Blood vessels are a network of tubes which transport blood to different parts of our body. They are a part of the circulatory system, along with the Heart. Their purpose is to take blood to and away from heart and circulate it to every organ, tissue and cell of the body, thus, taking nutrients, oxygen and other chemicals to the cells and organs, and carbon dioxide and toxins away from them. Blood vessels are of three main types, Arteries, Veins and Capillaries. Arteries are further sub divided into Arterioles and Veins into Venules. Arteries and Arterioles carry oxygenated blood away from the heart, while Veins and Venules carry deoxygenated blood back to the heart. Capillaries are microscopic vessels about the size of cells, which serve the most important purpose, exchanging oxygen and carbon dioxide and other nutrients to and from the cells and tissues.
The general structure of both the Arteries and Veins is same, in that they both have a wall and a lumen. Veins have valves to prevent the blood from flowing the wrong way. Blood in veins flow in pulses, with every beat of our heart (hence we get the pulse in our veins at wrist, neck, etc.) This is simply the pressure created by our heart as it pumps the blood out, due to which it flows through the Arteries out of the heart, and pushes the blood from Veins back to the heart. Arterioles and Venules, are smaller versions of Arteries and Veins, and reach to tissues and organs where major blood vessels cannot reach. The Aorta is the largest artery in our body and the Vena Cavae are the two largest veins. The Aorta is further divided into the Carotid artery, Subclavian artery, Celiac Trunk, Mesenteric
arteries, Iliac artery and the Renal artery. Veins are sub divided into Renal vein, Subclavian vein, Iliac vein and the Jugular vein. Although the arteries carry oxygenated blood and veins the deoxygenated blood, there is an exception to this rule. The Pulmonary artery carries deoxygenated blood to the lungs for oxygenation, which is then brought back to the heart by the Pulmonary vein after being oxygenated. This shows, that one can only generalise to an extent, that arteries only carry blood away from the heart, while veins carry blood to the heart.
The pressure required to circulate the blood through these blood vessels is called blood pressure and is created by the expansion and contraction of our heart, which pushes blood out and takes blood inside it. Blood in the veins, however, is pushed up by the movement of skeletal muscles.
Reference: Introduction to Human Anatomy and Physiology, by Tortora
