Question 4 of 6: Three Mechanisms of Thermoregulation

Question 4: Three Mechanisms of Thermoregulation (20 marks)

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.

1) Vasomotor (circulatory) mechanisms. Example: cutaneous vasoconstriction in a beef animal standing in a cold winter wind. Sympathetic nerves constrict the smooth muscle of cutaneous arterioles and open/close arteriovenous anastomoses, controlling how much warm core blood is routed to the skin surface versus shunted through deeper, more insulated venous return pathways (countercurrent heat exchange with the adjacent artery). Constriction in the cold minimizes blood flow to the skin, so heat is retained in the core and the skin surface is allowed to run cool (increasing the animal's effective peripheral insulation); dilation in heat maximizes skin blood flow so core heat can be carried to the surface and lost by radiation/convection/evaporation. This mechanism works because blood is the body's main convective heat-transport medium — controlling its distribution controls where heat is allowed to go.

2) Metabolic (chemical thermogenesis) mechanisms. Example: shivering and non-shivering (brown-adipose-tissue) thermogenesis in a newborn piglet exposed to a cold farrowing-house floor. Skeletal muscle can be driven into rapid, asynchronous, largely-uncoordinated contraction (shivering) that does little external mechanical work but converts most of the ATP hydrolysed into heat. Brown adipose tissue (abundant in neonates and cold-adapted mammals, much reduced in most adult farm-animal species) uses uncoupling protein-1 to short-circuit the mitochondrial proton gradient, releasing the energy that would otherwise drive ATP synthase directly as heat instead. Both raise metabolic heat production above the basal/resting level, which is the only way an animal can defend T_core once ambient temperature has fallen below its lower critical temperature and vasomotor insulation alone is no longer sufficient.

3) Evaporative heat-loss mechanisms. Example: panting in a dairy cow under summer heat stress. Species with few or non-functional cutaneous sweat glands (cattle have some, but rely heavily on respiratory evaporation; species such as swine and poultry rely on it almost exclusively) increase respiratory rate and depth (rapid, shallow, open-mouth breathing) to move large volumes of air across the moist mucosa of the upper respiratory tract. Water evaporating from that surface (and, in sweating species, from the skin) absorbs a large latent heat of vaporization per gram, carrying heat away from the body even when ambient temperature approaches or exceeds T_core, at which point radiative and convective loss to the environment become ineffective or even reverse direction. This mechanism works purely on the physics of the liquid-to-vapour phase change and is the animal's last line of defence against heat stress, at the cost of both water and (for panting) some additional metabolic heat from the respiratory muscle work itself.