body fluids

Body Fluids
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.
If you have any comments or queries, please email them to me.
Fluid compartments
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.

Ionic Composition
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.

Forces acting upon the ions
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
• the force of the chemical gradient

• the force of the electrical gradient

where
Nernst Equation
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".
Transport across membranes
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.
Osmotic pressure
• Just as ions tend to move across semi-permeable membranes - from areas of high concentration to areas of low concentration
• so does water - to dilute stronger solutions
• this is osmosis - the pressure which withstands the movement is osmotic pressure
Hydrostatic pressure
• If you use a hose pipe, the water pressure at the tap end, is greater than the water pressure at the nozzle.
• These are hydrostatic pressures - which "try" to force the water out of the pipe
• In a blood vessel, the hydrostatic pressure at the arterial end is usually higher than at the venous end.
Starling's Hypothesis
• In a capillary, there are two pressures to consider, the hydrostatic pressure, which is a composite of the force provided by the heart, and the forces due to gravity. Hydrostatic pressure is the force which "tries" to force the fluid out of the capillary.
• The plasma proteins can not normally leave the capillary, so they exert an osmotic pressure, which tends to draw fluid into the capillary.
• The consequence is that fluid tends to leave the capillary at the arterial end, and be drawn back in at the venous end. Any surplus fluid will either produce oedema, or be taken up by the lymph ducts, and returned to the circulation.
• Altering either the hydrostatic or osmotic pressure will disturb the fluid balance across the capillary wall.