Corpus: Arteriole

Arterioles are small arteries that represent the transition from the arteries to the capillaries in the vascular system.

The corresponding venous blood vessels that lead the blood away from the capillary bed are called venules. In some tissues there are also so-called metarterioles between arterioles and capillaries.

Arterioles are the finest blood vessels visible to the naked eye. On the sclera of the eye, for example, they appear as fine, red episcleral vessels. They emerge from the arteries through a flowing transition.

The wall structure of the arterioles is similar to that of the arteries. A distinction is made between intima, media and adventitia. However, the wall consists only of a thin, one to two-layered layer of smooth muscle cells. In addition, reticulin fibres and a non-fenestrated endothelial lining on the lumen side can be seen. In contrast to the arteries, the membrana elastica interna lies directly under the endothelium. The membrana elastica externa typical of larger arteries is missing.

The arterioles gradually merge into the capillaries in the direction of blood flow, losing the muscle cells and the internal elastic membrane.

Arterioles are resistance vessels that can almost completely restrict the flow to the capillary bed of a tissue through their contraction. Even slight changes in their lumen have a major influence on local blood flow and blood pressure. In the event of major blood loss, the constriction of the arterioles in the periphery leads to a centralisation of the blood volume and thus to a concentration on the blood supply to vital organs.

The arterioles are innervated by sympathetic nerve fibres, which mediate vasoconstriction. Vasodilatation is achieved by decreasing sympathetic activity; a dedicated parasympathetic innervation of the arterioles can only be detected in a few tissues, for example in the corpus cavernosum of the penis.

Hypoxaemia, a reduced pH value and the local release of histamine lead to dilatation of the arterioles. Conversely, adrenaline, angiotensin II or serotonin cause these vessels to constrict by increasing the tone of the smooth muscles.