In this post, I will look at the structure and function of the three main types of blood vessel in the human circulatory system.  Although this is not the most difficult topic, there are a few things that can catch out even A* GCSE students in the heat of an exam.

Arteries are the blood vessels that take blood away from the heart.  Because the blood is coming straight from the ventricles of the heart, it will be at a high blood pressure and will flow in pulses.  This means that arteries need a thick wall to withstand this high blood pressure.  All arteries apart from one carry oxygenated blood.  Can you remember which artery is the exception to this rule?

The artery wall has a narrow lumen (the space where the blood flows) as this helps to maintain the high blood pressure within.  There are also many elastic fibres in the middle tissue (tunica media) of the artery wall.  This elastic tissue is important because the blood flows in pulses.  The artery wall needs to stretch as the pulse of blood passes and the elastic recoil of the wall helps to push the blood along in between heart beats.

The tunica media also contains a lot of smooth muscle.  Why do arteries need muscle in their walls?  When this muscle contracts it narrows the lumen of the vessel.  This will increase the blood pressure and so one reason for muscle in arteries is to regulate the blood pressure.  But there is something more…  Arteries carry blood into the organs of the body and the pattern of blood flow to different organs can vary depending on the conditions.  For example, when you are running, you need more blood to go to your skeletal muscles (to carry oxygen for respiration and to remove heat and carbon dioxide) and less to go to the digestive system.  This is brought about by the smooth muscle in the artery taking blood to the intestines and stomach contracting so that less blood can flow through the vessel.  The smooth muscle in the arteries in the exercising muscles will relax so that more blood can pass.  This shift in the pattern of blood flow is the second key significance of arteries having lots of muscle in the walls.

Veins have the same tissues in their walls as arteries but they are much thinner.  The blood is flowing at a much lower pressure in veins as all the pressure from the heart has been lost in the extensive capillary beds in the tissues.  Veins return blood to the heart and all bar one (the pulmonary vein) contain deoxygenated blood.  As there is low blood pressure in veins, this can cause problems moving blood back to the heart especially when against gravity.  Veins contain valves which only allow blood through in one direction thus preventing the blood falling back.  The thin walls of veins also mean that they can be compressed by the action of skeletal muscles.  When the muscles that move the skeleton contract, they can squeeze on veins and help to return blood to the heart.

Capillaries are the smallest of the three types of blood vessel.  They are found in the tissues throughout the body and are beautifully adapted to ensure the exchange of materials between the cells of the body and the blood.  The lumen of a capillary is less than the width of a red blood cell and so red blood cells pass through capillaries in single file and only be squeezing along.  This ensures the speed of blood flow in capillaries is very very slow.

The lining of a capillary is made up of a single layer of cells called the endothelium.  Arteries and Veins have an endothelium too but in the capillary the endothelial cells have gaps between them called pores.  This allows the fluid component of blood and various white blood cells to leak out of capillaries to form tissue fluid.  This leaky nature of capillaries is very important as it provides the fluid that bathes the tissues of the body.