How our bodies keep warm
As you are sitting quietly reading this, your body is busy keeping you warm. Although you can warm yourself in the sunshine or by a radiator or fire, almost all the heat in your body comes from your food. Inside your food, energy is stored. When you eat and digest your food, all parts of the body use up the energy in it to stay alive. As they do this, some of the energy changes to heat. About half the heat energy in your body is produced by the processes that keep you alive. Just over a tenth of the heat is produced by one organ in the body. This is called the liver. It produces a lot of heat because it does many things. Here are some examples of the things the liver does. It helps digest food, store some of the food and helps to clean the blood. All these activities produce heat. The rest of the heat in your body is produced by the muscles as they work.

Even though different parts of your body are producing heat, all of your body is kept at the same temperature. This is due to the blood. It flows all over the body and picks up heat from the hot parts and delivers it to the cool parts. As the blood flows through the brain it passes through a part which acts like a thermostat on a central heating system. If it detects that the blood is becoming cool, it makes the body produce more heat. If it detects the blood is becoming too hot, it makes the body lose heat.

The skin is a major part of the body that controls body heat. It is packed with tiny tubes through which the blood passes. If the blood is too cool, the thermostat in the brain shuts down the tubes a little so that less blood can flow through them. This means that less heat reaches the surface of the skin and escapes. More heat is kept inside the body to warm it up. If the blood is too hot, the thermostat in the brain opens the tubes in the skin wider so more blood can flow through them. This increases the amount of heat reaching the skin and escaping. By losing heat in this way the body cools down.

All these changes take place automatically. If a sudden draught blows through your room more heat will be lost from your skin. The thermostat in your brain responds straight away by shutting down the tubes in your skin to help you save heat. If you have a hot drink, your blood will warm up and your thermostat will open up the tubes in your skin again to lose the extra heat. These tiny changes keep your body at a steady 37°C - the temperature at which all parts of your body work best.

Why do doctors take your temperature when you fall ill?
It helps them decide what is wrong with you. Some illnesses make the body get too hot. The body is then said to have a fever. The doctor can tell if you have a fever by taking your temperature. If you have a fever, the doctor can tell how bad it is by measuring your body temperature.

Have doctors always used thermometers?
No. They used to decide what was wrong with you by feeling the warmth in your skin. Galileo invented the thermometer in 1593 but it was not suitable for taking the temperature of the body. As time passed many scientists worked on making better thermometers. In the 1850s it was discovered that the study of fevers was very useful in treating patients. At this time thermometers were still too large and too long to be used easily to take the body's temperature. A thermometer had to be placed in the patient's mouth for twenty minutes before it gave a steady reading. This meant that the patient had to suffer a long period of discomfort while the temperature was being taken. In 1867 Thomas Allbutt made temperature-taking a much easier process when he invented the clinical thermometer. This was much smaller than other thermometers and gave a steady reading in five minutes.

Why is a clinical thermometer shaken after it has been used?
If you are taking the temperature of something, the bulb of the thermometer must stay in contact with it as you make your reading. If you remove the thermometer to read it you just get the temperature of the air around the bulb. The clinical thermometer is designed so that you can take it out of the patient's mouth and still read it correctly. This design was made possible because the scale on the thermometer is only short. It runs from 35°C to 43°C. If the doctor had to read this scale while the thermometer was still in the patient's mouth it would be uncomfortable for both the doctor and the patient as their heads would have to be very close. In the tube of the clinical thermometer is a narrow bend through which the mercury must pass as it expands out of the bulb. When the thermometer is taken out of the patient's mouth, the bend stops the mercury flowing back. It stays in place and shows the temperature of the patient. The bend allows the doctor to read the thermometer more comfortably - away from the patient's face. The thermometer is shaken to make all the mercury go back round the bend so it is ready for use again.

Why do you sweat when you are hot?
You sweat to lose heat. Sweat is mostly water. It forms a thin layer over your skin. This layer of liquid changes into a vapour by a process called evaporation. It needs energy to make this change and takes energy, in the form of heat, from the blood. As the sweat uses up heat to evaporate, the skin and the body become cooler.

Why do some animals pant when they are hot?
Panting lets water on the tongue and the lining of the mouth evaporate. This uses up heat in the blood and makes it cooler. Evaporation also takes place inside the lungs during panting. This further cools the blood and brings down the body temperature to normal.

How high can the body temperature of a human safely go?
The temperature can rise to 43°C. Above this temperature the body suffers from heatstroke and dies.

How cold can the body temperature of a human safely go?
When the body temperature falls to 35°C the person's sight and speech will be affected. By the time the temperature has fallen to 32°C the person will be unconscious and will die if not warmed up again.

When does a person suffer from hypothermia?
When their body temperature falls below 35°C.

What can cause hypothermia?
Hypothermia can be caused in several ways. It can be caused by staying in cold water too long or by being in wet clothes in a cold, windy place such as a mountaintop. Hypothermia can also occur if you are standing in cold surroundings for a long time. The people most at risk from hypothermia are babies and old people who sleep in unheated rooms in the wintertime.

How can you help someone recover from hypothermia?
You should wrap them in a warm blanket and let them take sips of a warm drink.