The human cardiovascular (CV) system has three types of blood vessels: arteries, capillaries and veins. Circulation through the CV system goes from the heart through the arteries to the capillaries, on to the veins and back to the heart. The arteries and veins are pathways only, transporting blood to and from the capillaries respectively. It is at the capillary web or bed, a lace-like network of capillaries, that substances transfer back and forth between the CV system and the organs and tissues of our bodies. Our CV system has two closed circuits, the pulmonary that takes blood from the heart to the lungs and back, and the systemic that takes blood from the heart through the rest of the body and back.
There are three types of arteries: elastic, muscular and arterioles. Elastic are adjacent to the heart, such as the aorta, cardiac artery and pulmonary arteries. They can expand considerably to take the sudden surge of blood pumped by each heart beat, then constrict to force it on with a relatively uniform blood pressure. Muscular arteries are smaller, dividing to take blood to all the organs and tissue of the body. Their walls have a high concentration of smooth muscle that can contract to reduce the diameter of the artery if triggered by nerve impulses or hormones, an action called vasoconstriction. Arteriole literally means small artery, each muscular artery divides into multiple arterioles that then divide into multiple capillaries.
There are two types of capillary: thoroughfare channels and true capillaries. These are identical in their anatomical structure but different in purpose. Thoroughfare channels are straight connections between arterioles and venules, the smallest veins. When blood pressure is high, blood can pass quickly down these channels to relieve the pressure in the capillary bed. The true capillaries are more meandering, spreading as a network throughout the tissue before joining together again to flow into the venules.
There three types of veins: venules, veins and vena cavae. The venules are the smallest veins formed by the joining of several capillaries. They in turn, join together to form veins that join in turn, sometimes several times, leading to the largest veins, the vena cavae. Veins are larger than their equivalent arteries, but have thinner, less muscled walls; 65 percent of the body's blood is in the veins at any given moment. The veins have valves in the walls that stop backflow, particularly important in the legs. The vena cavae are the largest veins, but do not have valves in the walls, allowing the smooth flow of blood into the right atrium of the heart. The lymphatic system empties into the vena cavae, increasing the plasma level of the blood and its carbon dioxide level before it goes to the lungs. The inferior vena cava carries all the blood from the lower body into the heart, the superior vena cava does the same for the upper body.
As far as the process of circulation is concerned, the capillaries are just as essential as the other blood vessels, the arteries and veins, in completing the circuits. However, if we consider the purpose of circulation, we may perceive them as more important. The circulatory or CV system has one function, to transport substances from one part of the body to another in a speedy manner. Imagine how long it would take carbon dioxide produced in a muscle in your foot to reach your lungs to be expelled from the body, if there was no circulatory system to carry it along.
The arteries are important because they are the flexible "pipes" that quickly carry blood from the heart to where it needs to be. The veins are important because they slowly return the blood to the heart, which allows them to act as the primary blood reservoir of the CV system as well as the return "pipes". But the capillaries are most important because they leak, they are the "faucets" and "drain inlets" of the CV system, where substances enter and leave. Without capillaries, there would be no point in having a circulatory system.
Capillaries are the smallest of blood vessels, only wide enough to allow the passage of erythrocytes (red blood cells) in single file, forming a web or lace that spreads through the tissue that they serve. Arterioles supply relatively high pressured blood to the capillaries. The walls of capillaries are just one cell thick, the blood pressure at the end attached to the arteriole forces fluid out between the cells of the wall. The fluid is thinner than plasma, the non-blood cell component of blood, because the larger blood proteins can't fit through.
Nutrients and dissolved gases are carried through the cell wall with this fluid, to mix with the interstitial fluid that surrounds the capillary and the cells of the tissue around it. The blood pressure within the capillary eases the further you get from the arteriole end, as the fluid leaks out. The flow of fluid from the arteriole end increases the pressure of the interstitial fluid fractionally, so that fluid leaks back into the capillary towards the venule end.
The result of this fluid exchange is that any substance that was in greater concentration within the blood, such as oxygen in the systemic circuit, will leave an increased amount behind in the interstitial fluid, while any substance that was in greater concentration in the interstitial fluid, such as carbon dioxide, will pass more into the blood, reducing the amount in the interstitial fluid. This is a continuous process, occurring with every beat of your heart, every day of your life.