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.

AVP and the regulation of osmolality

Specialized cells in the hypothalamus sense differences between their intracellular osmolality and that of the extracellular fluid, and adjust the secretion of AVP from the posterior pituitary gland. A rising osmolality promotes the secretion of AVP while a declining osmolality switches the secretion off (Fig 7.2). AVP causes water to be retained by the kidneys. Fluid deprivation results in the stimulation of endogenous AVP secretion, which reduces the urine flow rate to as little as 0.5 mL/minute in order to conserve body water. However, within an hour of drinking 2 L of water, the urine flow rate may rise to 15 mL/minute as AVP secretion is shut down. Thus, by regulating water excretion or retention, AVP maintains normal electrolyte concentrations within the body.

Sodium

The total body sodium of the average 70 kg man is approximately 3700 mmol, of which approximately 75% is exchangeable (Fig 7.3). A quarter of the body sodium is termed non-exchangeable, which means it is incoporated into tissues such as bone and has a slow turnover rate. Most of the exchangeable sodium is in the extracellular fluid. In the ECF, which comprises both the plasma and the interstitial fluid, the sodium concentration is tightly regulated at around 140 mmol/L.

Sodium intake is variable, a range of less than 100 mmoL/day to more than 300 mmol/day being encountered in Western societies. In health, total body sodium does not change even if intake falls to as little as 5 mmol/day or is greater than 750 mmol/day.

Sodium losses are just as variable. In practical terms, urinary sodium excretion matches sodium intake. Most sodium excretion is via the kidneys. Some sodium is lost in sweat (approximately 5 mmol/day) and in the faeces (approximately 5 mmol/day). In disease the gastrointestinal tract is often the major route of sodium loss. This is a very important clinical point, especially in paediatric practice, as infantile diarrhoea may result in death from salt and water depletion.

Urinary sodium output is regulated by two hormones:

Regulation of volume

It is important to realize that water will only remain in the extracellular compartment if it is held there by the osmotic effect of ions. As sodium (and accompanying anions, mainly chloride) are largely restricted to the extracellular compartment, the amount of sodium in the ECF determines what the volume of the compartment will be. This is an important concept.

Aldosterone and AVP interact to maintain normal volume and concentration of the ECF. Consider a patient who has been vomiting and has diarrhoea from a gastrointestinal infection. With no intake the patient becomes fluid depleted. Water and sodium have been lost. Because the ECF volume is low, aldosterone secretion is high. Thus, as the patient begins to take fluids orally, any salt ingested is maximally retained. As this raises the ECF osmolality, AVP action then ensures that water is retained too. Thus, aldosterone and AVP interaction continues until ECF fluid volume and composition return to normal.