Vénules

Vénules are small veins in the circulatory system. They are the smallest veins and are formed by the convergence of capillaries. Vénules collect deoxygenated blood from the capillaries and transport it towards larger veins, eventually leading back to the heart.

There are two main types of vénules: postcapillary vénules and muscular vénules. Postcapillary vénules are the smallest and are directly connected to the capillary beds. They have a thin wall with a limited amount of smooth muscle. Muscular vénules are slightly larger and have a more developed layer of smooth muscle in their walls, allowing them to control blood flow to some extent.

The walls of vénules are composed of three layers, similar to arteries and larger veins, but significantly thinner. The tunica intima, the innermost layer, consists of endothelium. The tunica media, the middle layer, contains smooth muscle cells and elastic fibers, though it is much thinner in vénules than in arteries. The tunica externa, the outermost layer, is made of connective tissue.

Vénules play a crucial role in the inflammatory response. Their thin walls and the presence of specific adhesion molecules on their endothelial cells allow leukocytes (white blood cells) to exit the bloodstream and migrate to sites of infection or injury. This process is known as diapedesis.

The pressure within vénules is very low, much lower than in arteries. This is because the blood has already passed through the high-resistance capillary network. The slow flow of blood in vénules, combined with their thin walls, makes them susceptible to dilation and increased permeability during inflammation.