These are notes to support lectures given by The Reverend Dr. David C.M. Taylor on body fluids. For copyright reasons the thumbnail images lead to pages that can only be accessed by people using the Liverpool University Server. Use the "Back" button to return to this page.
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The fluid compartments in healthy, normal men and women differ, because weight for weight the female body contains more fat.
When we consider body fluids, it is essential to distinguish between intracellular (inside cells) and extracellular (outside cells) fluids. The extracellular fluid in turn comprises both interstitial (between cells) and plasma compartments. Normally it is the interstitial concentration of substances which concern us.
It is important to realise that proteins are large organic anions, that can not, under normal circumstances, leave the cell body, or for that matter the blood vesels.. The intracellular protein ions are responsible, in part, for the distribution of ions across the cell wall. The crucial concentrations are the interstitial and intracellular concentrations.
Although the underlying principles of what follows are more important than the details, it is often helpful to try and analyse the forces which are acting across the membrane. If this treatment seems too complex, then you might like to link to one of my other pages.
Since there are different concentrations of ions on either side of the cell membrane, there are two forces acting
where
It follows that, if the cell is permeable (here to potassium), the equilibrium potential will be
or
If you are confident with maths, you will realise that this means that, if the membrane is only permeable to potassium, the inside of the cell will be negative with respect to the outside. This is the situation which obtains in excitable tissues when the membrane is "at rest".
The different concentrations of ions either side of the membrane mean that there are several concentration gradients to consider, and these allow for a number of different transport processes.
The most important pump in the whole system, and one of the greatest users of energy in the body at rest, is the sodium/potassium ATPase, also known as the sodium pump. The sodium gradient set up by this pump provides the chemical energy to allow many substances to enter or leave the cell against their concentration gradient.
