We all get thirsty at times, especially during intense physical exertion. This is a normal reaction of the body when it is getting low on water. Dehydration occurs when the body loses more water than it is takes in, usually through sweating and open mouth breathing. Thirst is an annoyance; dehydration is a serious problem that may lead to leg cramps, upset stomach, severe headaches, and death. Dehydration is not a single event; it is a combination of events over time. Therefore, proper intake of fluids is an integral part of the training process. Training without replacing fluid and electrolyte losses does not condition, make you tougher, or improve your endurance, but it may kill you.
During even light exercise, serum (the liquid part of blood) volume is slightly reduced. During intense exercise, serum volume is reduced even more. During normal conditions, blood flow to muscles is relatively small. However, exercise dramatically increases blood flow to the muscles, which decreases volume flow to the rest of the body. The body compensates by reducing flow to the organs least necessary during exercise, e.g. the indigestion system. As the level or exercise increases, blood flow through the heart is modified since it depends on the return of blood from the body to supply what is needed for the next beat. As the exercise level increases, we sweat, which further reduces serum volume, which results in blood that is more viscous. The thicker blood reduces the efficiency of the heart.
The body has now reassigned as much of its blood supply as possible. If you suddenly do something that further reduces the volume of blood flow to the heart, then the heart cannot maintain enough blood pressure to get blood to its primary user, the brain, so you pass out.
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