The Circulation - the Circulatory System 3
 

Now that the cells have received their Oxygen and nutrients, they use them for energy. In the cells, the breakdown of sugar and oxygen to carbon dioxide and water to produce energy is called "Respiration".

However, when a lot of cells do this - in other words a tissue or organ - the breakdown of oxygen and nutrients to waste products and water to make energy to be used by the tissue or organ, we often call this "metabolism".

Therefore the cells in the tissues get their energy from this chemical process, but have to get rid of the waste products - mainly carbon dioxide and water.

These waste products pass into the extracellular fluid where they defuse back through the very thin capillary walls and back into the blood.

When the blood has not got so much oxygen in it (called deoxygenated blood), it appears darker - more of a bluish red - so in medicine are usually draw this as blue.

The capillaries pass this deoxygenated blood into very small vessels called venules. These join together to make small veins, which join together to make larger veins and eventually all join to make the largest vein in the body, the vena cava.

Whereas the oxygenated blood is pumped through the arteries by the heart, there is no obvious pump to get the venous blood back to the heart from the tissues. The understanding of how this happens is fairly complex - if you are interested, you should look at www.veins.co.uk where there is a section about how veins work.

However as far as understanding the circulatory system goes, we can just accept that the deoxygenated blood gets back to the heart through the veins as outlined above.

To move on to the next page please click here - Circulation 4

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For more information about vascular and circulatory diseases and disorders, visit our Vascular specialist websites page

(information provided by Mark Whiteley - www.TheWhiteleyClinic.co.uk)