DESCRIBE THE STRUCTURE OF ARTERIES AND VEINS AND CAPILLARIES AND RELATE THIS TO THEIR FUNCTION

Humans like all multicellular organisms need oxygen for cellular respiration, without this process occuring then we would die.
Oxygen needs to be able to travel to every cell of the body quickly and efficiently. Blood also needs to remove unwanted substances from the body and to distribute substances that the body needs, such as nutrients. This major task can only be carried out due to the vast array of blood vessels that the body has. These blood vessels make a large transport network throughout the body together with the heart and the blood itself they form the circulatory system.
However there are 3 main types of blood vessels; arteries, veins and capillaries. They all are part of the circulatory system, but they are all structurally different but have some similarities such as; they all are hollow tubes that have some form elasticity. They are different for a reason; they needed to be different because they all have different functions.



Image found at http://www.hcc.uce.ac.uk/physiology/circulation02.htm

Arteries
The function of arteries is to carry oxygenated blood through the body’s systemic circulatory system.
Arteries have thick muscular walls. This allows them to be strong and stay open when blood is pumped into them from the heart at high pressure. The lumen (the hollow centre) of the arteries is quite small. However, the endothelium (the lining of the lumen) is folded. The endothelium being folded is necessary. In conjunction with the elastic tissue that is encased in the walls of the artery, this allows for expansion. Expansion is needed when blood is pumped through the arteries due to the high pressure of the blood. Expansion may also be needed if the cardio vascular system needs to work harder, such as during times of exercise.
Veins
The function of veins is to transport deoxygenated blood back to the heart during systemic circulation.
Veins are structurally different to arteries. Although they contain some elasticity, there is not an abundant amount of elastic fibres in the veins wall, unlike arteries. They also have only a thin muscle wall and not a thick muscle wall like arteries. They do not need their walls to be tolerant of high pressure blood flow. This is because deoxygenated blood does not travel at high pressure; it has not just left the heart. The lumen is large. It has a smooth endothelium, since there is no need for expansion. However, when blood is returning back to the heart, it has to defeat gravity. This is done by the veins due to the use of valves. These valves stop the blood flowing backwards.
Capillaries
. According to Ivy Rose (2010) capillaries function “Is to supply tissues with components of, and carried by the blood and also to remove waste, from the surrounding cells as opposed to simply moving the blood around the body (in the case of other blood vessels)”.

These are the smallest blood vessels of the circulatory system. There are thousands of capillaries in the human body and usually found in close proximity to each other. Due to this fact they are commonly referred to as capillary beds. To achieve their function, capillaries are only one cell thick. This thickness is necessary for diffusion of metabolic substances and other substances. For diffusion to occur and be effective the diffusion pathway needs to be short i.e. one cell thick.
Due to capillaries being the smallest blood vessels, this also aids diffusion. Blood cells can only travel through capillaries in single file. This enables optimum diffusion in and out of the cells. Due to their function capillaries carry oxygenated blood and deoxygenated blood.

It is important to point out there are other blood vessels that are present in the systemic circulatory system. There are arterioles and venules. Arteries are structurally identically to arteries, just narrower and venules are structurally the same are veins, just narrower.
The pathway of blood flow, from the heart is
Arteries-Arterioles-Capillaries-Venules-Vein