Water and electrolyte balance is dealt with fully in this section. About 1 L of water is required in the daily diet to balance insensible losses, but much more is usually drunk, the kidneys being able to excrete large quantities. The daily RNI for sodium is 70 mmol (1.6 g) but daily sodium intake varies in the range 90-440 mmol (2-10 g). These are needlessly high intakes of sodium which are thought by some to play a role in causing hypertension
In health, the volume and biochemical composition of both extracellular and intracellular fluid compartments in the body remains remarkably constant. Many different disease states result in changes of control either of extracellular fluid volume, or of the electrolyte composition of extracellular fluid. An understanding of these abnormalities is therefore essential for the management of a wide range of clinical disorders.
Distribution and composition of body water
In normal persons, the total body water constitutes 50-60% of lean bodyweight in men and 45-50% in women. In a healthy 70 kg male, total body water is approximately 42 L. This is contained in three major compartments:
- the intracellular fluid (28 L, about 35% of lean bodyweight)
- the interstitial fluid that bathes the cells (9.4 L, about 12%)
- plasma (4.6 L, about 4-5%).
In addition, small amounts of water are contained in bone, dense connective tissue, and epithelial secretions, such as the digestive secretions and cerebrospinal fluid.
The intracellular and interstitial fluids are separated by the cell membrane; the interstitial fluid and plasma are separated by the capillary wall. In the absence of solute, water molecules move randomly and in equal numbers in either direction across a permeable membrane. However, if solutes are added to one side of the membrane, the intermolecular cohesive forces reduce the activity of the water molecules. As a result, water tends to stay in the solute-containing compartment because there is less free diffusion across the membrane. This ability to hold water in the compartment can be measured as the osmotic pressure.
Plasma osmolality, Osmoregulation