Blood vessel

Blood vessels are the tubular structures of a circulatory system transporting blood in animal bodies. Blood vessels transport blood cells, nutrients, and oxygen to most of the tissues of a body, and also transport waste products and carbon dioxide away from the tissues. Some tissues – such as cartilage, epithelium, and the lens and cornea of the eye – are not supplied with blood vessels, so are termed avascular.

There are five types of blood vessels: the arteries, which carry the blood away from the heart; the arterioles; the capillaries, where the exchange of water and chemicals between the blood and tissues occurs; the venules; and the veins, which carry blood from the capillaries back towards the heart.

The word, vascular, is derived from the Latin vas, meaning vessel, and is used in reference to blood vessels.

The arteries and veins have three layers. The middle layer is thicker in the arteries than it is in the veins:

Capillaries consist of a single layer of endothelial cells with a supporting subendothelium consisting of a basement membrane and connective tissue. When blood vessels connect to form a region of diffuse vascular supply, it is called an anastomosis. Anastomoses provide alternative routes for blood to flow through in case of blockages. Veins can have valves that prevent the backflow of the blood that was being pumped against gravity by the surrounding muscles. In humans, arteries do not have valves except for the two 'arteries' that originate from the heart's ventricles.

Early estimates by Danish physiologist August Krogh suggested that the total length of capillaries in human muscles could reach approximately 100,000 kilometres (62,000 mi) (assuming a high muscle mass human body, like that of a bodybuilder). However, later studies suggest a more conservative figure of 9,000–19,000 kilometres (5,600–11,800 mi) taking into account updated capillary density and average muscle mass in adults.

There are various kinds of blood vessels:

They are roughly grouped as "arterial" and "venous", determined by whether the blood in it is flowing away from (arterial) or toward (venous) the heart. The term "arterial blood" is nevertheless used to indicate blood high in oxygen, although the pulmonary artery carries "venous blood" and blood flowing in the pulmonary vein is rich in oxygen. This is because they are carrying the blood to and from the lungs, respectively, to be oxygenated.^{[citation needed]}