Hypothermia

This is the body's reaction to loss of heat, when its own metabolism can not keep the body temperature up. The body tries to keep the temperature of vital organs up by shutting down others. Blood flow to the brain eventually ceases causing amnesia. Death is not far away. 

In stage 1, when the body temperature has dropped by 1-2°C, the blood flow to extremities reduces to stop further heat loss. fingers and toes become numb. Goose bumps raise the hairs on the body to trap an insulating layer of air. The body starts shivering to generate energy from the muscles to warm it up.

In stage 2, when the body has cooled by 2-4°C, shivering becomes violent. Surface blood vessels close to their minimum size, and lips, ears, fingers and toes may appear blue as oxygen is no longer supplied. In a normal body most heat loss is through the head, so the body reacts to cold by starting to shut down blood flow to the brain & the victim becomes mildly confused.

In stage 3, when the body has cooled by more than 5°C, there is so little blood to the brain that amnesia starts to appear, and control of the lungs is reduced which leads to slower shallow breathing. Many cellular metabolic processes can not function, both because of the cold and because of the lack of oxygen. Even the heart is vulnerable to arrhythmias, especially fibrillation, which can be fatal. Eventually major organs fail, and death occurs, although slowly because of the reduced metabolic rate.

Summary
Stage 1 - the body tries to recover, you can warm the victim externally
Stage 2 - important body functions start to shut down, keep the victim still, insulated and horizontal while warming externally
Stage 3 - vital body functions start to shut down, professional treatment is essential to prevent heart failure.

Wind and water increase heat loss to over 30 times what it is when you are dry, so you will lose over 1000 Calories/hour. Very few people can work that hard when they are dry, let alone when they are cold and wet, so every competitor will start cooling down. At Tough Guy we have seen that people who are not fit simply can not work hard enough to keep their bodies warm in the cold and wet. Also they are out there for longer, giving their bodies longer to cool down.

So train hard and do not be tempted to slow down on the day.

Do not drink alcohol for at least 24 hours before competing, because it dilates the blood vessels in your skin where the heat gets out. 

When Tough Guy marshals ask how you are, they are trying to make sure you have not reached stage 2 hypothermia, so give them a loud clear answer or you risk being pulled off the course.