Most of the fluid we lose each day is in the form of urine. Healthy people make at least 0.5 ml of urine per kg body weight per hour. We also lose some fluid each day in stool, sweat, and in our breath. This amounts to about half-liter per day. In hot conditions, when we have a fever, or when we exercise, this increases.
When we exercise, need to replace fluid loss and the sodium, potassium, and other minerals that we lose in sweat. When we become hypovolemic several things happen. First, the heart speeds up; it pumps less volume per beat but it beats more often to maintain cardiac output. Second, the blood levels of catecholamines (epinephrine, norepinephrine, and dopamine) increase, which causes blood vessels in the critical organs, such as heart, brain, liver, and kidneys, to dilate, and the vessels in the less critical areas, such as gut and muscles, to constrict. Changes in the output from our sympathetic nervous system act to reinforce this. The net effect is reduced blood flow to the muscles and gut and more to the brain etc. Thirdly, receptors that measure our blood pressure, sodium level, etc. tell the brain to increase our fluid volume, which triggers the thirst mechanism. Fourthly, the atria of our heart, since they are smaller in diameter, reduce their production of ANP, a peptide hormone that stimulates the kidneys to make urine. Thus, the kidneys reduce their secretion of sodium and urine.