Veins: Structure and function. Introduction

The cardiovascular system is a system of organs that ensures the delivery of substances necessary for the vital activity to all tissues and cells of our body.

It consists of three components:

The heart (cor in Latin), which acts as a kind of pump for blood

Vessels (vasa), which perform a transport function by distributing blood throughout the body
Vessels (vasa)
And blood itself, which contains oxygen, as well as nutrients.
One of the types of vessels are veins (venae).
Veins (venae)
They drain venous oxygen-depleted blood from tissues and organs, and then carry it towards the heart, and therefore the blood pressure in them is lower, and the blood flow is slower.
Just like arteries, veins have different calibers, that is, they differ in wall thickness and lumen diameter. In their course, the veins usually accompany the arteries of the same name.
Let’s consider the main differences between veins and arteries:
1. Larger diameter of the lumens
2. Smaller thickness of the walls
3. A small number of elastic fibers and smooth muscle cells in the wall, as well as a more pronounced tunica externa
4. The volume of blood in the lumen of the veins is greater than in the lumen of the arteries, and is approximately 70%
5. The number of veins in the human body is greater than the number of arteries
6. They form a large number of anastomoses between themselves.
The vein wall consists of three tunicas.
The tunica externa or adventitia (tunica adventitia or tunica externa) mainly contains connective tissue elements
Adventitia (tunica adventitia or tunica externa)
The tunica media (tunica media) contains muscle cells
Tunica media (tunica media)
The tunica intima (tunica intima) consists of a special type of epithelial tissue called the endothelium
Tunica intima (tunica intima)
There are multiple capillaries in the walls of the veins. These capillaries feed all three tunicas of the vessel, unlike arteries.
The walls of the veins differ from each other in the composition of the tunicas. The following types of veins exist:
Muscle-free veins, which virtually do not contain muscle elements in their wall. At the same time, the walls of such vessels do not collapse due to being located in the thickness of the organs.
Muscular veins contain a different number of muscle elements in their wall, on the basis of which they are divided into:
Veins with a high content of muscle elements.
Veins with an average content of muscle elements
Veins with a low content of muscle elements

It should be noted that veins can be paired and single, superficial and deep. Superficial veins communicate with deep veins by means of anastomoses, which are called perforating or perforator veins (vv. perforantes).

Perforating or perforator veins (v. perforans)

Veins located nearby may communicate with each other by anastomoses, forming venous plexuses (plexus venosus).

Venous plexuses (plexus venosus sacralis)

Major veins branch into smaller venous tributaries, which in turn branch into venules, and venules branch into venous capillaries, which are a component of the microcirculatory bloodstream. Gas exchange occurs between the venous and arterial capillaries.

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