The function of arteries and veins is to carry blood over relatively long distances from one organ to another, whereas capillaries form branching networks to carry blood over relatively short distances within organs. Arteries carry blood away from the heart, veins carry blood from the tissues and return it to the heart. All three have a similar structure, however it differs in terms of the layers of tissue in the walls of each along with the size of the lumen – each vessel is structurally modified such that is best carries out its transport function.
- Capillaries consist of an endothelium which is only one cell thick
- Walls of arteries and veins consist of 3 layers
- The inner layer consists of a thin layer of endothelial cells
- The middle layer is made up of smooth muscle with some elastic fibres. This layer controls the diameter of the vessel and hence the amount of blood and its rate of flow
- The outer layer is composed of connective tissue, this holds the blood vessels in place in the body
- The walls of arteries are much thicker as it carries blood away from the heart at high pressure
- Major arteries close to the heart also have thick layers of smooth muscle in their walls to withstand the increases in pressure as the heart pumps
- The walls also have a large proportion of elastic fibres in both the inner and middle layers – this allows for the arteries to stretch according to the increases in volume of blood. As the heart relaxes the artery walls return to their original position, hence pushing the blood along – maintaining a constant flow in one direction.
- Arteries near the surface of the skin, the changes in the arteries diameter can be felt as a pulse.
- The walls of veins are thinner than the walls of arteries, as the blood they receive from the capillaries is at a much lower pressure.
- The walls have fewer elastic fibres and the lumen is wider (to allow for easier blood flow)
- Veins have two mechanisms for keeping the blood flow constant and in one direction. Firstly many veins are close to muscles, hence when the muscles contract they compress the walls of the vein – pumping blood forwards. Veins also have valves (small pocket like folds of the endothelium lining the lumen of veins), they are spaced along regular intervals in veins. They work much like one-way swinging doors – as the blood is forced through the valve opens, however once the pressure drops and the blood flow decreases, the valve shuts – preventing backflow of blood.
- They are extremely tiny microscopic vessels that bring blood into close contact with the tissues, for the exchange of chemical substances between cells and the bloodstream.
- The one cell thick endothelial layer is a continuation of the lumen arteries and veins
- Diffusion is a relatively slow process and hence the structure of capillaries is suited to slowing down the flow of blood
- In order to maximize the exchange of substances between the blood and cells capillaries have, thin walls (more efficient diffusion) a small lumen (forces blood cells to pass through in single file, slowing down the rate of flow and maximizing their exposed surface area)
- They form an expansive blood flow network, such that no cells are far from blood supply