composition of the body By far the largest component of the body is water, accounting for some 60% of the total weight, which in a 70 kg person amounts to about 40 litres. Of the remaining 40% of body weight typical proportions would be 15% protein, 18% fat, and the remaining 7% mineral, largely in the form of bone; but there are large individual variations.

Water is divided into intracellular fluid (that contained within the living cells of the body) and extracellular fluid (that which bathes the cells from the outside). These fluids account respectively for 40% and 20% of the total body mass. The extracellular fluid is further subdivided into the 5% contained within the blood and 15% which is outside the vascular system. The compositions of the intra- and extracellular fluids are very different; further, the blood fluid (plasma) is subtly different to the rest of the extracellular fluid, which is termed interstitial, as it fills the interstices between the cells. Most of the material dissolved within these body fluids consists of charged molecules or ions and clearly there must be the same number of positively-charged molecules (cations) as negatively-charged molecules (anions) to maintain electroneutrality. A major difference between intra- and extracellular fluids is that the main salt in the former is potassium chloride, while outside the cells it is sodium chloride. The body uses a great deal of energy to maintain this difference — the so-called sodium pump removing sodium ions which penetrate the cells and bringing in potassium ions at the same time. All cells have sodium pumps in their membranes; these consume energy by hydrolysing adenosine triphosphate (ATP).

The differing composition of intra- and extracellular fluids is of vital importance for many bodily processes. For example, the temporary increase in membrane permeability which occurs when nerves are activated allows a flow of sodium ions into, and potassium ions out of the nerve cells. This is the mechanism by which nerve impulses flow along nerve cells, carrying messages from one place to another within the body. After the impulse has passed the sodium pump restores the original resting condition.

The ionic composition of the intracellular, interstitial, and plasma fluids is given below.

The Ionic composition of the Intracellular, Interstitial, and plasma fluids is given below, showing values in milliEquivalents per litre

Notice that the total cations are balanced by the total anions in all three fluids. Values in milliEquivalents per litre represent number of molecules x valency, and thus allow assessment of electroneutrality. In other contexts concentrations of substances in the body are more commonly quoted in millimoles per litre.
Intracellular fluid
Cations	Potassium	157	Anions	Bicarbonate	10
	Sodium	14		Phosphate	113
	Magnesium	26		Protein	74
Interstitial fluid
Cations	Sodium	143	Anions	Chloride	117
	Potassium	4		Bicarbonate	27
	Calcium	5		Phosphate	2
	Magnesium	3		Sulphate	1
				Organic acid	6
				Protein	2
Plasma
Cations	Sodium	152	Anions	Chloride	113
	Potassium	5		Bicarbonate	27
	Calcium	5		Protein	16
	Magnesium	3		Phosphate	2
				Sulphate	2
				Organic acid	6

Additionally, interstitial fluid and blood plasma contain some non-electrolytes such as glucose. There are many other substances required by the healthy body but their total amounts are generally very small; an exception is iron, which is an essential component of haemoglobin contained in red cells, and hence is essential for the carriage of oxygen from the lungs to the tissues. In addition there are vitamins and trace elements (including cobalt, manganese, copper, zinc, iodine, and bromine).

Disturbances of body composition occur most obviously in dehydration or starvation. Estimation of the concentrations of electrolytes and other substances in the plasma is a routine part of many diagnostic investigations, since they can change in characteristic ways in a variety of conditions; in studies of nutritional status and obesity, the fat and lean fractions of body mass are measured and compared to standard values for males and females at different ages; estimation of the bone mineral is important in the assessment of osteoporosis.

Alan W. Cuthbert

See also blood; body fluids; body weight; bone; metals in the body; minerals; salt; water balance.