The sweat gland is constantly producing fluid that is reabsorbed before we see it form on the skin. However, there is some water that evaporates in a vapor that we do not see that we call insensible water loss. This is estimated at about 600 ml/day (1 liter=1000 ml and very close to 1 quart).
In an active exercise state, we can sweat several liters/hour! This occurs by the sweat gland not reabsorbing as much of the water and releases it on the skin so that the body has to evaporate the water, thus cooling our body.
This is primarily water, but there are other elements in it. These elements (solutes) are primarily sodium and chloride that make up the white rim on the hat, clothes and body. We also loss calcium, magnesium, urea and lactic acid. The latter two are also components in urine. Yucky!
Understanding how much water is lost and what solutes are contained in sweat will help us determine what we need for hydration before, during and after an exercise event.
Next post is discussing how much water is lost in different activities.