Transport In Mammals Essay

Transport in Mammals

A recurring theme in biological systems is the surface area to volume ratio. All cells require nutrients and most require oxygen as well. Wastes also need to be removed.

With a small organism this demand can be met by simple diffusion over the body surface but larger or very active organisms need a transport system with a pump to ensure that the supply meets the demand of all cells, even those deep within the body.

In mammals, the pump is the heart. Substances are carried in a transport medium of the blood. The blood is contained within vessels, with substances being released out of, or into the blood as it flows through certain vessels called capillaries.

Blood

These allow substances to leave the blood and bathe the cells of the tissues. The fluid made up of plasma and dissolved substances is called tissue fluid.

Tissue fluid

Tissue fluid is formed because of the high hydrostatic pressure of the blood at the arteriole end of the capillary that pushes fluid out of the blood.

The blood contains plasma proteins giving the blood a relatively high solute potential (and therefore a low water potential), tending to draw water into the blood. Since the hydrostatic pressure has a greater effect than the solute potential at the arteriole end, the net effect is that fluid leaves the capillary. No blood cells or large proteins leave as they are too big to fit through the gaps.

At the venule end of the capillary, since fluid has been lost, the hydrostatic pressure of the blood is lower and the solute potential is higher. Because of this, fluid drains back into the blood. At this stage, the useful materials such as amino acids and glucose will have been taken up by the cells and the tissue fluid will now contain

A graph of the percentage saturation of blood with O2, i.e. the amount of HbO2 as opposed to Hb at different pO2 is shown below. It is called an oxygen dissociation curve:

It is S-shaped because of the behaviour of the Hb in different pO2.

The first molecule of O2 combines with an Hb and slightly distorts it. The joining of the first is quite slow (the flatter part of the graph at the beginning) but after the Hb has changed shape a little, it becomes easier and easier for the second and third O2 to join. This is shown by the curve becoming steeper. It flattens off at the top because joining the fourth O2 is more difficult.

Overall, it shows that at the higher and lower end of the partial pressures, there isn't a great deal of change in the saturation of the Hb, but in the middle range, a small change in the pO2 can result in a large change in the percentage saturation of the blood.

The effect of pH - The Bohr effect

The amount of O2 carried and released by Hb depends not only on the pO2 but also on