When you exercise vigorously, your body faces challenges it doesn’t face at rest. Contracting muscles generate heat that must be dissipated and exercising muscles require more blood to supply oxygen and remove metabolites. To meet these demands, your body changes the way it circulates blood and also secretes fluid from the blood as sweat to cool the skin. A consequence of these actions is that more water is lost in sweat than in urine.
During vigorous exercise, contracting muscles produce heat. This heat must be removed from the body because it’s essential to maintain internal temperature within a narrow range. Your cardiovascular system is responsible for removing metabolic heat by changing the way it circulates blood. Heat from active muscles is transferred to the blood and blood is redirected to the skin where heat can be dissipated to ambient air by radiation and convection. Heat is also removed by evaporation, which requires the excretion of sweat. Some of the water for sweat comes from the fluid portion of your blood. Athletes performing high-intensity exercise in the heat commonly have sweat rates of 1.0 to 2.5 liters per hour.
Osmolarity is a measure of the number of particles in a solution. Much like internal temperature, it is important for your body to maintain the osmolarity of blood within a narrow range. When water is lost from your blood due to sweating, osmolarity increases, which means that regulatory mechanisms must be available to restore proper balance. This can involve either increasing the fluid being absorbed in your GI tract or decreasing the amount being expelled by your kidneys.
In addition to dissipating heat, another challenge your cardiovascular system faces during vigorous exercise is distributing blood to areas of need. At rest, only 21 percent of blood being circulated is directed to muscles, whereas during vigorous exercise, muscles receive as much as 88 percent. Opening blood vessels in contracting muscles to allow for this dramatic increase would result in a catastrophic fall in central pressure if blood was not simultaneously shunted from other organs. An example is blood flow to the GI tract, which is decreased to as little as 1 percent during vigorous exercise. A consequence of this redistribution is that as exercise intensity is increased, the ability to absorb fluids in the GI tract is compromised.
During vigorous exercise, sweating reduces blood fluid volume and redistributed blood compromises fluid absorption. This means that it is essential for your body to conserve water. There are a number of hormones that regulate water and solute concentrations in your blood. An important one is ADH, which is released into your bloodstream when osmolarity increases above a certain level. ADH causes your kidneys to retain water by reducing the amount they excrete in urine. During vigorous exercise, circulating ADH levels can increase up to 10-fold and urine output is reduced by 50 percent to 70 percent.