Your veins and capillaries

Your heart, blood vessels and blood make up your circulatory system, the huge network of pathways that your blood takes around your body.  The pathways of your circulatory system reach every single cell in your body, bringing nutrients and oxygen to your cells and taking away waste products.

Your veins are the blood vessels that carry blood to your heart.  They are similar to your arteries that carry blood away from your heart but they are not as thick because they do not have to cope with a lot of pressure from your heart pumping.  As your veins return from all over your body, they get larger as they get closer to your heart.  Your veins have three different layers – a very tough outer layer, a layer of smooth muscle in the middle and a layer of epithelial cells, very smooth cells that form a lining on the inside.  This inside layer is very smooth so that there are no humps or bumps to block your blood as it flows through your arteries.  The middle of your arteries, the hollow bit of the tube, is called the lumen. 

Your veins have something your arteries do not have - valves!  Valves are like little doors in your veins, they close off after blood has passed through so that your blood can only flow towards your heart.  Your veins have valves so that your blood will only flow in one direction, it cannot flow backwards!  You have different numbers of valves in different areas of your body because different areas of your body are under different levels of pressure.  The veins in the lower half of your body have more valves because they have further to go to get to your heart and they have to work against gravity!  Sometimes these valves stop working and blood does flow the wrong way.  This is a condition called varicose veins and it can be painful.  You can see varicose veins appear on the legs of people who are affected.   

The blood in your veins is a dark red (almost purple or blue colour!) because it does not carry any oxygen - it is de-oxygenated.  The oxygen has already been exchanged for waste products like carbon dioxide from your cells.  The smallest veins are called venules and these are connected to your capillaries.  Your capillaries are where the oxygen from the oxygenated blood carried in your arteries is exchanged for the waste products like carbon dioxide. 

Once your veins carry the de-oxygenated blood through your lungs, your blood becomes a bright red colour again because it swaps waste products for oxygen.  This bright red oxygenated blood continues its journey to your heart and gets pumped back around to all the different cells in your body, transporting oxygen where it is needed.  Your circulatory system is a great big circle that keeps your cells working!

Did you know…? 

Your veins are filled with about 70 percent of your blood at any one time!

There is only one vein in your body that does not have a valve – it is the vena cava that brings deoxygenated blood back from your body to your heart!