Arteriole, venule and capillary histology

The circulatory system consists of two functional parts, the lymphatic system and the blood circulatory system.

The microvasculature is part of the blood circulatory system and consists of the arterioles, capillaries, and venules.

This is the portion of the circulatory system that’s responsible for the exchange of gases, nutrients, fluids, and metabolic waste.

The majority of this exchange occurs in the capillaries.

The arterioles are continuations of the small muscular arteries that can constrict in order to control the amount of blood that flows into the capillary beds.

There are also muscular precapillary sphincters between the arterioles and capillaries called precapillary sphincters, that also help control how much blood flows to the capillary beds.

The capillaries drain the blood into the postcapillary venules, then the collecting venules, and finally the small muscular venules.

The small muscular venules gradually increases in diameter and eventually drain into small veins.

Alright, this image is an example of an arteriole from the abdominal mesentery.

Similar to most blood vessels, the arteriolar wall has three main layers, the tunica intima, tunica media, and tunica externa.

The tunica intima consists of an internal elastic membrane or lamina and an inner lining of flat endothelial cells with round nuclei.

The tunica media consists of 1 to 2 layers of circularly arranged smooth muscle.

The tunica externa surrounding the smooth muscle is a very thin layer of fibrous tissue that can be difficult to identify in images like this.

Here’s an example of a smaller arteriole.

We can see 3 nuclei from endothelial cells and two smooth muscle nuclei.

In smaller arterioles like this one, the internal elastic lamina is not present.

After the arterioles, blood flows into the capillaries, which are the blood vessels with the smallest diameter in the body.

The walls of the capillaries are composed of only a single layer of endothelium surrounded by a very thin basement membrane.

There are a few types of capillaries, but the most common type is the continuous capillary.