Your arteries 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 arteries are the blood vessels that carry blood away from your heart.  These blood vessels are tiny but they have very thick walls so that when the blood is pumped from your heart, the arteries don’t burst!  Remember, your heart is a pump and it squeezes blood out of its chambers at very high pressure.  If the arteries didn’t have thick walls, they would leak or collapse when your heart pumped, which wouldn’t be very good for you!

Even though your arteries are tiny, they are very tough!  They are made up of 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. 

The middle layer of smooth muscle in your arteries is very stretchy and very strong so your arteries move as your heart pumps blood through them.  As your heart pumps blood into your arteries, they expand and when the blood passes, your artery contracts.  This is your pulse!  Your pulse rate is at the same rhythm as your heart beats, so not only can you feel your blood flowing through your arteries, you can measure your heart rate by taking your pulse!   

There are lots of different types of arteries, some as thick as your thumb and some smaller that a hair on your head!  As blood travels away from your heart, your arteries get smaller and smaller.  When they are very small, they are called arterioles and finally, when they are really tiny, they are called capillaries.  Capillaries are so small that blood can usually only get through them one cell at a time.  As each blood cell moves through your capillaries, the oxygen carried by red blood cells is taken by your body’s cells and exchanged for waste products like carbon dioxide which is removed by the blood.

The blood that is carried away from your heart by your arteries is bright red in colour.  This is because it is oxygenated – it has lots of oxygen in it.  The oxygen is carried by haemoglobin in your red blood cells and when it gets to your capillaries, this oxygen is given to every cell in your body.  There is an exception to this rule though - your pulmonary artery, one of the most important arteries in your body, is a little different.  It carries de-oxygenated blood (blood with no oxygen in it) away from your heart to your lungs (pulmonary just means to do with your lungs).

Once your arteries become small enough to be called capillaries and the exchange of oxygen for carbon dioxide takes place, your capillaries start to get bigger and become veins.  Veins bring blood back to your heart, picking up oxygen from your lungs along the way.  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 from your arteries 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 makes sure everything is connected! 

What happens if you have a problem with your arteries?  Sometimes people have a little weak spot in their arteries, at any point in their body.  When the artery is put under a lot of pressure, or even a little pressure for a long time, this weak spot can burst and cause problems.  This is called an aneurysm.  Other problems can happen when people have a lot of bad cholesterol in their diet or smoke a lot, in combination with other things like high levels of stress or high blood sugar (like in diabetes) for a long time.  The stretchy layer of the artery (the layer of smooth muscle) can become harder and less stretchy.  It is a condition called atherosclerosis and can lead to serious health problems.

When you hear that you have to ‘look after your heart’, don’t forget that this also means your arteries!  If you remember to eat a healthy diet, keep fit and avoid things like smoking, you will do the best you can for your heart!