An artery is a blood vessel which carries oxygenated blood from the heart left ventricle to the rest of the body. It supplies every tissue and organ with oxygen and nutrients. Its smooth muscle, which forms its middle layer, makes it elastic as its walls can expand outwards when blood pressure increases. It has to be like this, since it must withstand the pressure exerted by the pump (heart), when the ventricular muscle of the heart suddenly contracts during the systole to send blood to every part of the body. Thus, when blood pressure is measured, it is always done on an artery, never on a vein. In anatomical drawings and diagrams, arteries are red colored, while veins are light blue.
The main artery of the body is the aorta, which arises from the left ventricle, ascending and forming an arch, which gives off important branches that supply the head and the upper limbs. Then it runs down parallel to the spinal column as it sends secondary arteries that supply the internal organs. At the base of the abdomen, the aorta divides into two important branches, which are the common iliac arteries. The main artery of the lower limb (thigh and leg) is the femoral artery, while the main artery of the upper limb (arm) is the brachial artery. Meanwhile the head, which include the brain, is supplied by the common carotid and basilar artery.
As it extends throughout the body, the artery keeps giving off secondary branches, just like a tree, forming a network, as it gets smaller and smaller in size, until it becomes so thin that it is called arteriole, which can be thinner than a hair. Finally, the arteriole ends up in a capillary, which is the tiniest blood vessel of the circulatory system. In order to efficiently supply an organ, an artery sends off lateral side branches that link up with another nearby artery; this is called anastomoses, which form a network of arterial blood vessels supplying an organ tissue.
Structure
The artery is made up of three layers of three different tissues: 1) the tunica intima, which is the innermost layer that lines the lumen (opening) of the artery; 2) the tunica media, which is the middle layer; and 3) the tunica adventitia, which is the external coating of the artery. The tunica intima is an endothelial layer, which means it is made up of endothelium (flat epithelial cells that line the inner walls of blood vessels and organs). The tunica media is formed by smooth muscle, which gives the artery elasticity. Meanwhile, the tunica adventitia consists of connective tissue, which give the artery form and consistency.
Innervation
The smooth muscle of tunica media is innervated by the peripheral nervous system (sympathetic and parasympathetic fibers), which means that its constriction and dilation do not depend on our will (cerebral cortex), but on adrenaline and acetylcholine, which are neurotransmitters of the peripheral nervous system.
Below, a diagram of the systemic blood circulation. Arteries are in red, while veins are in blue. The artery coming out of the heart left ventricle is the aorta.
