Arteriole, venule and capillary histology

Definitions & Key takeaways

Arterioles, venules, and capillaries are all small blood vessels that play an important role in the cardiovascular system. Each type of vessel has a different structure and function, but they all work together to transport blood throughout the body.

Arterioles are the smallest arteries in the body and they are responsible for regulating blood flow by constricting or dilating in response to various signals. Venules are small veins that collect blood from the capillaries and return it to the larger veins, whereas capillaries form a network between arterioles and venules, and provide a path for nutrients, oxygen, carbon dioxide, and wastes exchange between the bloodstream and tissues.

The histology of each type of vessel is different, but all are lined with a layer of endothelial cells. The smooth muscle cells that make up the walls of arterioles and venules are arranged in layers, while the capillaries only have a single layer of endothelial cells.

Chapters:

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Introduction0:00–1:04

Now, let's get a closer look. 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 gasses, nutrients, fluids and metabolic waste.
The majority of this exchange happens 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 increase in diameter and eventually drain into the small veins.
All right. This image is an example of an arteriole from the abdominal mesentery, similar to most blood vessels.

Arterioles1:04–1:56

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 three nuclei from endothelial cells in 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.

Capillaries1:56–5:25

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 to 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 capillary 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 red blood cell, which can be a useful tool to gauge the size of surrounding structures in an image.
Since red blood cells normally have a diameter of 6 to 8 microns in longitudinal sections of the capillaries, the nuclei will look more elongated compared to cross sections.
Occasionally, you can also see flattened supportive cells that surround the capillary called parasites. In this image, we can see the parasite 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 continuous capillaries. The basement membrane is also continuous with no perforations.
Fenestrated capillaries are found in organs where molecular exchange with the blood is important like endocrine organs, intestinal walls and choroid plexus.
The pores or fenestrations in the endothelial cells are too small to be seen with light microscopy but they can be seen using electron microscopy.
This image of a fenestrated capillary using electron microscopy was colored to show the endothelial cells in red in the basement membrane.
In light blue, the fenestrations are small circular pores about 60 to 80 nanometers in diameter that penetrate the thinner portions of the endothelium.
But they can be seen in this image as small gaps in the endothelium with a thin black membrane that spans the gap. The membrane allows small molecules and proteins to cross from one side of the endothelium to the other larger molecules can still be transported across the endothelium by small pinocytotic vesicles that can transport molecules in both directions.
Sinusoidal capillaries are another type of capillary that have walls with much larger holes or breaks in both the endothelium and basement membrane.
This allows for more exchange of macromolecules but with less control or regulation of what molecules are exchanged. These capillaries will also have larger diameters of about 15 to 30 microns and will have a more irregular shaped lumen like the one in this electron microscopic image of a sinusoidal capillary from the liver.
The lumen is the mostly white area of the image which also contains a large specialized macrophage called a cup for cell colored green and a platelet that's been colored orange.
The large breaks in the endothelium can be easily seen. Since the discontinuous endothelium has been colored red.

Venules5:25–6:07

After the capillaries, the blood then drains into postcapillary venules like the ones in this section taken from a lymph node.
The venule on the left is a longitudinal cut of the blood vessel and the one seen in the lower right is a cross section.
These vessels have large lumens with only a thin wall of simple endothelial cells and no smooth muscle. The postcapillary venules then converge into collecting venules that have even larger lumens and still don't have a layer of smooth muscle.
These venules gradually increase in size. Once smooth muscle is present, they're considered muscular venules, which can have 1 to 2 layers of smooth muscle surrounding the endothelium.
In this image, we can see that the venule is at least partially surrounded by smooth muscle. All right, as a quick recap, the blood circulatory systems.

Review6:07–7:29

Microvasculature includes arterioles, capillaries and venules. Starting with the innermost layer.
The arteriolar walls are aligned by squamous endothelial cells with round nuclei and a surrounding layer called the internal elastic lamina.
The next layer consists of circularly arranged smooth muscle that can be 1 to 2 cells thick. The outermost layer is the tunica externa that consists of a thin layer of fibrous tissue capillaries are the blood vessels with the smallest diameter in the body and of walls that consist of only a single layer of endothelial cells.
And its basement membrane. Continuous capillaries have diameters that are only slightly larger than the diameter of a single red blood cell which is approximately 6 to 8 microns.
Fenestrated capillaries have capillary walls that show small fenestrations. Sinusoidal capillaries have capillary walls with a discontinuous endothelium and basement membrane.
These capillaries also typically have larger diameters that are usually between 15 to 30 microns. The capillaries then drain into postcapillary venules which have larger lumens and a thin wall of endothelial cells.
These venues converge to form an even larger collecting venules that gradually increase in diameter and eventually become muscular venules which are the only venues that have 1 to 2 layers of circularly arranged smooth muscles surrounding the endothelium.