Dehydration

Fluid Intake and Fluid Loss

Fluid intake occurs principally during drinking (about 60%) and eating (about 30%); 10% of the fluid intake results from the metabolic processes within the body.

Fluid loss occurs through excretion from the kidneys (which in resting conditions amounts to approximately 60% of the total fluid loss), excretion from the large intestine (approximately 5%), evaporation of water from the respiratory tract, and sweat secretion (about 35%).

Sweat components are principally water and electrolytes; these help in regulating nerve and muscle function and in maintaining acid-base balance and fluid balance.

Fluid loss rates can vary greatly depending on many factors. These include environmental factors (e.g., radiant heat from surfaces such as black asphalt), climatic conditions (e.g., temperature, humidity, solar load, and wind velocity), and other factors concerning the specific characteristics of each individual (e.g., gender, age, health status, training status, psychological features, clothing and equipment, intensity of physical activity, degree of heat acclimatization, and exercise-induced metabolic body heat production). Moreover, all these factors may interact synergistically, stressing the physiological systems even more. However, even in cool conditions, sweat loss can be considerable: The highest sweat rate reported in the literature, [Page 351]measured in an athlete during his preparation for a marathon, is 3.7 liters/hour.

In fact, excessive fluid loss can be fatal, because water is essential for human life: It constitutes nearly 60% of body weight and about 72% of lean body mass and forms the basis for all body fluids, among them blood. As water is the principal constituent of the body, even a 9% to 12% fluid loss can have serious consequences.

The body fluids have vital functions: They contribute in the transportation and absorption of nutrients, allow muscle contraction, and help in eliminating waste. For this reason, it is essential to maintain the correct parameters of these fluids, especially blood parameters, such as blood volume and blood pressure. In effect, the body tries to defend blood volume as much as possible in hypohydration situations: Water passes from inside the cells to the bloodstream. Through this fluid exchange, dehydration causes a redistribution of body water, which largely derives from depletion of the water content of muscles and skin. Thus, depletion of body levels of fluid can significantly stress the body, impairing physical performance.

Consequently, dehydration should be limited and, better still, prevented. To prevent dangerous dehydration, the brain stimulates the thirst center, inducing the hypohydrating individual to drink more fluids. Yet if fluid intake cannot balance fluid loss, dehydration becomes more severe. The body tissues start to dry out, to such an extent that cells may shrivel and malfunction.

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