![major body fluid compartments major body fluid compartments](https://image.slidesharecdn.com/ch3compartmentscellsf07-150311201408-conversion-gate01/95/compartments-cells-5-638.jpg)
* Free ionic Ca 2+ is very low inside cells, total calcium may be much higher (1-2 mmol/l). The normal composition of the major body fluid compartments is approximately as follows ( mmol/l, except Ca 2+) Imagine 15 bags of sugar poured all over the floor. 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. If all the water were removed from a human body some 30 kg of assorted salts would remain. 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.
![major body fluid compartments major body fluid compartments](http://medtrng.com/anatomy%20lesson/bhp3_files/image004.gif)
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). Any fluid not contained inside a cell therefore comprises the extracellular compartment. The largest compartment is the intracellular compartment. In the human body (for example) there are several major fluid compartments each of which is subject to homeostatic regulation. 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. have all evolved to play their own part in the homeostasis of the whole organism. The lungs are essentially the result of the same principle applied to the problem of gaseous exchange. 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. Once past this hurdle, all sorts of specialisation amongst the cells that comprise an organism become possible. 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. Any arrangement other than being flat puts the innermost cells to far away from the outside to survive. 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 CO 2 diffuse out). Homeostasis may now occur at both the level of the single cell and at the level of the whole organism. In short the outer layer of the flatworm creates a comfortable internal environment for all the other cells to live in. 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. Flatworms do not have digestive or circulatory systems most of the animal is a flattened cylinder of cells. A flatworm is a good example of the next stage of development. However, the real advantages to being multicellular aren't apparent until component cells start to show specialisation. 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. A sponge is a collection of identical cells that exist as a colony. The simplest possible multicellular organism is a sponge. 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. Body Fluid compartments Body Fluid Compartments From single cell to multicellular organismīeing 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).