The transport of oxygen from air in the alveoli of the lungs to the cytoplasm of respiring muscle cells.

Fick's Law = Surface area x difference in concentration

Thickness of membrane(s)

The 3 ways that Fick's Law is obeyed:

* The thin walls of the alveoli and capillary (only 1 cell thick and made of squamous epithelia) maintain the small diffusion pathway.

* A large surface area for diffusion is obtained in humans by the structure of the alveoli, which enables them to have a large surface area in relation to volume ratio, and there are a large number of them in the lungs. A dense bed of capillaries surrounds the alveoli; this also keeps the surface area: volume ratio high.

* The difference in concentration of O2 and CO2 is maintained by the blood within the capillary beds constantly moving, so as soon as O2 has diffused from the alveolus to the blood, the blood moves onwards and more deoxygenated blood becomes present as a continuous process. The blood is however moving quite slowly in these capillary beds, in order to ensure that as much gas exchange as possible occurs.

Now that the oxygen is in the plasma, it passes into the red blood cells (through diffusion again) where it binds to haemoglobin.

Haemoglobin is a globular protein, the haem group is important in transporting oxygen. The tertiary structure of haemoglobin gives it a distinctive shape and this allows the oxygen molecules to bind onto its active site, as it has a complementary shape to the site. Therefore, another molecule would not be able to bind to that specific active site on the haemoglobin molecule, as it would not have the right shape.

Haemoglobin is what enables blood to be so efficient at transporting oxygen. In the lungs, it binds with oxygen to form oxyhaemoglobin (the enzyme-substrate complex of oxygen and haemoglobin), which is transported in the blood to the respiring tissues. Here the enzyme-substrate complex is broken down to haemoglobin and oxygen again; the oxygen diffuses into the cells of the body while the haemoglobin is transported back to the lungs.

When the body is exercising, the energy required for increased muscle contraction comes from respiration. More exercise means a faster rate of respiration and the need for more oxygen to be supplied to the muscles. This can be achieved by increasing the amount of blood flowing through the capillaries.

When blood passes through the blood vessels, it loses pressure. By passing through the left side of the heart before being sent round the body, the pressure can be increased, allowing oxygenated blood to reach the organs of the body rapidly (i.e. the double circulation of the heart).

The oxygen in the being sent to the muscle tissue breaks from the haemoglobin and diffuses into the cells of the muscle tissue. This is because they need oxygen to respire more as the body is undergoing exercise. For the oxygen to get into the cell it must pass through the cells membrane, which consists of a phospholipids bilayer (two layers of phospholipids). Phospholipids have a head that is hydrophilic (i.e. mixes with water) and a tail that is hydrophobic (i.e. does not mix with water), because of this the head is on the outside of the layer and the tail is on the inside.

Diffusion of a small molecule like oxygen requires no added energy; it diffuses freely across the cell's plasma membrane and into the cytoplasm of the respiring muscle cell.

Alana Russell Page 1 16/05/2007