Cell is a structural and functional unit of life, as being
a free-floating and self-replicating
molecule, it would appear to offer
survival advantages, after all there are lots of cells around now and very few non-cellular life forms. 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.


Total 45 litre

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+)

Constituent (1st)
Plasma (2nd)
Interstitial fluid (3rd)
Intracellular fluid (4th)

Na+ 142 139 14

K+ 4.2 4 140

Ca2+ 1.3 1.2 0

Mg2+ 0.8 0.7 20

Cl- 108 108 4

HCO3- 24 28.3 10

Protein 1.2 0.2 4

Glucose 5.6 5.6 absent

Amino acid 2 2 8

Urea 4 4 4

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

In the above overall osmolarity of all three
compartments is identical at
about 300 mOsmol/l.