BODY FLUIDS

BODY FIUIDS
Body fluids consist of the water of the body and substances dissolved in it. Water is the main component of the human body, and, in any individual, the body water content stays remarkably constant from day to day. In a 70 kg (154 lbs or 11 stone) man of average build, about 63% of the body weight is water, and hence the total body water (TBW) amounts to 45 litres. In a woman of the same weight, typically only about 54% is water, so the TBW is about 38 litres. The difference is due to the fact that women generally have more fat (adipose tissue) than men, and there is very little water in the adipose tissues. In both sexes, the proportion of the body weight that is water tends to decrease with increasing age. The functional tissue of the body can be regarded as fat-free, and the percentage of water in the fat-free tissue is very constant in any one individual from day to day, and between individuals. Water accounts for 73% of this ‘lean body mass’.

Life as we know it could not have evolved without water. All living things consist of aqueous solutions separated from each other by boundaries such as cell membranes. Water has many properties of biological importance. It is a liquid at the ordinary temperatures found on the earth. Compared with other liquids, it has a high specific heat (the amount of heat energy needed to produce a given increase in temperature): this tends to minimize changes in temperature when heat is produced by chemical reactions inside cells. Water has a high latent heat of evaporation (the amount of heat energy given out as a liquid evaporates): this provides the basis for an efficient mechanism for heat loss by sweating. Water is a good solvent for ionic compounds, which are essential components of all living systems.

The cells of our tissues and organs could not survive in the outside world. The immediate environment of the cells is the extracellular fluid. This internal environment (‘milieu intérieur’ — a term first used by the French physiologist, Claude Bernard) maintains the correct concentrations of oxygen, carbon dioxide, ions, and nutritional materials for the normal functioning of the cells. Maintenance of the constancy of the internal environment, which the American physiologist Walter B. Cannon termed ‘homeostasis’, is achieved by the actions of many body tissues and organs, including the cardiovascular, respiratory, and renal systems, and the liver. These actions are in turn regulated by nerves and hormones.


Input, production, and output of water

The body is continually exchanging fluid with the external environment. Water input into the body occurs by drinking (typically 1500 ml/day), by eating (500 ml/day of our water intake is contained in food), and by the metabolism of food (400 ml/day).The metabolically-derived water comes from the oxidation of food — glucose oxidation for example:

C6H12O6 + 6O2 → 6CO2 + 6H2O
glucose oxygen carbon dioxide water

The fluid output from the body occurs by several routes: from the lungs (400 ml/day), from the skin (400 ml/day), in the faeces (100 ml/day), and in the urine (1500 ml/day).

The loss from the lungs occurs because as air is breathed in it becomes saturated with water evaporating from the moist linings of the route to the lungs — mainly in the nose or mouth. Some of this water is restored to the same surfaces during exhalation, but much of it is lost. In hot, dry environments, or in sub-zero temperatures (when the air is very dry), the loss of water from the lungs can be considerably greater than 400 ml/day.

The water loss from the skin, termed ‘insensible perspiration’, is evaporative loss from the skin epithelial cells and occurs at an almost constant rate. It is not sweat. Sweating, or ‘sensible perspiration’ represents an additional and adjustable loss, which can exceptionally reach up to 5 litres hour. The fluid lost in faeces, normally 100 ml per day, can be increased to several litres per day by diarrhoea. The loss in urine can vary enormously — between 400 ml/day and 25 litres per day — being adjusted according to the needs of the body relative to intake. Total water loss can never be cut down to less than about 1200 ml per day, so survival without any water intake is possible for only a short time: generally less than one week.


The body fluid compartments

There are two main body fluid compartments; inside and outside the cells (intracellular and extracellular). The extracellular compartment is divisible into (a) the plasma, which is extracellular fluid within the blood vessels; (b) the interstitial fluid, which is extracellular fluid outside the blood vessels and separated from plasma by the walls of the capillaries; and (c) transcellular fluids, which are fluids with specialized functions. They include synovial fluid (which lubricates joints), cerebrospinal fluid (which cushions and nurtures the brain), and the aqueous and vitreous humours of the eyes (which maintain the shape of the eyeball and the integrity of structures within it). The transcellular fluids are separated from the plasma by a cellular membrane, which takes part in their formation, in addition to the capillary wall.

The volumes of the body fluid compartments, in a ‘typical’ 70 kg man, are: 30 litres inside cells and 15 litres outside cells, comprising 3 litres in the plasma, 11 litres in interstitial fluid, and 1 litre in transcellular fluids.


The solutes in the body fluid

The partitions between these compartments (cell membranes between extracellular fluid and intracellular fluid, capillary walls between plasma and interstitial fluid, and cellular layers between interstitial fluid and transcellular fluid) are permeable to water, and hence the osmotic concentrations of the solutes in the different compartments must be essentially identical — otherwise water will move through the barriers until this is the case. However, although the osmotic concentrations (osmolality) in the compartments must be almost identical, the solutes that make up the osmolality are different. The main difference is between intracellular and extracellular fluid.

The major ions of the extracellular fluid are: sodium (Na+: 142 mmol/litre), chloride (Cl-: 110 mmol/litre), and bicarbonate (HCO-3: 25 mmol/litre). In the plasma component there is a significant volume of proteins and lipids in colloid suspension, so Na+ concentration is in fact higher.

In intracellular fluid, there is a low Na+ concentration, but a high potassium ion (K+) concentration, and there are large amounts of negatively charged proteins. However, the enclosure of fluids of different compositions in multiple microscopic compartments (organelles) within the cells makes it rather difficult to generalize about intracellular fluid composition.


Exchanges between compartments

A continual exchange of water and solutes takes place between the compartments of the body fluids, which are in dynamic equilibrium with each other. This has an important bearing on the regulation of the body fluids.

Fluid moves between the plasma and the interstitial fluid through the walls of the capillaries, the smallest blood vessels. This movement occurs as a result of two forces, the hydrostatic pressure within the capillaries, pushing water and solutes out, and an osmotic gradient due to the plasma proteins in the capillaries, drawing water into the capillaries. (Since the capillary walls are permeable to other solutes but not to plasma proteins, it is the proteins alone that cause an osmotic pressure difference between plasma and interstitial fluid.) The volume of fluid leaving the capillaries slightly exceeds that re-entering. This excess interstitial fluid is taken up by the lymph vessels and returns to the vascular system at the base of the neck where the main lymph vessel, the thoracic duct, joins the venous system.


Regulation of volume and osmolity

How then is the volume of the body fluids regulated? For intracellular fluid, this is straightforward. Cell membranes are permeable to water, therefore water will cross the membranes if there are differences in solute concentration (osmolality) — and hence differences in osmotic pressure — between the two sides. Individual cells can thus regulate their volume by adjusting their membrane transport processes to increase or decrease their solute content; this will lead to corresponding increases or decreases in volume as water osmotically follows the solute.

For extracellular fluid, the regulation of volume is more complicated. Because the extracellular fluid solute concentration (osmolality) is kept constant, the water content will depend on the solute content. For example, if we increase the amount of Na+ in the body by eating salty food, this makes us thirsty so that we drink to bring the Na+ to the correct concentration, and we end up with an increased volume. Because sodium (Na+) and its associated negative ions are the main solutes of the extracellular fluid, the volume is regulated indirectly by controlling the Na+ content of the body. Sensors in circulatory system detect the blood pressure and the amount of blood returning to the heart from the rest of the body. Both of these tend to increase if the extracellular fluid volume increases, and nerve signals from the sensors, relayed to the brain, ultimately lead to changes in the concentrations in the blood of hormones that regulate Na+ excretion by the kidneys. The main hormones are angiotensin II and aldosterone. Both of these act to retain Na+ (and consequently water) so their secretion is inhibited when extracellular fluid volume increases. An increase of volume also has a more direct effect by diminishing secretion of the water-regulating hormone, vasopressin (antidiuretic hormone), the action of which is to promote retention of water in the kidneys.

The regulation of body fluid volume is thus inextricably linked with regulation of the concentration of sodium ions in the extracellular fluid. There are also many other solutes whose concentration in the body fluids are kept within necessary limits by a variety of mechanisms which ultimately adjust their retention or loss, mostly in the kidneys.