Exchange of materials

Blood travels via arteries until it reaches smaller vessels called capillaries. It is here that materials are exchanged between blood and the tissue cells.

1. The blood enters a capillary bed. These vessels are very leaky and are only wide enough for one cell at a time to pass through. The capillary walls are only 1 cell thick!
2. The blood pressure forces some of the blood plasma to leak out of the capillary. This fluid is high in nutrients and oxygen (from the red blood cells). Large objects like red blood cells and protein molecules cannot pass through the walls of the capillary. The fluid that is surrounding the tissue cells is called tissue fluid. It is from this fluid that materials will diffuse into the cells.
3. White blood cells are the only cells, which can leave the blood, so they can hunt down pathogens.
4. Waste materials like carbon dioxide and urea diffuse from the cells into the tissue fluid. This fluid is drawn back into the blood capillary by an osmotic pressure supplied by the large proteins in the blood.
5. Not all the tissue fluid flows back into the blood. If it did not return the tissues would swell with fluid. Sets of vessels, called lymph vessels, drain this tissue fluid and carry it away from the tissues. Eventually the fluid (called lymph) drains back into the blood.
6. The blood leaves the capillary beds and travels back to the heart via veins.

This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 2.5 License.

2005 Psionica