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Arterioles are the smallest branches of the arterial tree. Arterioles vary in diameter ranging from 30 mm (0.03 mm) to 400 mm (0.4 mm). Any artery smaller than 0.5mm in diameter is considered to be an arteriole. The intima of an arteriole is composed of endothelial cells lying on a basement membrane with an underlying fine internal elastic lamina in the larger arterioles.
The arteriolar media is composed of one or two layers of smooth muscle cells. As the arterioles get smaller, the continuous layers of smooth musclebecome progressively discontinuous. In the smallest arterioles, the endothelial cells have basal processes whichpierce the basement membrane and make junctional contacts with the smooth muscle cells. The adventitia of arterioles is insignificant.
The arterioles offer considerable resistance to blood flow because of their very small radius, and are the major site of resistance to flow in the vascular tree. Thus the total peripheral resistance, that is the total resistance to blood flow, is mainly determined by the radius of the arterioles. This area of high resistance to blood flow serves several functions: first, together with the elastic arteries, it converts the pulsatile ejection of blood from the heart into a steady flow through the capillaries; second, if no resistance were present and a high pressure persisted into the capillaries, there would be a considerable loss of blood volume into the tissue by transudation of fluid across the capillary wall. The pulmonary circulation is a low pressure circulation partly to prevent this happening; if pulmonary pressures increase for some pathological reason, pulmonary oedema may develop. The arterioles are also important in determining the blood supply to different tissues and regions. There are specialised regions near the junction between the terminal (smallest) arterioles and the capillaries known as pre capillary sphincters, which consist of a few smooth muscle cells arranged circularly. If the sphincters are relaxed and the lumen patent, the capillary beds distal to the sphincter are open and perfused. If the sphincters are partially constricted, blood flow to the capillaries will be reduced, and if fully contracted, no blood will flow through. In active muscle, for instance, many more capillaries are patent due to relaxation of the sphincters and thus blood flow is increased; this has the effect of greatly increasing the surface area available for exchange of substances and at the same time reduces the distance across which substances have to diffuse to reach the cells. The mechanisms that control the arteriolar radius and pre capillary sphincters will be considered in detail later. Altering the radius is the normal mechanism for controlling the resistance and altering blood flow, and both the sympathetic nervous system and local factors are involved. The microvasculature starts at the level of the arterioles. It is composed of small diameter blood vessels with partly permeable thin walls that permit the transfer of some blood components to the tissues and vice versa. The structure of the microvasculature varies in different tissues in order to meet specific functional requirements. Some tissues have a much more abundant network of capillaries than others. For example dense connective tissue has a poor capillary network when compared to cardiac tissue or that of the kidneys and liver. Most of this exchange between blood and tissues occurs in the extensive capillary network, the smallest arterioles (metarterioles) emptying into the capillary system. The capillary networks drain into the first components of the venous system, the venules. [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 Wednesday, 30 June 1999 15:05 +0100 |
Diagram of a medium
sized arteriole
Diagram showing a
small arteriole