Blood vessel

A blood vessel is a hollow, tube-like structure in the body through which blood flows. Blood vessels, the heart, and blood together make up the circulatory system. The heart pumps blood, while vessels distribute it to all the body's tissues.

The primary function of blood vessels is to deliver oxygen, nutrients, hormones, and other substances to tissues throughout the body, and to remove waste products such as carbon dioxide. They also help regulate body temperature and distribute immune cells.

Types

Three main types exist. Arteries carry blood from the heart to the body; their walls are thick and muscular to withstand high pressure. Veins return blood to the heart and often contain small valves that prevent blood from flowing backward. Capillaries are the smallest vessels; their walls are just one cell thick, and it is here that oxygen and nutrients are exchanged.

Wall structure

The walls of arteries and larger veins consist mainly of three layers: an inner layer of thin tissue (endothelium) through which blood flows, a middle layer of muscle and elastic fibers, and an outer connective tissue layer. The muscle in the middle layer constricts or relaxes the vessel, which affects blood pressure and flow.

Related conditions

Blood vessels can be affected by several conditions. Thickening of arteries and buildup of plaque in vessel walls (atherosclerosis) reduces blood flow and increases the risk of heart attack and stroke. High blood pressure strains the vessels. Varicose veins in the legs result from valves that fail to close properly. Blood clots (thrombosis) can block flow.