22-Agric-A1 Applied Plant, Animal or Human Physiology · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. 04-Agric-A1 Animal or Human Physiology, National Exams May 2013 — a three-hour closed-book examination; one of two approved calculator models (Casio or Sharp) is permitted. The cover page states that five (5) questions constitute a complete exam paper and that only the first five as they appear in the answer book are marked, that each question is of equal value, and that full marks are not available for a bare correct answer — the reasoning must be communicated clearly. All six printed questions are worked here, because the set is a study resource rather than a timed attempt.
Reference texts. S.E. Curtis, Environmental Management in Animal Agriculture (thermoneutral zone, lower/upper critical temperature and the factors that shift them, group and flooring effects); D. McDonald et al., Animal Nutrition, 7th ed. (gross/digestible/metabolizable/net energy partition, heat increment of feeding); M.K. Yousef (ed.), Stress Physiology in Livestock (external stressors and energy partition, thermoregulatory heat-flow pathways); P.D. Lewis and T.R. Morris, Poultry Lighting: the Theory and Practice (photoperiod versus light-intensity reciprocity); ASABE Standards (American Society of Agricultural and Biological Engineers) (metabolic body-size scaling and sensible heat production of livestock).
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.
Part (a) — the heat-flow paths. Metabolic heat generated in the body core has only two routes out to the environment: through the body wall to the skin surface, or through the respiratory tract on the air the animal breathes. Both routes then use the same four physical mechanisms — radiation, convection, conduction and evaporation — to make the final transfer to the surroundings.
Part (b) — adjusting the rate of heat flow. The animal has several independent controls, all of which change either the insulation between core and surface or which mechanism carries the load:
Vasomotor control. Blood vessels supplying the skin dilate or constrict to change how much warm core blood reaches the periphery. Vasodilation in the heat raises skin temperature and hence the radiative and convective gradient to the environment; vasoconstriction in the cold keeps warm blood in the core and lets the peripheral tissue act as an extra insulating layer, sharply reducing conduction outward.
Piloerection and posture. Raising the hair or feathers thickens the insulating air layer trapped against the skin (reducing heat loss in the cold); curling into a ball or huddling with other animals reduces the exposed surface area, while stretching out or standing to expose more surface increases it in the heat.
Evaporative mechanisms. Sweating and panting are recruited specifically when sensible (radiative + convective + conductive) loss cannot keep pace, most often above the UCT; their rate is adjustable (sweat gland activity, panting frequency) and gives the animal a controllable evaporative channel that does not depend on there being a favourable temperature gradient at all — evaporation can remove heat even when the air is hotter than the skin.
Countercurrent exchange in the extremities. In limbs and extremities, paired arteries and veins run close together so that outgoing warm arterial blood pre-warms the returning cool venous blood; this recovers heat that would otherwise be lost from the extremity tip and can be engaged or bypassed to trade off core heat conservation against keeping the extremity itself from freezing.
