|
|
|
|
Capillaries are specialised for diffusion of substances across their wall. Capillaries are the smallest vessels of the blood circulatory system and form a complex inter linking network. Capillaries have the thinnest walls of all blood vessels and are the major site of gaseous exchange, permitting the transfer of oxygen from blood to tissues, and carbon dioxide from the tissues to the blood. Fluids containing large molecules pass across the capillary walls in both directions. The capillary wall is composed of endothelial cells, a basement membrane, and occasional scattered contractile cells called pericytes. The capillaries form a dense network of narrow, short tubes; they can be as little as 3-4 mm in diameter (i.e. half the diameter of red blood cells), and up to 30-40 mm (these large blood spaces are usually known as sinusoids). On average, capillaries have a diameter of 6-8 mm and are approximately 750 mm long. The total number of capillaries in the body has been estimated to be of the order of 40,000-50,000 million. In the resting state, probably only about 25% of the capillary beds are patent. For exchange of substances to be efficient, it is necessary to have short distances for substances to diffuse, a large surface area (the total cross-sectional area of all the capillaries is about 700 times larger than that of the aorta), and a slow steady flow of blood, about 0.3-0.5 mm/s (the flow velocity is about 700 times lower in the capillaries than in the aorta because of the narrower vessels). This part of the circulatory system is often referred to as the microvasculature. The structure of the microvasculature is modified in different tissues to meet specific functional requirements. Different tissues have varying abundance of capillaries, e.g. dense connective tissue has a poor capillary network as compared to cardiac muscle. Electron microscopy has shown that the nature of the endothelium is not the same in all parts of the circulation. Three different kinds of capillary walls have been identified, and the terms continuous, fenestrated and discontinuous are used to describe them, according to the size of the intercellular gaps or pores present in each. Another modification in the structure of the microvasculature in tissues is the presence of arteriovenous shunts or arteriovenous anastomoses. These are direct connections between the arterial and venous systems that bypass the capillary beds. If these shunts are patent, blood can flow rapidly through the vessels, but does not serve any nutritive purpose. These short connecting vessels have strongly developed muscular control and are under sympathetic nervous control. They are found in many tissues and organs. In the skin, for example, they enable cutaneous blood flow to be increased to allow dissipation of heat from the body surfaces when exercising or in high environmental temperatures. The capillary network, whatever its form, drains into a series of vessels of increasing diameter to form venules and veins. [Human Biology Contents Page] [My Home Page] This page authored by John Ross. Please e-mail any comments or queries to johnross@cwcom.net or leave a message in the guest book. This page last updated on Friday, 09 July 1999 11:13 +0100 |
