22-Agric-A1 Applied Plant, Animal or Human Physiology · May 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. 04-Agric-A1 Animal or Human Physiology, National Exams May 2017 — a three-hour closed-book examination; one of two approved calculator models (Casio or Sharp) is permitted. The rubric states that five (5) questions constitute a complete exam paper and that the first five questions appearing in the answer book are marked (worth 20 marks each, 100 marks total); all six (6) printed questions are worked here as a complete study resource.
Reference texts. M.K. Yousef (ed.), Stress Physiology in Livestock, Vol. I — Basic Principles, CRC Press (thermoregulation, thermoneutral zone, endotherm/ectotherm physiology, external stressors); J.A. DeShazer (ed.) and ASABE Standards (American Society of Agricultural and Biological Engineers), Livestock Energetics and Thermal Environmental Management (sensible heat production, metabolic body-size scaling, animal housing design); P. McDonald et al., Animal Nutrition, 7th ed. (gross/digestible/metabolizable/net energy, feed-energy partition); K. Schmidt-Nielsen, Animal Physiology: Adaptation and Environment, 5th ed. (Bergmann's rule, comparative thermal biology, calorimetry); R.L. Curtis, Environmental Management in Animal Agriculture, Iowa State University Press (animal housing microclimate).
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
Body core temperature (Tcore) is a classic negative-feedback control loop with one integrating centre and two divergent effector branches, sketched below.
Sensors (afferent limb). Peripheral thermoreceptors in the skin detect surface (peripheral) temperature and skin blood flow, while central thermoreceptors located in the hypothalamus itself, the spinal cord, and the abdominal viscera detect core (deep-body) temperature directly. Both populations feed continuous afferent nerve signals into the central nervous system.
Integrating centre. The preoptic area/anterior hypothalamus acts as the thermostat: it compares the combined (weighted core-dominant) thermal input against an internal reference set-point (Tset, normally tightly regulated but itself resettable, e.g. upward during a febrile/pyrogen response) and computes an error signal, then routes efferent output down one of two mutually exclusive pathways depending on the sign of that error.
Hot-condition (Tcore > Tset) effector branch. The hypothalamus reduces sympathetic vasoconstrictor tone to cutaneous arterioles, producing cutaneous vasodilation: skin blood flow rises, carrying core heat to the body surface for radiative and convective loss. Simultaneously it drives sympathetic cholinergic output to eccrine sweat glands (species with functional sweat glands, e.g. humans, horses, cattle) and/or increases respiratory rate for evaporative panting (species with few/no sweat glands, e.g. dogs, pigs, poultry); evaporation of water from the skin or respiratory mucosa removes a large amount of latent heat per gram evaporated, and remains effective even when ambient temperature is close to or above Tcore, unlike radiative/convective loss. Behaviourally, the animal reduces voluntary activity and heat-generating feed intake, and seeks shade or water.
Cold-condition (Tcore < Tset) effector branch. The hypothalamus increases sympathetic vasoconstrictor tone to the skin, sharply reducing cutaneous blood flow and hence convective heat delivery to the body surface (increases the animal's effective peripheral insulation). It drives somatic motor output to skeletal muscle producing shivering thermogenesis (rapid, asynchronous, largely mechanically-ineffective contraction that converts chemical energy to heat), and sympathetic/hormonal output (catecholamines, thyroid hormone) driving non-shivering thermogenesis, most notably via uncoupling protein-1 (UCP1) in brown adipose tissue in neonates and cold-adapted species. Piloerection (arrector pili muscle contraction, sympathetically driven) fluffs the hair/feather coat to thicken the trapped insulating air layer. Behaviourally, the animal increases feed intake (added substrate for thermogenesis), huddles, and seeks shelter.
Closing the loop. Both effector branches act to move Tcore back toward Tset; as the error shrinks, the afferent signal driving the effector response weakens correspondingly, which is the defining property of a negative-feedback (self-limiting) control system — the corrective response itself removes the stimulus that produced it.
