Thermoregulation: Staying Near 37 °C from January to July

Tuque weather and heat warnings

Canadians live with some of the biggest temperature swings on Earth. Winnipeg can see −35 °C in January and +35 °C in July. Through all of that, your core temperature (the temperature of your brain, heart and other internal organs) stays close to 37 °C.

Why does that matter so much? Remember from the Biochemistry unit that enzymes have an optimum temperature. Human enzymes work best near 37 °C. Too cold, and reactions slow down so much that the heart and brain cannot keep up. Too hot, and proteins begin to denature, losing their shape. Controlling body temperature, called thermoregulation, is one of the most important homeostatic loops you have.

Heat in, heat out

Your body is always making heat as a by-product of metabolism. As you learned in Metabolic Processes, cellular respiration is not 100% efficient; much of the energy in glucose ends up as heat. Muscles at work make a lot of extra heat.

Your body exchanges heat with the surroundings in four ways:

The hypothalamus: your thermostat

Thermoreceptors in the skin detect surface temperature. Others, inside the hypothalamus itself, monitor the temperature of the blood flowing through the brain. The hypothalamus compares this information with its set point (about 37 °C) and sends instructions along nerves and through hormones.

When you are too warm:

• Vasodilation: smooth muscle in the walls of arterioles in the skin relaxes. The vessels widen, more warm blood flows near the surface, and more heat radiates away. (That's why faces look flushed after a run.)
• Sweating: sweat glands release water onto the skin. As it evaporates, it absorbs heat.
• Behaviour: you slow down, find shade, take off your hoodie.

When you are too cold:

• Vasoconstriction: skin arterioles narrow, so less blood reaches the surface and heat is kept in the core. Fingers and toes get pale and cold first.
• Shivering: skeletal muscles contract rapidly and involuntarily. The contractions do no useful work, so almost all the energy becomes heat.
• Goosebumps: tiny muscles pull body hairs upright. In furry mammals this traps a thicker layer of insulating air; in humans the effect is small, a leftover from our ancestors.
• Non-shivering thermogenesis: brown adipose tissue (brown fat), found in babies and in smaller amounts in many adults, has mitochondria that release energy as heat instead of making ATP. Over weeks of cold, the thyroid gland also raises metabolic rate.
• Behaviour: you put on a tuque, curl up, or go inside.

When the set point changes: fever

During an infection, white blood cells release chemical signals called pyrogens. In the hypothalamus, these lead to the production of prostaglandins, which raise the set point, for example from 37 °C to 39 °C.

The thermostat has not broken; it has been reset. Now a normal 37 °C feels "too cold" to the hypothalamus, so you get the warming responses: you feel chilly, your skin is pale, and you shiver (the "chills"). Body temperature climbs to the new set point. When the infection is controlled, the set point drops back to 37 °C. Now 39 °C is "too hot", so you get cooling responses: you feel hot, your skin flushes and you sweat. This is when people say the fever has "broken".

A moderate fever may help the immune system, because many bacteria and viruses multiply less well at higher temperatures. Common fever-reducing medicines such as acetaminophen and ibuprofen lower the set point by reducing prostaglandin production.

When thermoregulation is overwhelmed

Cold: In very cold or wet, windy conditions, heat can be lost faster than it is made. Frostbite is freezing of tissue, usually in fingers, toes, ears and noses, where vasoconstriction has cut blood flow. Hypothermia is a core temperature below about 35 °C. Early signs are intense shivering, clumsy hands and confusion; in severe hypothermia, shivering stops. Get the person somewhere warm, remove wet clothing, warm the core first with blankets, and call for medical help.

Heat: Heat exhaustion causes heavy sweating, dizziness, headache and nausea; rest somewhere cool and drink water. Heat stroke happens when cooling fails and core temperature rises above about 40 °C, with confusion or collapse. It is a medical emergency: call 911 and cool the person right away. Older adults, babies, outdoor workers and people with some health conditions are at higher risk during heat warnings.

The good news: both are very preventable. Dress in layers in winter, cover skin when wind chill warnings are issued, and in summer drink water, take breaks in the shade and check on older neighbours.

Endotherms, ectotherms and Canadian survival strategies

Endotherms (birds and mammals) make most of their heat from metabolism and keep a fairly constant temperature. This costs a lot of food energy, but it lets them stay active in the cold. Ectotherms (most fish, amphibians, reptiles and invertebrates) get their heat from the environment and use behaviour to regulate: a garter snake basks on a warm rock in the morning and moves into shade at noon.

Some adaptations from Canadian wildlife and people:

• Countercurrent heat exchange: in the legs of ducks and geese, warm arteries going down run right beside cold veins coming up. Heat passes from the artery to the vein, so it returns to the body instead of being lost through the feet standing on ice.
• Insulation: seals and whales have thick blubber. Caribou hairs are hollow and trap air. Many Inuit seamstresses today still make caribou-skin and sealskin clothing that uses these same insulating properties.
• Hibernation: a groundhog lowers its set point and body temperature dramatically through the winter, saving energy when food is scarce.
• Freeze tolerance: wood frogs in Ontario survive winter with much of their body water frozen; their liver floods their cells with glucose, which acts like antifreeze.