Body fluid compartments

The major body constituent is water. An ‘average’ person, weighing 70 kg, contains about 42 litres of water in total. Two-thirds (28 L) of this is intracellular fluid (ICF) and one-third (14 L) is extracellular fluid (ECF). The ECF can be further subdivided into plasma (3.5 L) and interstitial fluid (10.5 L).

A schematic way of representing fluid balance is a water tank model that has a partition and an inlet and outlet (Fig 6.2). The inlet supply represents fluids taken orally or by intravenous infusion, while the outlet is normally the urinary tract. Insensible loss can be thought of as surface evaporation.

Selective loss of fluid from each of these compartments gives rise to distinct signs and symptoms. Intracellular fluid loss, for example, causes cellular dysfunction, which is most notably evident as lethargy, confusion and coma. Loss of blood, an ECF fluid, leads to circulatory collapse, renal shutdown and shock. Loss of total body water will eventually produce similar effects. However, the signs of fluid depletion are not seen at first since the water loss, albeit substantial, is spread across both ECF and ICF compartments.

The water tank model illustrates the relative volumes of each of these compartments and can be used to help visualize some of the clinical disorders of fluid and electrolyte balance. It is important to realize that the assessment of the volume of body fluid compartments is not the undertaking of the biochemistry laboratory. The patient’s state of hydration, i.e. the volume of the body fluid compartments, is assessed on clinical grounds. The term ‘dehydration’ simply means that fluid loss has occurred from body compartments. Overhydration occurs when fluid accumulates in body compartments. Figure 6.3 illustrates dehydration and overhydration by reference to the water tank model. When interpreting electrolyte results it may be useful to construct this ‘biochemist’s picture’ to visualize what is wrong with the patient’s fluid balance and what needs to be done to correct it. The principal features of disordered hydration are shown in Table 6.1. Clinical assessments of skin turgor, eyeball tension and the mucous membranes are not always reliable. Ageing affects skin elasticity and the oral mucous membranes may appear dry in patients breathing through their mouths.