Artery

An artery is a specialized blood vessel found in humans and most other animals.

Its primary role is to transport blood away from the heart. In the systemic circulation, these vessels carry oxygenated blood to various parts of the body. However, there are two important exceptions to this rule. The pulmonary arteries carry deoxygenated blood from the heart to the lungs for oxygenation. The umbilical arteries carry deoxygenated blood from a fetus to the placenta.

Understanding arteries is essential because they are the high-pressure delivery system for the entire body.

The structure of an artery is a multi-layered, tube-shaped channel. This channel features a hollow internal cavity known as the lumen, where blood flows. The wall of the artery consists of three distinct layers called tunics. The innermost layer is the tunica intima, which is in direct contact with the blood. It is composed mainly of endothelial cells. The middle layer is the tunica media, which contains smooth muscle cells, elastic tissue, and collagen fibers. The outermost layer is the tunica externa, also called the tunica adventitia. This layer is made of collagen fibers and elastic tissue. In the largest arteries, the tunica externa even contains small blood vessels called vasa vasorum to supply the vessel walls.

Arteries can be categorized into different types based on their size and composition. The systemic arteries are divided into muscular and elastic types. Elastic arteries are generally larger, with a diameter greater than 10 mm. These vessels rely on high amounts of elastic tissue to handle blood flow. Muscular arteries are smaller, typically ranging from 0.1 mm to 10 mm in diameter. As the arterial system branches, it leads to smaller vessels called arterioles. Arterioles act like adjustable nozzles for the circulatory system. They use smooth muscle to regulate local blood flow and overall blood pressure. Finally, the system reaches the capillaries, which are the smallest vessels. These are only about 5 micrometers in diameter, which is even smaller than a red blood cell.

History shows how our understanding of these vessels has changed significantly over time. In ancient Greece, before the time of Hippocrates, all blood vessels were called phlebes. At that time, the word arteria actually referred to the windpipe. This misunderstanding occurred because people found arteries in cadavers that were devoid of blood. Later, thinkers like Diogenes of Apollonia believed arteries transported a substance called pneuma, or air. In the medieval period, some believed arteries carried "vital spirits" instead of blood. This changed in the 17th century when William Harvey described the modern concept of the circulatory system. Later, in the 20th century, Alexis Carrel developed techniques for vascular suturing. His work with anastomosis allowed for successful animal organ transplantations and paved the way for modern surgery.

The mechanics of blood flow involve significant pressure changes. Arteries maintain a much higher pressure than other parts of the circulatory system. This pressure fluctuates during the cardiac cycle. It reaches its peak when the heart contracts and its lowest point when the heart relaxes. This variation in pressure creates the pulse that can be felt in areas like the radial artery. The amount of blood ejected by each heartbeat, known as stroke volume, helps determine pulse pressure. Arterial blood pressure is also determined by the combination of cardiac output and systemic vascular resistance. This resistance is the collective effort of all the arterioles in the body to slow or direct blood flow.

The aorta is the most notable example of a major artery.

It is the largest artery in the human body and serves as the root systemic artery. The aorta receives blood directly from the left ventricle of the heart through the aortic valve. The first branches to leave the aorta are the coronary arteries, which provide oxygenated blood to the heart muscle itself. Other early branches include the brachiocephalic artery, the left common carotid, and the left subclavian arteries. From the aorta, blood travels through peripheral arteries into smaller arterioles, and eventually into the capillaries for nutrient exchange.

Maintaining the health of these vessels is vital for survival. Over time, various factors can damage the endothelial cells and the arterial walls. These factors include high blood pressure, stress, smoking, and elevated blood sugar from diabetes mellitus. Such damage can lead to a disease called atherosclerosis.

Atherosclerosis is characterized by the hardening of the arteries. This happens when an atheroma, or plaque, builds up in the wall. This plaque is made of cell debris, lipids like cholesterol, calcium, and fibrous connective tissue. If an artery is cut, the high pressure can cause a rapid, life-threatening arterial gush. Keeping the tunics strong and the lumen clear is essential for a healthy circulatory system.