Blood vessel

From ArticleWorld

A blood vessel is a tubular structure that, along with other blood vessels in the body, forms the means of transportation of blood. In almost all animal species, some form of blood vessel or blood vessels exists and together, they form an important part of the circulatory system.

Types

In humans and other higher animals, there are several types of blood vessels, each defined by size and/or function. The arterial system contains arteries, of which the largest is the aorta. Arteries have thick walls and carry blood away from the heart. Almost all arteries carry oxygen-rich blood with the exception of the pulmonary artery which leaves the right ventricle of the heart and carries oxygen-depleted blood to the lungs. Small arteries are called arterioles.

A capillary is the smallest type of blood vessel. It carries the important function of exchanging molecules, nutrients, oxygen and carbon dioxide between the blood and the surrounding cells. Capillaries are generally thin-walled, having walls of only one cell in thickness.

The venous system consists of the blood vessels that pick up the blood after the capillaries have exchanged oxygen for carbon dioxide. Veins, generally, carry the oxygen-depleted blood back to the heart, along with the end-products of metabolism. Small veins are called venules and the largest veins are the superior vena cava and the inferior vena cava which both drain directly into the heart. The pulmonary vein, however, does not carry oxygen-depleted blood and instead carries the oxygen-rich blood from the lungs to the left atrium of the heart.

On a smaller scale, blood vessels generally have an inner layer, called the endothelium, a middle layer, containing smooth muscle cells, and an outer layer, made of connective tissue and nerve endings. Capillaries differ somewhat, in that they have only an endothelium and a small amount of supportive connective tissue.

Physiology

In general, the blood flows through blood vessels as a result of the pressure generated by the pumping of the heart and the blood vessels are passive conduits. Arteries, however, can help somewhat in the transportation of blood by using the smooth muscle layer to contract or relax, leading to “vasoconstriction” or the narrowing of a blood vessel, or to “vasodilatation”, when the smooth muscle is relaxed and the blood vessel is dilated.

Various factors play a role in the permeability or “leakiness” of a blood vessel. In situations where there is localized infection or inflammation, the affected area releases histamine, prostaglandins and interleukins that open the capillaries and small blood vessels to allow cells and molecules important in the process of fighting infection or inflammation to enter the tissues.