Water and sodium balance

Published on 01/03/2015 by admin

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Last modified 01/03/2015

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Water and sodium balance

Body water and the electrolytes it contains are in a state of constant flux. We drink, we eat, we pass urine and we sweat; during all this it is important that we maintain a steady state. A motor car’s petrol tank might hold about 42 L, similar to the total body water content of the average 70 kg male. If 2 L were lost quickly from the tank it would hardly register on the fuel indicator. However, if we were to lose the same volume from our intravascular compartment we would be in serious trouble. We are vulnerable to changes in our fluid compartments, and a number of important homeostatic mechanisms exist to prevent or minimize these. Changes to the electrolyte concentration are also kept to a minimum.

To survive, multicellular organisms must maintain their ECF volume. Humans deprived of fluids die after a few days from circulatory collapse as a result of the reduction in the total body water. Failure to maintain ECF volume, with the consequence of impaired blood circulation, rapidly leads to tissue death due to lack of oxygen and nutrients, and failure to remove waste products.

Water

Normal water balance is illustrated in Figure 7.1.

Water intake largely depends on social habits and is very variable. Some people drink less than half a litre each day, and others may imbibe more than 5 L in 24 hours without harm. Thirst is rarely an overriding factor in determining intake in Western societies.

Water losses are equally variable and are normally seen as changes in the volume of urine produced. The kidneys can respond quickly to meet the body’s need to get rid of water. The urine flow rate can vary widely in a very short time. However, even when there is need to conserve water, man cannot completely shut down urine production. Total body water remains remarkably constant in health despite massive fluctuations in intake. Water excretion by the kidney is very tightly controlled by arginine vasopressin (AVP; also called antidiuretic hormone, ADH).

The body is also continually losing water through the skin as perspiration, and from the lungs during respiration. This is called the ‘insensible’ loss. This water loss amounts to between 500 and 850 mL/day. Water may also be lost in disease from fistulae, or in diarrhoea, or because of prolonged vomiting.

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