Heat and the Body – Heat Loss
Your internal organs digest the food you eat. This process creates heat. Without systems to shed some of this heat, the body temperature would rise too high. The systems are:
- Breathing. You need to breathe in oxygen to aid the process of burning calories. At the same time, the hot air in the lungs is replaced with cooler air from outside, thus cooling your body. What happens on a hot day when the outside air is warmer than the air in the lungs? There is another process happening at the same time – evaporation.
The moisture in your lungs converts to water vapour through evaporation, but this can only take place if the air is not already saturated with vapour. Breathing, therefore, will do very little to cool down your body if you breathe hot and humid air. Under normal circumstances we breathe through the nose and if this isn’t enough to cool the body, we breathe or pant through the mouth.
- Circulation. The blood carries a large amount of heat to all parts of the body, including the skin. On the surface of the skin three additional forms of heat exchange take place: radiation, convection and conduction.
During hot days or while exercising, the normal blood circulation is not sufficient. Our thermostat, the hypothalamus, instructs the heart to work harder and beat faster. At the same time, the blood vessels widen to compensate for the increased blood flow. You will notice a reddening of your skin.
The hot skin radiates some of the excess energy into the surroundings. The hot air near your skin rises, thus carrying energy away from your body, a process called convection. Dip into the pool, and you lose even more through conduction.
- Sweating. Below 20°C outside temperature, the body loses most of its heat through radiation and conduction. Above this temperature, evaporation through sweating begins to aid the heat transfer. Two to five million sweat glands in your skin will open on command and shed great quantities of water. A person exercising and sweating profusely dissipates three-quarters of their body heat and can lose up to two litres of water per hour. Top athletes become miniature waterfalls that can shed up to three litres per hour.
The rate of heat loss through sweating is very much determined by the surrounding air’s humidity. If the air is already saturated, or contains a large amount of water vapour, sweat won’t or will only slowly evaporate. Non-porous clothing will further hinder evaporation. Sweat contains essential body salts (electrolytes), and excess perspiration can severely disrupt their balance in the body, and lead to certain medical conditions.
Heat disorders develop when the three cooling systems are insufficient to keep your body temperature at its normal level. A body temperature rise above 41°C causes weakness or exhaustion. A further rise to levels above 42.5°C may result in serious health problems or even death.
You step from the aircraft on to the tarmac where the heat and humidity hits you like a sledgehammer. If you had a knife you could carve the air. The legs are heavy like lead, almost as heavy as the sweat-soaked clothes. Why did you think of fleeing the bitter winter and accepting a job in this tropical country? It’s really no problem eventually; you acclimatize.
The process stresses your body, though. For the first few days you probably feel exhausted and your work performance suffers. A healthy person requires up to 14 days to fully acclimatize. In the first week, the cardiovascular system adapts slowly: your blood volume increases by up to 20% and your heart rate decreases by between 15% to 25%. This is a temporary adjustment and should normalize by the end of two weeks.
Long-term adaptations include an improved ability to lose heat more efficiently. The blood flow to the skin increases, aiding heat loss due to radiation, convection and conduction. Dormant sweat glands become active and increase the sweat rate. The sweat, however, is less salty because the kidneys learn to prevent excessive loss of electrolytes. Your metabolism also changes. It limits the process of calorie burning, thus reducing heat generation.
Luckily, you can use your brain to assist the body – and choose to consume cooling food and drink during long exposure to heat. A low-calorie diet means fewer calories to burn. Of course, wearing appropriate clothing is another major factor – you won’t need the fur coat in Hawaii.
Unfortunately, you lose the benefits of acclimatization very quickly when you return to the previous climatic conditions. All the hard work vanishes within a few days.
Heat Stress Risk Factors
Excessive heat affects everybody. Certain age groups, conditions and activities, however, will pose a higher risk of developing heat-related illnesses. The following list provides some examples, but is not exhaustive:
- Children. In infants, the hypothalamus, and its heat regulation mechanism, is underdeveloped, while their sweat glands are small and operate less efficiently. Less blood flows to the outer skin and children acclimatize slower. Their metabolism creates more heat as compared to an adult performing the same amount of activity.
- The aged. The hypothalamus degenerates with age, so that the heat regulation mechanism responds slower to body signals. Sweating and blood circulation becomes less efficient. The elderly end up with a diminished sense of thirst and, as a consequence, may not take in enough fluid. In addition, the elderly have more chronic conditions and are more often forced to remain in bed or at home during hot conditions.
- Obesity. Fatty tissue blocks sweat glands and impairs blood circulation in the skin. Also, a layer of body fat acts as an insulator and traps heat.
- Excessive physical activity. An athlete or outdoor worker can lose up to three litres of sweat per hour. Unfortunately, the person can’t replace this amount immediately, as the body only absorbs approximately one litre of imbibed liquid per hour. The net loss leads to dehydration.
- Medical conditions. Underlying health problems will increase the risk of heat-related illness. For example
High blood pressure (hypertension)
Damaged or diseased skin
Diseased heart or blood vessels (cardiovascular disease)
Excess of thyroid hormones, increased metabolic rate (hyperthyroidism).
- Medication and drugs can increase the body’s stress level during heat, which is undesirable if there’s a risk of dehydration. Antihistamines and some tranquillizers can interfere with sweating. Beverages such as tea, coffee and alcohol are diuretic, meaning they stimulate the body to loose fluid. Recent findings show that the combination of heat and the use of recreational drugs can also lead to fatalities.
- Other factors. The lack of air conditioning and unsuitable housing or work environments will exacerbate heat wave effects. Some workers or athletes have to wear heavy protective clothing despite unfavorable environmental conditions.