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The body is pushed hard by running long distances, as the death of four people in the Great North Run has shown. So what happens to it during a race?
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Any exercise is a challenge to the body, but long-distance running is bigger than most.
The outward effects are obvious - breathlessness, redness, sweating - but what goes on inside?
When a person runs, the body makes sure the muscles cope by keeping them supplied with oxygen to help burn fuel to make the energy they need. One of the key physiological attributes of a good distance runner is the body's capacity for oxygen consumption - known as VO2max.
The runner starts to breathe quickly and more deeply and their heart beats faster to provide more and more oxygen for the muscles. The arterioles - the small terminal branches of an artery - also widen to stop their blood pressure getting too high.
Blood that would usually go to organs like the gut and liver is diverted to the muscles, to make the most efficient use of the runner's blood supply.
Contracting muscles also squeeze the veins, forcing blood back to the heart more quickly, which stretches and then contracts more strongly to pump even more blood.
As the muscles work, they generate heat which warms the blood. It is then diverted closer to the skin to allow the heat to escape and avoid overheating. This makes people go red and start to sweat, which helps keep them cool. Drinking fluids also helps lower body temperature.
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But if a run is done in humid weather the sweat may not evaporate, reducing the efficiency of the system and causing heat stroke. It can cause a person to collapse, or, in extreme case, lose consciousness or even die.
It's been suggested that unseasonably warm weather contributed to the deaths of the four men who died in Sunday's Great North Run. However, post-mortem examinations on two of them found they died of natural causes. Examinations of the other two will take place on Tuesday afternoon.
Levels of glycogen - energy stored in muscles - start to run low and the body begins to consume its own protein and fat for the energy needed to carry on.
The best way to restore glycogen levels is to take on water and energy drinks. This, however, only has a partial effect as the body does not like to absorb fluids during exercise.
Stomach cramps can kick in as oxygen-rich blood is diverted from the digestive system to the muscles. At this point the runner faces "hitting the wall", when the body starts running on empty.
Glycogen levels have been exhausted, so blood sugar levels are very low. As a result a runner can feel faint and woozy. Lactic acid levels may be high and the levels of important salts in the body, known as electrolytes, may be very low. Pain and muscular cramps could occur.
The constant pounding of running also causes tiny micro-damage and undetectable tears in muscle tissue, making the joints - especially the knees - sore. As the body tries to prevent further damage and repair what has been done, swelling can occur.
Refuel with carbs and protein
Keep warm and keep moving
Take vitamins to boost immune system
The runner's body is reaching exhaustion point and at this stage they are most likely to suffer a heart attack. The heart rate has soared, while blood pressure and core body temperature are also dangerously high.
One of the more surprising effects of long-distance running is the temporary loss of height. Evidence suggests marathon runners can finish a race 2cm shorter than they were when they began because of the impact of their feet hitting the ground.
After a race, blood pressure plummets and a runner can feel faint. The heart rate and core temperature should go back to normal fairly quickly. On a cold day a runner should put on something warm or they could suffer from hypothermia, when the body cools down too much.
Nevertheless, running is widely credited with numerous health benefits, from helping weight loss, fighting disease and ageing and relieving depression.
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