The circulatory system consists of two functional parts, the lymphatic system and the blood circulatory system.
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.
These capillaries have many tight junctions joining the endothelial cells together in order to form a very thin tube.
As a result, larger molecules and proteins leaving or entering the capillary are regulated by the endothelial cells, since any substances that leave the capillary will have the pass through the layer of endothelial cells.
This image shows a cross-section of a few continuous capillaries surrounded by adipose tissue.
The cytoplasm of the endothelial cells are difficult to see with light microscopy, but we can still see the nuclei of the endothelial cells in the upper left and upper right capillaries stained dark purple, as well as a single red blood cell or RBC in each of their lumens.
In general, the diameter of the capillaries are about the same as a single RBC, which can be a useful tool to gauge the size of surrounding structures in an image, since RBCs normally have a diameter of 6 to 8 µm.
In longitudinal sections of capillaries, the nuclei will look more elongated compared to cross-sections.
Occasionally, you can also see flattened supportive cells that surround the capillary called pericytes.
In this image, we can see a pericyte along the bottom of the capillary.
Fenestrated capillaries also have tight junctions, but perforations or fenestrations through the endothelial cells allow greater exchange across the endothelium.
Similar to the continuous capillaries, the basement membrane is also continuous with no perforations.
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