The circulatory system consists of two functional parts, the blood circulatory system and the lymphatic system.
The blood circulatory system or cardiovascular system consists of a circuit that transports blood to and from tissues throughout the body.
The heart pumps the blood through a series of arteries that branch into smaller and smaller blood vessels that supply the tissues with blood.
The smallest arteries branch further to become arterioles, which drain the blood into capillaries.
The capillaries form a network of tiny blood vessels that perfuse the tissue.
The veins continue to join or converge with one another, forming larger and larger veins that eventually drain back into the heart.
Arteries and veins have walls that share a common overall structure that consists of three layers: the tunica intima, tunica media, and tunica adventitia.
The inner layer or tunica intima is lined with a single layer of a specialized epithelium called the endothelium, which acts as a semipermeable barrier.
The subendothelial layer is the layer beneath the endothelium and is still part of the tunica intima.
This layer, which consists of loose connective tissue and occasionally smooth muscle as well.
In some arteries, the tunica intima will also have a very thin layer of elastic tissue along the outer edge of the subendothelial layer called the internal elastic lamina.
The internal elastic lamina has holes throughout the layer, which allow substances from the blood to more easily diffuse through this layer, deeper into the wall.
The middle layer or tunica media consists mostly of smooth muscle.
Arteries will also contain a lot more elastic fibers in comparison to veins.
Both the smooth muscle and elastic fibers are arranged in circular or concentric layers surrounding the lumen of the blood vessel.
Similar to the tunica intima, in arteries, the tunica media may also have a layer of elastic tissue that surrounds it, but in this case it’s called the external elastic lamina.
The tunica adventitia is the outermost layer of connective tissue that consists mostly of longitudinally arranged collagen and elastic fibers.
Generally, arteries will typically have a thicker tunica media and smaller lumen; whereas veins will have a larger lumen and the thickest layer is the tunica adventitia.
Large arteries are categorized as either elastic arteries or muscular arteries.
These arteries have an outer diameter that’s typically more than 10 mm.
Muscular arteries, such as the femoral and radial arteries, are slightly smaller, with an outer diameter between 1-10 mm.
As you might expect, the elastic arteries will have significantly more elastic fibers in the tunica media compared to muscular arteries and the muscular arteries have more layers of smooth muscle.
Although, it’s the muscular arteries that actually have a more prominent internal elastic lamina, and larger muscular arteries can also have a prominent external elastic lamina as well.
This section of tissue was taken from the wall of the carotid artery and stained with Hematoxylin and Eosin, or H&E for short.
The lumen is on the right side of this image, which means the very thin layer next to it is the tunica intima.
This layer has small folds that allow arteries to easily expand when blood pressure increases.
For example, when the heart contracts during systole, blood pressure within the carotid arteries will increase significantly.
The endothelium is composed of simple specialized cells, but don’t have their typical flat appearance because of the folds.
Along the outer edge of the intima, we can see portions of the very thin layer of elastic tissue called the internal elastic lamina.
Now, let’s move to the thickest layer in arterial walls, the tunica media.
This layer is composed of circularly or concentrically arranged elastic fibers that more or less alternate with layers of smooth muscle.
With an H&E stain, the smooth muscle is stained a combination of purple and pink; and the elastic fibers are seen as wavy sheets that either stain pink or are unstained.
The majority of the elongated nuclei are from the smooth muscle cells, and the rounder nuclei are more likely to be from the fibroblasts.
In order to visualize the elastic fibers more easily, a specialized stain such as aldehyde fuchsin can be used.
This stain won’t stain smooth muscle, but will stain the elastic fibers dark purple.
Individuals that have chronic high blood pressure will actually have an increased number of elastic lamellae or layers in order to compensate for the increased pressure.
Surrounding the entire artery is the tunica adventitia, which mainly consists of dense irregular connective tissue, but in this image we can also see some small blood vessels that supply the tunica adventitia and tunica media with oxygen and nutrients.
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