Circulation System

In hot weather, the heart hsa to pump harder to supply blood to the skin which acts as a cooling system.  With perspiration and water loss, the volume of blood in circulation declines and the heart works even harder.

Brain

Prolonged exertion can lead to a change in the proportion of chemical messengers in the brain – an increase in serotonin versus a decrease in dopamine.  This is believed to lead to sensations of tiredness or lethargy.

Liver

This is one of the body's major storage sites for glucose and glycogen, which helps to store energy.  These resources are depleted in 60 to 90 minutes into an endurance event and must usually be refuelled with isotonic drinks.  If the muscle glycogen store is depleted, blood sugar levels drop.

Skin

It is susceptible ti sunburn in hot weather. Blisters, especially on the feet, can spoil a runner's race.  When ther skin is constantly wet, chafing occurs at places such as the nipples and underarms.  The repeated stubbing of toenails within a runner's shoe can also lead to bleeding below the nails.

Kidneys

They have to work extra hard to conserve water and salt as these are lost in perspiration.  In extreme cases, when the muscles break down, molecules such as myoglobin are released into circulation, which may impair the kidney's functioning.  This may even precipitate acute renal failure.

In ultra-endurance evetns, the body loses a great deal of salt.  If the runner replenishes this with water only, rather than isotonic drinks, his kidneys may lose their ability to generate the salt content in the body.

Too litlle salt in circulation may also lead to swelling of the brain, which may result in loss of consciousness or even death.

Musculoskeletal System

The muscle, especially in the leg area, start to lose their elasticity and tighten up.  This can lead to msucle cramps and muscle strain.  Prolonged running also causes microscropic tears in the muscle which may result in sreness after the race.  As the muscles get tired, they don't function well as shock absorbers and transfer the repeated impact when running to the tendons, bones and cartilages.

Tendons may tear or become inflamed; bones may suffer from stress injuries such as stress fractures; cartilages may soften and fray; joints may begin to swell.

[tags]Marathon[/tags]