Temperature Regulation: Keeping Body Temperature in Range

Thermoregulation means controlling body temperature. In humans, the variable is core body temperature, not the temperature of the room or the skin surface by itself.

Human core temperature sits near 37.0 °C and is held within roughly 36.5 °C to 37.5 °C during normal activity. That narrow band is not arbitrary. Almost every reaction in a cell is catalysed by an enzyme, and an enzyme works because its active site holds a precise three-dimensional shape.

Raise the temperature too far and the weak bonds holding that shape begin to break. The enzyme denatures, the active site no longer fits its substrate, and the reaction rate falls even though plenty of substrate is still available. Above about 40 °C in humans this becomes dangerous, which is why heat stroke is a medical emergency rather than just discomfort.

Cooling causes a different problem. Enzymes and their substrates move more slowly, so they collide less often and reaction rates drop. Nothing is destroyed, but metabolism slows, and below about 35 °C coordination, judgement and heart rhythm are all affected.

So homeostasis here is not about hitting one perfect number. It is about keeping the variable inside a range where enzyme-controlled reactions continue at a workable rate. That is why the syllabus talks about a tolerance range rather than a set point alone.

When temperature rises, negative feedback activates responses that reduce the rise. The response works in the opposite direction to the original change.

The pathway above names the effectors. This card explains how each one physically moves heat out of the body, which is the difference between naming a response and explaining it.

Sweating, which is evaporative cooling

Sweat glands in the dermis secrete a dilute salt solution onto the skin surface. The cooling comes from evaporation, not from the presence of liquid. Converting water from liquid to vapour requires energy, roughly 2.4 kJ per gram, and that energy is drawn from the skin and from the blood flowing through it. Blood returning to the core is therefore measurably cooler.

This is also why humid heat feels so much worse. When the air is already close to saturated, evaporation slows sharply, so sweat pools on the skin and the mechanism largely fails even though sweat production continues. The effector is still working; the physics is not.

Vasodilation

Smooth muscle in the walls of the arterioles supplying the skin relaxes. The vessels widen, so a greater volume of warm blood flows through capillary beds close to the body surface, where heat transfers to the environment by radiation, conduction and convection. Flushed skin during exercise is vasodilation you can actually see.

Watch the causation in exam answers. Vasodilation does not create cooling by itself. It increases the volume of warm blood delivered to a surface from which heat can be lost.

Reduced heat production

In sustained heat, voluntary activity falls and metabolic rate is turned down slightly. This does not remove heat already in the body, it reduces how much new heat is being added. Treat it as a supporting response rather than a main effector.

Endotherms use metabolic heat to maintain a relatively stable body temperature. Ectotherms rely more on external heat sources, but they still regulate using behaviour and body features.

Both groups regulate temperature. The difference is where the heat comes from, and that single difference explains nearly every adaptation that follows.

Endotherms

An endotherm generates its own heat internally through metabolic activity, largely as a by-product of cellular respiration in the liver, brain, heart and skeletal muscle. Because the heat source is internal and can be adjusted, an endotherm holds a stable core temperature even when the surrounding environment swings widely. Mammals and birds are endotherms.

The cost is energy. A mammal can spend a large share of its daily energy intake simply maintaining body temperature, which is why endotherms must eat far more often than ectotherms of similar mass.

Ectotherms

An ectotherm does not generate meaningful metabolic heat. Its body temperature is determined mainly by the environment, so it rises and falls with the surroundings. Reptiles, amphibians, fish and insects are ectotherms.

The benefit is efficiency. An ectotherm needs a fraction of the food an endotherm of the same mass requires, which is one reason reptiles do so well across arid Australia where food supply is unpredictable.

The distinction that earns marks

Ectotherm does not mean unregulated. A desert lizard can hold its body temperature within a few degrees across a day, which is tighter control than the air around it manages. It achieves that by moving, not by metabolising. State it as a difference in method: endotherms regulate mainly physiologically, ectotherms mainly behaviourally.

Every temperature adaptation you will be asked about falls into one of three categories. The category sets the speed of the response, its energy cost, and whether the animal has to do anything at all.

Two cautions. First, structural adaptations cannot be adjusted quickly, which is why a thick-furred animal caught in an unseasonal heatwave is in genuine trouble. Second, many real adaptations span categories, as the kangaroo example showed. Naming the category is the start of an answer, not the whole answer.

When core temperature falls below the tolerance range, the hypothalamus activates responses that either add heat or conserve it. The logic is identical to cooling. Only the direction reverses.

Shivering

The hypothalamus sends rapid alternating signals to skeletal muscle groups, producing involuntary contractions that do no useful external work. Because muscle contraction is inefficient, most of the chemical energy consumed appears as heat, which is exactly the point. Shivering can lift heat production to several times the resting rate, but it is expensive and cannot be sustained indefinitely.

Vasoconstriction

Arterioles supplying the skin narrow, reducing blood flow through the superficial capillaries. Less warm blood reaches the surface, so less heat is lost to the environment. The core is effectively prioritised over the extremities, which is why fingers, toes and ears turn pale and lose sensation first in the cold.

Piloerection

Arrector pili muscles contract and raise the hairs. In a well-furred mammal this traps a thicker layer of still air against the skin, and still air conducts heat poorly, so insulation improves. In humans the same reflex survives as goosebumps, with almost no insulating benefit because we no longer have the fur to raise.

Increased metabolic rate

Over longer periods thyroxine raises baseline metabolic rate, and adrenaline can drive a faster short-term increase. Both add internal heat by increasing the rate of cellular respiration.

Trace: Start at arteriole diameter in each panel and follow the causal chain to heat transfer. Then identify which response changes heat loss and which response generates heat.

Insulation

Fur, feathers and blubber all work by trapping a layer that conducts heat poorly, either still air or lipid, between the core and the environment. This slows heat transfer in both directions, which is useful in the cold and a liability in the heat. Insulation is always on, so an animal cannot shed it when conditions change quickly.

Countercurrent heat exchange

In the limbs of many endotherms, arteries carrying warm blood outward run alongside veins carrying cool blood back. Heat passes from the artery to the adjacent vein along the whole length of the vessel, so warmth is returned to the core instead of being lost at the extremity. The foot or flipper is allowed to sit close to environmental temperature while the core stays warm. Penguins standing on ice and dolphins in cold water both depend on this arrangement.

Body shape and surface area

Heat exchange happens across a surface, so the ratio of surface area to volume matters. Animals in cold climates tend to be more compact and rounded, with shorter extremities, reducing relative surface area and therefore heat loss. Animals in hot climates tend to have larger extremities that shed heat, which is why desert-adapted species often have conspicuously large ears.