oestrus and, by positive feedback on the hypothalamus, the LH surge that causes ovulation. Typical cycle lengths are about 21 days in the cow, buffalo, sow, doe and mare and about 17 days in the ewe, each with a normal range. The cow ovulates after the end of standing oestrus, whereas the mare ovulates towards the end of a longer oestrus. The queen, rabbit and camelids are induced ovulators, in which mating triggers the LH surge. The bitch is monoestrous, with a long interoestrous interval and a luteal phase of similar length whether or not she is pregnant. Season also matters: the mare and queen are long-day breeders, whereas the ewe and doe are short-day breeders; melatonin mediates both.
If the animal is not pregnant, the uterus releases prostaglandin F2-alpha, which lyses the CL. In ruminants PGF2-alpha passes from the utero-ovarian vein to the closely applied ovarian artery by a local countercurrent route. Maternal recognition of pregnancy prevents luteolysis and differs by species: interferon-tau from the trophoblast in ruminants, oestrogens from the conceptus in the pig, and movement of the conceptus throughout the uterus in the mare. In the mare, the endometrial cups secrete equine chorionic gonadotropin (eCG), which causes supplementary corpora lutea. Placental types by tissue layers are epitheliochorial (pig, horse), synepitheliochorial (ruminants), endotheliochorial (dog, cat) and haemochorial (primates, rodents). The source of pregnancy progesterone differs: the ewe and mare shift to placental production, whereas the goat, sow and bitch depend on the CL throughout, so luteolysis at any stage ends their pregnancy.
Typical gestation lengths, which vary with breed and individual, are about 9 months in the cow (commonly around 280 to 285 days), about 10 months in the buffalo, about 5 months in the ewe and doe, about 114 days in the sow, about 11 months in the mare and about 63 days in the bitch and queen. In ruminants, parturition is initiated by the fetus: maturation of the fetal hypothalamic-pituitary-adrenal axis raises fetal cortisol, which induces placental enzymes that convert progesterone to oestrogens. Falling progesterone and rising oestrogen increase uterine contractility, oxytocin receptors and PGF2-alpha.
In the hen only the left ovary and oviduct develop. The ovum is caught by the infundibulum, where fertilisation occurs; albumen is added in the magnum, shell membranes in the isthmus and the shell in the shell gland (uterus), where the egg spends most of the roughly one-day journey.
The mammary gland is a modified skin gland. Its secretory unit is the alveolus, lined by epithelial cells and surrounded by contractile myoepithelial cells; alveoli drain through ducts into the gland and teat cisterns. The number and site of glands vary: the cow has four quarters, each with its own teat and duct system; the ewe and doe have two; the mare has two glands each usually with two duct systems; the sow and bitch have several pairs.
Mammogenesis, the growth of the gland, is driven by oestrogen (ducts), progesterone (lobuloalveolar development), prolactin, growth hormone, glucocorticoids and, in some species, placental lactogen. Lactogenesis, the onset of copious secretion, is triggered near parturition by the fall in progesterone in the presence of prolactin and glucocorticoids. Galactopoiesis, the maintenance of lactation, needs regular milk removal and hormonal support; in ruminants growth hormone (somatotropin) is especially important, while prolactin dominates in several other species. Recombinant bovine somatotropin raises milk yield; it is permitted in some countries and prohibited in others, including the European Union and Canada.
Lactose is the main osmotic component of milk, so it largely determines milk volume. It is made in the Golgi apparatus from glucose and galactose by lactose synthase, which is galactosyltransferase modified by alpha-lactalbumin. Because the ruminant absorbs little glucose, milk yield depends on hepatic gluconeogenesis from propionate and amino acids. Milk fat is made from two sources: short- and medium-chain fatty acids synthesised in the gland from acetate and beta-hydroxybutyrate (in ruminants), and long-chain fatty acids taken up from blood lipoproteins and non-esterified fatty acids. Diets high in rapidly fermentable starch and low in effective fibre depress milk fat, largely because altered rumen biohydrogenation produces fatty acid intermediates, such as trans-10, cis-12 conjugated linoleic acid, that inhibit mammary fat synthesis. Milk proteins are mainly caseins, secreted as micelles with calcium phosphate, and whey proteins such as beta-lactoglobulin in ruminant milk and alpha-lactalbumin. Large volumes of blood must pass through the udder for each litre of milk formed; figures of several hundred litres per litre are commonly quoted for dairy cows.
Milk secreted between milkings is stored in the alveoli and cisterns, but most alveolar milk is only available after milk ejection (let-down). Stimulation of the teat by suckling or milking sends afferent signals to the hypothalamus, which releases oxytocin from the posterior pituitary; oxytocin contracts the myoepithelial cells within about a minute and milk moves into the cisterns. This is a neuroendocrine reflex. Fear or pain releases adrenaline, which inhibits ejection both centrally and by constricting mammary blood vessels, so the effect of oxytocin on the gland is reduced.
Colostrum is the secretion of the first days after parturition. It is rich in immunoglobulins, especially IgG1 in ruminants, which is selectively transferred from blood into the gland in the last weeks of pregnancy, and it contains more protein, vitamin A and cells, and less lactose, than mature milk. Species whose placentas do not transfer antibodies, notably ruminants, pigs and horses with their epitheliochorial and synepitheliochorial placentas, are born almost without circulating antibody. Their neonates absorb intact immunoglobulin by pinocytosis across the small intestinal epithelium for a limited time after birth, after which the gut closes to macromolecules; absorption efficiency falls over the first day. Failure of passive transfer therefore follows late or inadequate colostrum feeding.
Milk yield rises after parturition to a peak, typically some weeks into lactation in the dairy cow, then declines slowly; the rate of decline defines persistency. Early lactation is a period of negative energy balance because intake lags behind milk output, and body fat is mobilised. A dry period before the next calving allows the gland to regress and regenerate. Milk composition varies greatly between species, breeds, stage of lactation and diet, so milk from different sources is compared on a common basis such as 4% fat-corrected milk, commonly calculated as 0.4 times milk weight plus 15 times fat weight.
Growth is an increase in body mass and size; development is change in form and function. Both begin with prenatal growth, first in the embryonic period, when organs form, and then the fetal period, when most prenatal weight is gained, much of it in the last third of pregnancy. Birth weight depends on genotype, number of offspring in the litter, maternal nutrition and placental size. Postnatal growth of the whole animal follows a sigmoid curve: slow at first, then a phase of accelerating absolute gain, a point of inflection near puberty, and a slowing towards mature size.
Tissues do not grow at the same rate. In the classical description attributed to Hammond, tissues mature in the order nervous tissue, bone, muscle and finally fat, and the body grows in waves from head to loin and from the distal limbs towards the trunk. Early-maturing breeds reach a given composition at a lighter weight than late-maturing breeds. Relative growth between a part and the whole is described by the allometric equation y = a times x to the power b, where b above 1 means the part grows faster than the whole.
Growth is measured practically as average daily gain (weight gained divided by days), feed conversion ratio (feed eaten divided by weight gained; lower is better), feed efficiency (the reciprocal), and specific growth rate (100 times the natural log of final over initial weight, divided by days), which is used for fast-growing young animals and fish. Energy requirements for maintenance scale not with body weight but with metabolic body size, weight to the power 0.75. Kleiber's relation, basal metabolic rate of about 70 kilocalories times weight in kilograms to the 0.75 per day, explains why small animals need far more energy per kilogram of body weight than large ones.
The central axis is growth hormone (GH) and insulin-like growth factor 1 (IGF-1). GH, secreted by the anterior pituitary in pulses under GHRH stimulation and somatostatin inhibition, acts on the liver to produce IGF-1, which mediates most of GH's effect on bone and muscle growth. GH also acts directly: it is lipolytic, spares glucose and is anti-insulin. Nutrition modifies the axis; in undernutrition GH rises but hepatic IGF-1 falls, uncoupling the axis. Thyroid hormones are essential for normal growth and for the maturation of the brain and skeleton. Insulin is anabolic. Androgens promote muscle growth, which is why entire males grow faster and leaner than castrates, while oestrogens, at puberty, promote closure of the growth plates and so end longitudinal bone growth.
Longitudinal bone growth takes place at the epiphyseal growth plate, where chondrocytes proliferate, hypertrophy and are replaced by bone (endochondral ossification). Muscle growth after birth is mainly hypertrophy of existing fibres rather than an increase in fibre number, which is largely fixed before birth; satellite cells supply extra nuclei. Myostatin is a negative regulator of muscle mass; loss-of-function mutations in its gene cause double muscling in some cattle breeds, such as Belgian Blue and Piedmontese, with more muscle fibres and less fat, and an increased rate of dystocia.
Adipose tissue grows by hyperplasia early in life and by hypertrophy later; fat is deposited in the order visceral, intermuscular, subcutaneous and intramuscular (marbling), the last being late-maturing. Leptin from adipose tissue signals energy stores to the hypothalamus and is one of the signals that permit puberty. Growth promoters such as anabolic steroid implants and beta-adrenergic agonist repartitioning agents are permitted in some countries and banned in others, including the European Union; their use is a matter of national law and residue regulation, not physiology alone.
Mammals and birds are homeotherms: they keep core body temperature within a narrow range by balancing heat production against heat loss. Heat is produced by metabolism, including digestion; in ruminants rumen fermentation adds a substantial heat increment. Heat is exchanged with the environment by four routes. Radiation, conduction and convection are sensible heat routes that depend on a temperature gradient, so they can add heat to the animal when the surroundings are hotter than the skin. Evaporation of water from skin (sweating) and respiratory surfaces (panting) is a latent heat route; it removes about 2.4 kilojoules for each gram of water evaporated and is the only route that still works when air is hotter than the body. High humidity reduces evaporation, which is why hot, humid weather is more stressful than hot, dry weather.
Within the thermoneutral zone, heat balance is maintained with minimal metabolic cost, by adjusting skin blood flow, posture and coat. Below the lower critical temperature, the animal must increase heat production, by shivering and by non-shivering thermogenesis. Above the upper critical temperature, it must spend energy on evaporative cooling, and heat production eventually rises as body temperature climbs. The width and position of the zone depend on species, age, coat or fleece, feeding level, production level and acclimatisation. Newborn animals have a narrow zone and a high lower critical temperature.
Species cool themselves differently. The horse sweats profusely; its sweat contains the protein latherin, which helps spread sweat over the hair coat. Cattle sweat moderately and pant; Bos indicus cattle generally tolerate heat better than Bos taurus, with more sweat glands, a sleek coat, loose skin and lower metabolic heat production. Pigs have few functional sweat glands and rely on behaviour, especially wallowing, and on panting; buffalo, with few sweat glands and dark skin, also rely on wallowing and shade. Dogs pant and have sweat glands mainly on the footpads. Birds have no sweat glands; they pant, and some species also use gular flutter. Sheep, goats and cats have a carotid rete, a network of arteries bathed in cooled venous blood from the nasal passages, which allows selective brain cooling.
Heat stress occurs when heat gain exceeds the animal's capacity to lose heat. Early responses are increased respiratory rate, sweating, reduced activity, shade-seeking and more drinking. Feed intake falls, which reduces metabolic heat but also production. Blood flow is redirected to the skin, so less reaches the gut and reproductive tract. In dairy cattle, heat stress reduces milk yield, shortens and weakens oestrous behaviour, impairs oocyte quality and early embryo survival, and so lowers conception rates, with effects that continue after the hot period because follicles developing during it are damaged. Panting causes loss of carbon dioxide and respiratory alkalosis. In laying hens this alkalosis reduces the bicarbonate available for eggshell formation and plasma ionised calcium, so eggshells become thinner; broilers show reduced growth and increased mortality in heat waves.
Environmental heat load is commonly expressed as the temperature-humidity index (THI). One widely used form is THI = (1.8T + 32) minus (0.55 minus 0.0055RH) times (1.8T minus 26), where T is air temperature in degrees Celsius and RH is relative humidity in percent. Several other formulae exist, and the thresholds at which production falls vary with species, breed, yield and climate; for high-yielding dairy cows, thresholds lower than the traditionally quoted values have been proposed.
Cold stress is countered by insulation (coat, fleece, subcutaneous fat), vasoconstriction, huddling and increased heat production. Non-shivering thermogenesis in brown adipose tissue, whose mitochondria contain uncoupling protein 1 (thermogenin), is important in neonatal lambs and calves; newborn piglets lack functional brown fat and depend on shivering and on a warm environment, which is why creep heating matters. Countercurrent heat exchange in the limbs of birds and many mammals reduces heat loss to cold ground or water. Acclimatisation is the physiological adjustment to a changed environment over days to weeks, including increased sweating capacity, lower heat production and altered thyroid activity; heat shock proteins protect cellular proteins during heat exposure. Adaptation is the genetic suitability of a breed to its environment.
Muscle can store only a small amount of ATP, which it regenerates by three systems. The phosphagen system transfers phosphate from creatine phosphate to ADP through creatine kinase; it supports maximal efforts for seconds. Anaerobic glycolysis converts glycogen or glucose to pyruvate and then lactate, yielding ATP quickly but little per glucose; it supports intense efforts for tens of seconds to a few minutes and generates hydrogen ions that contribute to fatigue. Aerobic metabolism oxidises carbohydrate and fat in mitochondria; it yields far more ATP per substrate and supports prolonged work, but at a lower maximal rate. All three contribute at any time; their proportions depend on intensity and duration. Fat is the main fuel in low-intensity endurance work; carbohydrate becomes more important as intensity rises.
During exercise, cardiac output rises by increases in heart rate and stroke volume, sympathetic activity redistributes blood to working muscle, and ventilation rises. Maximal oxygen uptake (VO2max) is the highest rate of oxygen consumption, set by oxygen delivery and use. The horse is an exceptional athlete: its mass-specific VO2max is far higher than that of most other mammals because of a large heart, a large lung, and a spleen that contracts under sympathetic stimulation to release a large reserve of red cells, raising packed cell volume sharply during exercise. The dog also has a contractile splenic reserve. At the canter and gallop the horse breathes once per stride, locomotor-respiratory coupling. Exercise-induced pulmonary haemorrhage occurs in many racehorses and reflects very high pulmonary vascular pressures during maximal effort.
Fitness in horses is monitored by heart rate and blood lactate during standard exercise tests. V200, the speed at a heart rate of 200 beats per minute, and V4, the speed at which blood lactate reaches 4 millimoles per litre, both increase with training. Training increases mitochondrial density and oxidative enzymes, capillary density, muscle glycogen stores, stroke volume and the capacity to buffer acid; resting heart rate may fall, and fibre-type proportions shift towards oxidative types within the limits of the individual's genetics.
Fatigue has several causes: depletion of creatine phosphate and glycogen, accumulation of hydrogen ions and inorganic phosphate that impair cross-bridge function and calcium handling, and dehydration and hyperthermia in prolonged work. Only about a quarter of the chemical energy used by working muscle becomes mechanical work; the rest becomes heat. Exercising animals therefore face a large heat load, and horses in endurance rides can lose large volumes of sweat containing sodium, potassium and chloride. Dehydration, electrolyte loss and hypochloraemic metabolic alkalosis can cause exhaustion, and in some horses synchronous diaphragmatic flutter (thumps), linked to low ionised calcium and alkalosis. Exertional rhabdomyolysis (tying-up) is muscle damage during or after exercise, with stiffness, pain and myoglobinuria.
Draught animals, mainly cattle, buffalo, horses, donkeys, mules and camels, still provide farm power in many countries. The sustainable draught force of working cattle is usually quoted as roughly one-tenth or a little more of body weight, with the exact fraction depending on species, training, harness, speed, duration and condition. Mechanical power is force times speed; for example, a 400 kg bullock pulling at one-tenth of body weight (about 392 newtons) at 0.8 metres per second develops about 314 watts, roughly 0.42 horsepower. Work in joules is force times distance. Because muscle is only about 25% efficient, the metabolic energy cost is about four times the mechanical work done, and the extra feed must be supplied or the animal loses condition. Buffalo tolerate heat poorly during work because they sweat little, so work is scheduled in the cooler parts of the day with access to water and wallowing.
Ethology is the study of animal behaviour in its natural context. Physiologically, behaviour results from sensory input, internal state and learning, integrated in the brain. The hypothalamus generates motivational states such as hunger, thirst, thermoregulatory behaviour, sexual and maternal behaviour; the limbic system, including the amygdala and hippocampus, attaches emotional value and memory to events; the amygdala is central to fear. Dopamine pathways from the midbrain to the nucleus accumbens signal reward and wanting; endogenous opioids contribute to pleasure and pain relief; serotonin influences mood and impulsivity; oxytocin promotes social bonding and maternal behaviour; testosterone increases sexual and many forms of aggressive behaviour, which is one reason castration reduces aggression in males. Circadian rhythms are generated by the suprachiasmatic nucleus of the hypothalamus and entrained by light, with melatonin as the night signal.
Learning modifies behaviour through experience. Habituation is a decline in response to a repeated harmless stimulus; sensitisation is an increased response after a strong or noxious one. Classical (Pavlovian) conditioning pairs a neutral stimulus with one that already produces a response, as when cows come to the parlour at the sound of the gate. Operant (instrumental) conditioning links a behaviour with its consequence: positive reinforcement adds something the animal wants, negative reinforcement removes something aversive (the release of pressure in horse training), and punishment reduces a behaviour. Imprinting is rapid learning during a sensitive period early in life, classically the following response of precocial birds such as ducklings and chicks. Socialisation periods in puppies and kittens, in the first weeks of life, shape later fearfulness towards people and novelty.
Domestic animals are mostly social species. Groups form dominance hierarchies that reduce fighting once established; mixing unfamiliar animals, as with pigs at weaning, causes fighting until a new order forms. Handling uses the flight zone, the space around an animal which, when entered, causes it to move away, and the point of balance, usually at the shoulder, which determines whether the animal moves forward or back.
Sexual behaviour is hormone-dependent. In the cow, standing to be mounted is the most reliable sign of oestrus; mounting other cows, restlessness and mucus discharge support it. The oestrous sow shows the standing (lordosis) response to back pressure, strongest in the presence of a boar, whose saliva contains the pheromone androstenone. The oestrous mare shows tail raising, urination and eversion of the clitoris (winking) when teased by a stallion. Males of many species show the flehmen response, lip curling that draws pheromones towards the vomeronasal organ.
Maternal behaviour starts at parturition. In the ewe, stimulation of the vagina and cervix at birth releases oxytocin in the brain, which opens a short period in which she learns the smell of her own lamb and accepts it; she then rejects others. Licking the newborn dries it, stimulates breathing and helps bonding. The sow builds a nest before farrowing when given material.
Stress is the response to challenge. The sympathetic-adrenal medullary system releases catecholamines within seconds; the hypothalamic-pituitary-adrenal axis raises cortisol over minutes. Short-term stress helps survival; prolonged stress impairs immunity, growth and reproduction. Abnormal behaviours signal welfare problems: stereotypies are repetitive, invariant behaviours without obvious function, such as crib-biting and weaving in horses, bar-biting in sows and pacing in confined carnivores; redirected behaviours such as tail-biting in pigs and feather-pecking in hens relate to barren environments, diet and genetics. Welfare is assessed with frameworks such as the Five Freedoms and the Five Domains model, combining physiological, behavioural and health indicators, because no single measure, including cortisol, is sufficient on its own.
In the second year you will use this subject to understand why a cow bloats on lush legume pasture, why a calf must get colostrum within hours, and why a dog with a spinal injury has brisk reflexes behind the lesion. Build your notes around species comparisons (ruminant, horse, pig, dog and cat, fowl) and around control loops: the stimulus, the sensor, the hormone or nerve, and the effect. Practise the short calculations until they are automatic: VFA and methane, Nernst potential, average daily gain, feed conversion, fat-corrected milk, THI and draught power. These carry reliable marks in entrance and licensing examinations.
After five years in practice, this physiology is behind daily decisions. You time insemination from the cow's standing oestrus and the known interval to ovulation, choose a luteolytic drug knowing which species depend on the corpus luteum throughout pregnancy, and advise on transition-cow rations to prevent milk fever and ruminal acidosis. You check colostrum management when calves scour, localise neurological lesions by reflexes, and warn clients about heat stress when temperature and humidity rise. In equine and small animal work you read fitness, endocrine and neurological findings through the same principles.
With twenty-five years of experience, physiology shapes herd and system decisions rather than single cases. You set heat-abatement plans for dairy units using THI and production records, judge whether a fertility problem is nutritional, thermal or managerial, and design weaning, mixing and handling routines that cut stress and injury. You advise on feeding strategies that reduce methane per unit of product, on breed choice for hot climates, on draught-animal welfare where animal power remains important, and on the national rules governing growth promoters and somatotropin. You also train younger colleagues to reason from mechanism rather than protocol.
A researcher with a PhD and long field experience uses this physiology to ask questions that change practice: how rumen microbial pathways can be steered to lower methane without losing production, how heat stress damages oocytes and how to protect fertility, what controls the GH-IGF-1 axis under undernutrition, how the mammary gland partitions nutrients, and which physiological and behavioural indicators validly measure welfare. The work demands command of quantitative methods, including degradability kinetics, energy scaling, and endocrine sampling designs that respect pulsatile secretion, and an honest view of where species comparisons break down.
This manual gives mechanisms and species comparisons, not full reference intervals: normal hormone concentrations, rumen pH values, milk composition and physiological measurements vary by species, breed, method and laboratory, and are left to the laboratory and the specialist text. It does not yet cover fish, camelid or laboratory-animal physiology in depth, sensory physiology beyond the essentials, detailed intracellular signalling, the endocrinology of synchronisation and superovulation protocols (covered with gynaecology), or quantitative models of heat balance. Duration of spermatogenesis and heat stress thresholds are described in general terms because published values differ between sources.
Original practice questions written to the syllabus and pattern of the exams named in each question's examTags. They are not past papers and do not reproduce any board's questions. Numerical answers were computed by script; reference values vary by species, breed and laboratory.
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