Artery and vein 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.

The capillaries then drain into venules, before converging to form small veins.

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.

Elastic arteries include some of the largest arteries in the body, such as the aorta and carotid arteries.