What Are Arteries?

The layers of the arteries are tunicae. The outermost layer is the tunica externa, though some researchers refer to it as the tunica adventitia. This layer consists primarily of collagen, which anchors the arteries to nearby organs for stability. The layer also has bundles of the protein elastin, which form elastic fibers. In addition, the tunica externa contains the vasa vasorum, a network of small blood vessels that supply the walls of bigger blood vessels. Elastic arteries such as the aorta receive blood and nourishment from the vasa vasorum.

sitox / Getty Images

The tunica externa surrounds the tunica media. In smaller arteries, the tunica media consists primarily of smooth muscle fibers. Most small arteries have a single layer of these fibers, though larger arteries can have up to six. The femoral and other large arteries have a combination of elastic fibers and collagen that alternate with the smooth muscle layers. The largest arteries, such as the aorta, have a significant amount of elastic tissue.

alex-mit / Getty Images

The innermost layer of an artery is the tunica intima, a layer of endothelial cells that filter fluids, assist with blood clotting, and cause inflammation. An internal elastic lamina made of elastin fibers supports the tunica intima and separates it from the media. In the elastic arteries, this layer has a thin supporting layer of collagen and myointimal cells, smooth muscle cells and fibroblasts that collect lipids.

7activestudio / Getty Images

Large arteries such as the aorta and pulmonary artery are elastic arteries. They contain a large amount of collagen and elastin fibers that allow them to stretch more than the other arteries. This elasticity also allows the arteries to maintain a consistent pressure. When the heart contracts, the walls stretch to hold a large amount of blood. When the heart rests, the arteries constrict to maintain blood flow. The elastic arteries connect to the muscular arteries and transfer blood to them.

Bangkokerz / Getty Images

Muscular arteries are also called distributing arteries; these medium-sized vessels draw blood from the elastic arteries and distribute them to the smaller vessels. The femoral and coronary arteries are examples of muscular arteries. The walls consist of large amounts of smooth muscle, allowing them to contract and dilate. However, since these arteries have less elastin, they cannot stretch as much as the large elastic arteries.

gecko753 / Getty Images

Blood travels from the distributing arteries to the arterioles, smaller arteries that control blood flow to even smaller blood vessels -- the capillaries. Smooth muscle can contract or dilate the arterioles, making them the primary source for vascular resistance. This resistance must be overcome to create blood flow. Because there is a decrease in blood velocity between the arterioles and the capillaries, blood pressure rises.

wildpixel / Getty Images

Though there are many arteries, the aorta stands out as one of the most important. It is the root systemic artery and receives blood directly from the left ventricle of the heart. The aorta is so large that anatomical sources frequently divide it into sections. Some medical sources divide the aorta based on its location in the body, while others base the sections on the flow of blood. The aortic arch loops over the left pulmonary artery. It contains special cells that relay information about blood pressure, pH levels, and carbon dioxide levels to the brain.

Raycat / Getty Images

When the arteries stiffen, they can restrict the flow of blood. Atherosclerosis occurs when fats, lipids, and other substances build up along the walls of the arteries. Though many people consider this a heart issue, atherosclerosis can occur in any artery of the body. In cases of extreme buildup, blood clots form. Symptoms are gradual and may be difficult to notice unless very little blood can travel through the arteries. Atherosclerosis can cause pain, numbness, high blood pressure, or difficulty speaking.

metamorworks / Getty Images

Rarely, the arteries narrow abnormally and prevent blood flow. When this occurs in arteries that don’t supply the heart or brain, peripheral artery disease develops. This disease primarily affects the legs, though any artery can narrow. Affected individuals may experience cramping, discomfort, or pain. A person may also feel that one arm or leg is significantly colder than the other. The causes of peripheral artery diseases are similar to the causes of atherosclerosis -- anything that could cause a buildup in the arteries, including smoking, diabetes, and hypertension.

colematt / Getty Images

The arteries are blood vessels, just like the veins. Medical professionals rely on the veins to transport fluids and medication they administer intravenously. However, it is possible to confuse an artery for a normal vein. When an intra-arterial injection occurs, there can be serious side effects. Paresthesia is an abnormal dermal sensation such as tingling, burning, or tickling -- a common symptom of intra-arterial injection. Death of cells and skin tissue can also occur. Treatment requires rest, antiplatelet agents, and anti-sludging agents to clear the artery of any obstructions.

fotografixx / Getty Images