| Water is an essential constituent of the human body, accounting for approximately 60% of the body weight in the adult. This changes with age: it is about 75% in the newborn and decreases to less than 50% in older individuals. Water content is greatest in brain tissue (about 90%) and least in adipose tissue (10%).
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| Both a relative deficiency and an excess of water impair the function of tissues and organs because the stability of subcellular structures and activities of numerous enzymes are dependent on adequate cell hydration. Water balance, and the distribution of water between the extracellular (vascular and interstitial) and cellular compartments are closely controlled.
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| Distribution of water within the body depends on hydrostatic and osmotic forces acting across biological membranes. Osmotic activity depends on the concentration gradients of electrolytes and other substances that cannot freely diffuse through cell membranes. Sodium is the most abundant extracellular cation and, with its associated anions such as chloride and bicarbonate, is fundamental for determining water distribution across cell membranes.
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| An imbalance between the amounts of water and sodium in the extracellular fluid leads to changes in osmolality. The consequent movement of water is associated with contraction or expansion of the cell volume and underlies many clinical disorders.
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| The total amount of water and electrolytes in the body depends on the balance between their intake and loss. The kidneys are a major regulator of this balance because they control the loss of body water and electrolytes. Consequently, impaired renal function leads to the water and electrolyte imbalance. This chapter describes the aspects of the water and electrolyte balance which are most relevant to clinical practice.
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| Note that water and electrolyte balance is closely linked to nutrition (Chapter 21), and particularly to the issues associated with enteral and parenteral feeding.
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