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FLUID REPLACEMENT AND HEAT STRESS
(Ladell, 1965, p. 284). This may be seen as at least one explanation of why thirst is inadequate, and there are others.
Common sense would dictate that it is quite useless to estimate a “normal” fluid intake because we are dealing with a homeostatic system designed to equate water requirements with the various losses (respiratory, urinary, skin, and sweat). Let us look briefly at these as a partial inventory of our water demand. Under normal conditions, respiratory water loss is about 200 ml/day, but it can be around 350 ml/day for men working in a dry climate and can approach 1,500 ml/day for men working at high altitudes in cold air (Ladell, 1965). The insensible perspiration may be as low as 500 ml in a moist climate, and with a minimum urine volume (<300 ml/day), a person can barely meet obligate losses on 1,000 ml of water per day. The obligate urine volume varies with the diet and is high on a high-protein diet and low on a carbohydrate diet. A more reasonable figure for urine volume represents a maximum of 1.4 osmol of metabolic end products (mostly urea and surplus electrolyte) per liter of urine on a mixed European-style diet. Thus, the greatest rate of water loss, by far, is represented in a healthy individual by eccrine sweating, which most physiologists would agree can be sustained at something over 1 liter/h. This makes sense, because the maximum rate of gastric emptying has been estimated between 15 and 20 ml/min or 900 to 1,200 ml/h (Davenport, 1982).
According to Ladell, “. thirst is primarily a sensation, which often serves as a drive to drink, but the drive and the sensations are not necessarily identical ” (Ladell, 1965, p. 271). Ladell (1965) has further introduced a concept of free circulating water, equivalent to some 2 liters, which does not appear to participate in the osmotic balance of the body. This suggests that the drive to drink would not come into play until this free circulating water is expended. This interesting notion actually delivers two important ideas: (1) There is an inherent delay in the onset or drive of thirst. If this could be explained, it would then be more accurate to describe thirst as delayed rather than inadequate. (2) The delay is a manifestation of the body's osmotic control.
HYPERTONICITY, ANTIDIURETIC HORMONE RELEASE, AND THIRST
Although the solute composition of the extracellular compartment is markedly different from that of the intracellular space, the total osmolalities (solute concentration, not content) (Conway and McCormack, 1953) are very similar. This is because most cell membranes are freely permeable to water. Thus, one can approximate intracellular fluid osmolality by measuring the