This structure of the aorta and large arteries is important to their function, which serves as a blood reservoir, or transport system, of oxygenated blood from the heart to the rest of the body. Arteries, because of their elastic nature can stretch with systole or relax with diastole. Their elastic property allows the arteries to oppose the force of the flow. The arteries can “control” (help form a pulsatile) the flow so that when blood reaches the arterioles and eventually the capillaries the stream of blood is milder. The wall of the arterioles contains less elastic fibres because the blood pressure within the arterioles is lower. Arterioles have more smooth muscle cells than large arteries, which allow blood to pass through them and on to the capillaries efficiently because friction is reduced.
Veins
Veins and venules are similar in structure to arteries and arterioles, however veins have a wider inner diameter, which means that they can hold a larger volume of blood. Since it would be unworkable to get all the blood back to the heart as quickly as it left the heart (because there is no pumping mechanism to do so); veins have to be able to hold more blood than arteries. The veins provide a “storage area” for our blood, which is why the diameter of the veins is larger than the artery diameter. At any given time, the majority of our blood is within our veins.
Veins have a thinner smooth muscle layer that allows for the expansion of the vessels and minimize resistance by reducing friction to the flow of blood. Because blood pressure is lower in veins, since the blood has had to travel some distance through the many capillaries, the diameter of muscle walls are needed to contain blood within the veins. Since the blood pressure is lower, the blood has to be pushed towards the heart via skeletal muscles and venous valves. Skeletal muscles surround the veins; and contract to squeeze blood along the veins. Venous valves keep blood moving in only one direction.
Venous valves are formed from the endothelium (vein lining), in places where the endothelial cells bend into the lumen of the vein. Blood is allowed to go toward the heart, but it cannot go back the other way. These types of valves are principally important in getting blood back from the legs.
Capillaries
The capillary structure shows a diameter just large enough to permit the gentle flow of red blood cells to squeeze through in single file. The capillary wall is formed of a single layer of endothelial cells resting on a membrane. The wall of the capillary is only one cell thick and has a large surface area, thereby facilitating rapid diffusion of respiratory gases and soluble food materials between blood and tissues. The major function of the capillaries is to promote exchange of nutrients and metabolic end products between the blood and the interstitial tissues. Such exchanges are facilitated by the presence of specialized junctions, gaps or fenestrations, which increase surface area.
The capillaries have precapillary sphincters (see diagram), which contract to constrict the flow of blood within the capillaries. For example after eating, more blood is required in the digestive system to carry away dissolved food materials, so blood needs to be “directed” from elsewhere, such as the muscles. Therefore, when more blood is needed elsewhere the sphincters can stop the blood flow into some arteries leaving more blood is available to other parts of the circulatory system.
From a functional point of view, the two most important parts of the circulatory system are the heart and capillaries. The heart is obviously important because it is responsible for pumping blood around the body. Capillaries represent the place where exchange of materials takes place, which is the principal function of the circulatory system.
The structural differences between arteries and veins are all based in their relationship to the heart. Since arteries receive blood from the heart, the blood they receive is under a lot of pressure. At the same time, this pressure helps the blood move through the arteries- even when the arteries are opposing gravity (like the carotid artery running towards the head).
In conclusion, the arteries carry oxygenated blood away from the heart, and due to the high pressure of this blood the arteries have thick wall, which contain many muscle fibres. The veins carry deoxygenated blood to the heart and because blood is of a lower pressure, have thinner walls consisting of less muscle fibres. Capillaries are the principal part of the circulatory system; they allow substance exchange.
