WATER BALANCE

1. What is meant by water turnover?

2. What is the derivation of metabolic water? Why does 5 g of fat yield more metabolic water than 5 g of protein or carbohydrate?

3. What are examples of insensible water loss?

4.

From day to day in any one animal the water content of the body remains relatively constant, with a balance between gains and losses. Water turnover is that amount of water gained by an animal to balance that which is lost. Typical values for lactating and nonlactating cows under moderate environmental conditions are shown in Table 2-4. The water turnover for the nonlactating cow is 29 L/ day and.for the lactating cow is 56 L/ day. The water intake in both cases is equal to the output; there is water balance. The “pool size” (liters) stays constant, but the water in the pool changes (water turnover). The output of the lactating cow has increased, not only because of the obvious milk production, but also because of the greater fecal output associated with eating nearly twice as much and because of greater urine and vapor losses associated with increased metabolism.

Water Gain

Water gains occur by ingestion of water in food and drink and from metabolic water. The food eaten by animals contains a variable amount of water; the usual drink is water or, in the very young, milk. Metabolic water is derived from the chemical reactions of cellular metabolism in the mitochondria. At the end of the electron transfer chain, hydrogen is combined with oxygen to form water; this is metabolic water (see Chapter 1).

The metabolism of proteins, carbohydrates, and fats requires different amounts of cofactors, with the greatest amounts required for fats. Accordingly, the yield of metabolic water is greater for a certain amount of fat than for an equal amount of protein or carbohydrate. For example, the metabolic water yield from each of 100 g of protein, carbohydrate, and fat is 40, 60, and 110 mL, respectively. Energy in the form of adenosine triphosphate (ATP) is formed during the transfer of electrons. The amount of metabolic water formed varies but could be substantial under certain conditions. In domestic animals it is said to average about 5-10% of daily water gain, and can approach 100% of the water gain for some small desert rodents.

Water Loss

Water loss from the body is classified as either an insensible loss or a sensible loss. Insensible losses are associated with vapor losses and occur constantly by evaporation from the skin and by loss of water vapor in exhaled air. Inhaled air becomes saturated with water vapor in the respiratory passages and lungs, but there is no body mechanism to remove moisture from the respiratory gases before exhalation. Sensible losses are the visible losses; they are part of the urine, feces, and body secretions that leave the body and are not subject to evaporation. Sensible losses can become excessive in certain conditions, such as diarrhea, and threaten body stores of water.

Water Requirements

No linear relationship exists between basal water needs and body weight. Accordingly, a 500 kg cow does not require 10 times more water than a 50 kg calf. However, the basal daily needs for water (that needed to maintain water balance) are related to caloric expenditure. Under basal metabolism conditions (e.g., resting animal, thermally neutral environment, fasting state), caloric expenditure is related linearly to body surface area. The cow might require only three to four times more water than the calf because her body surface area is three to four times greater. If the ECF (20% of body weight) is considered to be that from which emergency water is drawn, the 500 kg cow has 100 kg of ECF and the 50 kg calf has only 10 kg of ECF. Therefore, the cow has considerably more reserve on which to draw to supply basal needs for water than does the calf. In other words, the cow has ten times more reserve water to supply her needs, and her needs are only three to four times greater than that of the calf. It is because of the more limited reserves associated with their relatively higher needs that calves become distressed more quickly in conditions of uncontrolled water loss (such as diarrhea). It should also be noted that because of the greater surface area relative to the body weight of calves, they will also lose body heat quicker than the cows.

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