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Organ system histology
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.
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.
Arteries are blood vessels that carry oxygen-rich blood from the heart to the body tissues, whereas veins are blood vessels that carry oxygen-poor blood from the body tissues back to the heart. The walls of both the arteries and veins are made up of three layers: the tunica intima, the tunica media, and the tunica adventitia.
The tunica intima is the innermost layer and is made up of a thin layer of endothelial cells that line vessels' lumen. The tunica media is the middle layer and is composed of smooth muscle cells and connective tissue. The tunica adventitia is the outermost layer and consists of connective tissue that anchors the vessel to surrounding tissues. Arteries typically have a thicker tunica media and a narrow lumen, compared to veins which have thinner tunica media, and a wide lumen.
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