Thermoregulation in endotherms and ectotherms

Why core temperature is regulated

Metabolic reactions are enzyme-catalysed, and temperature acts on them in two ways, through the optimum and denaturation. Below the optimum, molecules have less kinetic energy, collisions between substrate and active site are less frequent and less energetic, and the rate falls: roughly halving for every 10 °C. Above the optimum, vibration breaks the hydrogen and ionic bonds holding the tertiary structure, the active site stops being complementary to its substrate, and the enzyme denatures.

A mammal is therefore limited in both directions. Cold does not denature its enzymes but slows every reaction they catalyse, including those that release heat, so hypothermia worsens once established. High temperature acts faster: a core temperature of 40 °C causes confusion, and around 42 °C denaturation becomes general and is not reversed by cooling. Human core temperature is regulated within a narrow range around 37 °C, although it varies with circadian rhythm, exercise, illness and measurement site.

One term in that paragraph needs care. Core temperature is the temperature of the blood and of the organs in the trunk and head, and it is the regulated variable. The temperature of your fingers, ears and skin is not regulated in the same way, and it is allowed to fall considerably, because reducing blood flow to the surface is one of the mechanisms that conserves core heat. Cold extremities are therefore a thermoregulatory response rather than a failure of one.

Thermoregulation

The control of body temperature within narrow limits, despite changes in the temperature of the surroundings.

Endotherm

An animal that maintains its body temperature chiefly by heat released from its own metabolism, and controls it physiologically.

Ectotherm

An animal whose body temperature depends mainly on heat gained from its surroundings, and which controls it chiefly by behaviour.

Core temperature

The temperature of the blood and of the deep organs, as distinct from that of the skin and extremities.

Two sets of thermoreceptors and one coordinating centre

The control loop has the same four parts as any other homeostatic loop. Two sets of receptors, in different places, detect different quantities.

Peripheral thermoreceptors are nerve endings in the skin, and separate populations of them respond to cold and to warmth. What they monitor is the temperature of the surface, which is to say the temperature of the environment, and they are particularly sensitive to a change. Step outside on a cold morning and they fire before the core has cooled. Correction therefore begins in response to a disturbance that has not yet reached the variable being regulated.

Central thermoreceptors lie in the hypothalamus itself and monitor the temperature of the blood flowing through it. These are the receptors that matter for the variable actually being controlled, because blood temperature is core temperature. The hypothalamus compares what they report with its set point, and acts on any difference.

The hypothalamus is the coordinator, and it is usually described as two centres: a heat loss centre that is stimulated when the blood is too warm, and a heat gain centre stimulated when it is too cold. Each sends impulses out along neurones of the autonomic nervous system to the effectors, so most of what follows happens without any decision on your part. Some of it is hormonal as well, and slower: thyroxine from the thyroid raises the basal metabolic rate over weeks of cold weather, which is the physiological basis of acclimatisation.

The loop stated in the standard vocabulary runs as follows. Receptors detect a deviation in core temperature, or a change at the skin that predicts one. The hypothalamus coordinates. Effectors in the skin, the skeletal muscles and the liver bring about a response. The response moves the temperature in the direction that removes the deviation, so the feedback is negative.

The effectors of thermoregulation

Most of the effectors are in the skin. Capillaries do not move within the dermis: they are tubes one cell thick, held in place by the surrounding tissue, and no mechanism exists that could raise or lower them.

Describe the drawing

Two panels of skin in section, drawn identically except for what the effectors are doing. On the cold side a hair stands upright on a contracted erector muscle, the sweat gland is dry, the arteriole running up to the surface is drawn thin and the shunt vessel below it is drawn thick. On the warm side the hair lies flat, droplets of sweat sit on the surface, the arteriole is thick and the shunt vessel thin, while the capillary loop keeps the same width in both. Four lines under each panel name the responses.

What actually changes is the width of two sets of vessels. The arterioles supply the capillary networks near the surface. The shunt vessels run beneath them, connecting the arterioles directly to the venules and bypassing the surface network altogether. In the cold, the muscle in the walls of the surface arterioles contracts and they narrow, which is vasoconstriction, while the shunt vessels dilate. Little blood reaches the surface, so little heat is lost by radiation and convection, and the skin goes pale. When the body is too warm, the arterioles dilate and the shunt vessels constrict. Far more blood flows through the capillaries close to the surface and heat is lost from it; an overheated person looks flushed.

Sweating is the effector that removes heat fastest. Sweat glands in the dermis secrete a dilute salt solution onto the skin, and evaporating it takes the latent heat of vaporisation from the surface it leaves. The dependence on evaporation makes humid heat dangerous: in saturated air the sweat runs off instead of evaporating, and sweat that drips cools nothing while still costing water and ions.

Two effectors work on heat production, not heat loss. Shivering is rapid involuntary contraction of skeletal muscle, doing no useful work. Nearly all of the energy released by the extra respiration appears as heat, and it can raise heat production several times over. Non-shivering thermogenesis is the slower route. Adrenaline and thyroxine raise the metabolic rate of the liver and other tissues, and in a human infant brown adipose tissue respires to release heat in place of making ATP. The liver is included because it is metabolically among the most active organs and one of the warmest.

The hair erector muscles require a qualification. Contracting them pulls each hair upright, trapping a thicker layer of still air next to the skin, and air is a poor conductor, so an animal with a dense coat gains a great deal of insulation this way. In humans the effect is negligible, because body hair is too sparse to trap a significant layer of air. State the trapped air as the insulating mechanism.

Behavioural regulation in an ectotherm

A lizard has enzymes with similar optima, but it lacks a metabolic rate high enough to heat itself. A reptile at rest produces a small fraction of the heat a mammal of the same mass produces, and loses it as fast as it is made. Its body temperature therefore depends largely on its surroundings.

Describe the drawing

A graph with air temperature from zero to forty degrees along the bottom and body temperature from zero to forty-five up the side. A nearly horizontal cyan line at about thirty-seven degrees is the endotherm, whose core barely moves as the air changes. A steep amber line rising from two degrees to forty is the ectotherm, whose body temperature tracks the air and crosses the endotherm's line once, at the air temperature that happens to suit both.

Depending on the surroundings does not mean being unable to regulate. An ectotherm regulates its temperature actively, by behaviour. It basks in the sun when it is cold, and moves into shade or down a burrow when it is hot. It presses its underside against a rock the sun has warmed, gaining heat by conduction, or lifts itself on straightened legs clear of ground that has become too hot. It shifts its activity to the time of day that suits it, so desert lizards are active in the morning and evening and desert mammals are more often nocturnal.

Posture is a control of its own. A lizard warming up turns its flank towards the sun and flattens its body against the ground, presenting the largest possible area to the radiation. An overheating one turns to face the sun so that only its head is exposed, and stands tall to catch the breeze. Many species darken when cold, since a darker surface absorbs more radiation, and pale as they warm. A lizard gaping in the heat is evaporating water from its mouth for the same reason a dog pants.

The steep line in the figure shows an ectotherm that cannot use behaviour. With it, over a day an active lizard's temperature is held in a band much narrower than the air's, because it spends the morning in the sun and the middle of the day in the shade. What it cannot do is hold that band on a cold, overcast day, and on such a day it is slow, cannot digest a meal properly and cannot escape a predator that is warm.

The two strategies compared:

Neither strategy is superior in all conditions. An endotherm maintains constant performance at a high energy cost. A mammal spends most of the energy it assimilates staying warm, and must find that food every day or every few days. An ectotherm is cheap to run. It can survive months without eating and can colonise habitats where food is too scarce to support a mammal of the same size. It also converts a far greater share of the food it eats into new tissue, so ectotherms dominate the animal biomass of many warm ecosystems. Its limitation is that on a cold, overcast day, and at night, it cannot sustain full activity.