Body Fluid Compartments

From single cell to multicellular organism

In the beginning...(again)

Being a cell, as opposed to being a free-floating self-replicating molecule would appear to offer survival advantages, after all there are lots of cells around now and very few non-cellular life forms (even if you count viruses as non-cellular life forms they don't make up much of the total biomass). Although it might be argued that bacteria represent the most successful cellular lifeform, because there are more of them then anything else, being a multicellular organism does seem to have some things going for it. The simplest possible multicellular organism is a sponge. A sponge is a collection of identical cells that exist as a colony. It is possible to disperse a sponge through a sieve, if you leave the pieces in a bucket for a few days they will reform back into a colony with apparently no ill effects. However, the real advantages to being multicellular aren't apparent until component cells start to show specialisation. A flatworm is a good example of the next stage of development. Flatworms do not have digestive or circulatory systems; most of the animal is a flattened cylinder of cells. Nevertheless, the cells on the outside of the cylinder are specialised and act to hold the animal together, to protect the inner layers and to absorb food from the environment. In short the outer layer of the flatworm creates a comfortable internal environment for all the other cells to live in. Homeostasis may now occur at both the level of the single cell and at the level of the whole organism. One of the main disadvantages of being a flatworm is that you have to be flat to allow oxygen to diffuse into the innermost layers of cells (and also to let CO2 diffuse out). Any arrangement other than being flat puts the innermost cells to far away from the outside to survive. Everything more advanced than a flatworm has discovered the way around this is to have a circulatory system to reach the parts that diffusion cannot reach. Once past this hurdle, all sorts of specialisation amongst the cells that comprise an organism become possible. The digestive system is a way of extending the surface area for digestion whilst simultaneously protecting it by internalising it, thus the same cells don't have to try and combine protective and digestive functions. The lungs are essentially the result of the same principle applied to the problem of gaseous exchange. The liver, the brain, the muscles etc. etc. etc. have all evolved to play their own part in the homeostasis of the whole organism. The part of the body that has charge of maintaining the ionic composition of the organism, a task analogous to that of the plasma membrane in a single cell, is the kidney.

In the human body (for example) there are several major fluid compartments each of which is subject to homeostatic regulation.

The largest compartment is the intracellular compartment. Any fluid not contained inside a cell therefore comprises the extracellular compartment. The extracellular compartment may be divided into an interstitial compartment (means literally 'in the spaces', in this case, the spaces between the cells) and a circulating compartment (the blood plasma and the lymph fluid). A 70 kg man (the figures are slightly different for women) contains about 40-45 litres of water divided into the different compartments as follows.

Total 45

Intracellular 27



Extracellular 18

interstitial 13



plasma 3.5



lymph 1.5


If all the water were removed from a human body some 30 kg of assorted salts would remain. Despite the impression given by Start Trek or Red Dwarf where dried people are represented by a tiny pile of salt crystals, 30 kg is a lot of material. Imagine 15 bags of sugar poured all over the floor.


The normal composition of the major body fluid compartments is approximately as follows (mmol/l, except Ca2+)




Interstitial fluid

Intracellular fluid












<10-6 mol/l*


















* Free ionic Ca2+ is very low inside cells, total calcium may be much higher (1-2mmol/l).


N.B. These are only approximate values; each textbook has its own set of approximate values that may vary from these by one or two mmol/l. It doesn't matter.


The overall osmolarity of all three compartments is identical at about 300 mOsmol/l


Capillaries and Starling forces

The intracellular and extracellular compartments are separated from one another by the plasma membrane of the cells (see above). The extracellular compartments (interstitial/plasma/lymph) are separated by a layer of endothelial cells surrounded by a basement membrane; the capillaries. To cross from the plasma to the cells or vice versa, substances must either cross both membranes of the endothelial cells or travel between the cells and then cross the basement membrane. Capillaries come in three main types distinguished largely on the permeability of their walls.


1) Continuous capillaries have a close connection between adjacent cells and will permit only small molecules < 10nm in diameter to cross. Continuous capillaries surround muscle, skin lungs, adipose tissue CNS, retina and mammary glands.

2) Fenestrated capillaries contain 'windows' that offer easy passage to larger molecules (10-100nm) and are found around the kidneys, pancreas, gallbladder and intestine.

3) Discontinuous have wide gaps between the cells and will allow practically anything (even cells) across. Discontinuous capillaries surround the liver, spleen, ovaries and some endocrine glands.

Capillaries act rather like a leaky hosepipe; although the bulk of the fluid continues along the pipe, the pressure forces some out of the walls. The fluid and soluble contents of plasma small enough to cross the capillary wall circulate into the interstitial fluid at the high pressure arterial end of the capillary bed and returns to the capillary, bringing with it small soluble waste products from the cells, at the low pressure venous end of the capillary bed (Oxygen and CO2 being lipid soluble, diffuse from the plasma across the capillaries and to and from the cells as necessary.). Hydrostatic (blood) pressure is not the only force acting to cause fluid movement in and out of the capillaries. The plasma proteins that cannot cross the capillary walls exert an osmotic pressure to draw water back into the capillaries which outweighs the hydrostatic pressure at the venous end of the capillaries.

The balance of hydrostatic and osmotic forces causing movement out and into the capillaries are known as Starling forces.

© Pete Smith 1998