The breakdown of proteins and nucleic acids releases nitrogen-containing wastes. The body must eliminate these compounds before they build up. Filtering wastes from the bloodstream is the job of the excretory system. Your body regulates excretion in response to changes in its environment.
The excretory system traps wastes like urea and excess salts and expels them from the body. In the process, it helps preserve a vital balance in the level of salts and fluids in the blood. A portion of your brain called the hypothalamus regulates excretion by producing anti-diuretic hormone (ADH), which acts to reduce the amount of water removed from the blood by the kidneys and thereby to reduce the rate of excretion.
When you exercise, you may begin to sweat as your body tries to keep down its temperature. Sweating removes water and salts from your body together with a small amount of urea. As the level of sodium in your bloodstream drops, ADH secretion also falls, and your kidneys produce urine that is more dilute.
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As you continue to exercise, your body loses more water. After a certain point, the hypothalamus begins to boost the release of ADH from the pituitary gland to conserve water as much as possible. As ADH levels rise, the kidneys produce more concentrated urine, thus causing the level of sodium in the bloodstream to further drop.
In extreme cases, an athlete may become dehydrated, or the decreased level of sodium in the blood can cause a condition called hyponatremia. Endurance athletes competing in grueling events like marathons must ensure they consume enough fluid to replace what they lose during the race.