The daily need for fluid for the population is difficult to calculate because of the large discrepancies in its loss due to physical exertion. Many textbooks determine the need for fluids for people with a sedentary lifestyle, in the amount of 2 liters per day. This minimum requirement (2 liters equals 8 glasses per day) can be satisfied by various sources, including milk, soft drinks, fruit juices, sports drinks, water, fruits, soups, etc. Physically active persons need a daily fluid requirement above 2 liters per day. For some athletes and workers it is higher than 10 liters per day. These high fluid requirements are caused by a huge amount of perspiration during physical exertion, which at times can exceed 3 liters per hour in well-trained and acclimated athletes. This rapid loss of fluid is often not accompanied by an equivalent volume of absorbed liquid, resulting in dehydration.
Periodically, the fluid is eliminated from the body by the kidneys (urine), the gastrointestinal tract (feces) and sweat glands, and constantly - from the respiratory tract and through the skin. The total volume of fluid loss per day is determined by the environmental conditions, the size (and surface area) of the individual, the intensity of its exchange and the volume of the released liquid. The insensible loss of water through the skin is relatively constant, and the insensible loss through the respiratory tract depends on the ambient temperature, the relative humidity and the volume of ventilation of the lungs. Passing through the respiratory tract, the inhaled air is moistened, its relative humidity reaches 100% (the vapor pressure is 47 mm Hg). In case of inhalation of warm and humid air, the imperceptible fluid loss is slightly reduced, since the inhaled air already contains water vapor. At sportsmen and workers inconsiderable losses of a liquid through respiratory ways more because of the general increase of intensity of respiration which accompanies physical activity. Air, inhaled during a cold load, contains a relatively small amount of water vapor, so as it passes through the respiratory tract, it heats up and is humidified, an additional loss of moisture occurs. For this reason, it is important to remember that even in cold weather conditions, fluid loss through sweat glands and airways can be quite high.
Losses in the urine of athletes and workers are less than those of people leading a sedentary lifestyle, even less in warm weather, as the body tends to maintain fluid. Motor activity leads to a reduction in urinary output, as the kidneys try to keep water and sodium in order to compensate for losses with sweat.
Even in the absence of a load, the daily fluid loss averages at least 2-3 liters. If athletes train or compete at high temperature, their daily fluid requirements are high. For example, an athlete who trains for 2 hours each day can easily lose an additional 4 liters of fluid, which increases the daily fluid requirement to 6-7 liters. Many people are in an active state for more than 2 hours every day, thereby increasing their fluid needs. Such losses create tension in the fluid regulation system, so thirst becomes an inadequate indicator for fluid absorption and the results of dehydration of the body.
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