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diameter. In the body, larger arteries and veins are divided into smaller ones. From
them, in turn, even thinner arterioles and venules depart (figure 18).

Figure 18. Arterioles and venules

The smallest blood vessels are the capillaries – the meeting place of arterioles
and venules. Capillaries differ from other vessels in a special structure: their walls
consist of a single layer of cells, which facilitates the exchange of gases and substances
with tissues. Oxygen and nutrients enter the tissues through the capillary wall, and cell
waste products enter the capillaries from the tissue fluid (figure 19).
The walls of the remaining vessels consist of three layers. External, internal and
medium muscle (smooth muscle tissue). The muscular wall of the veins is noticeably
thinner than that of the arteries.
The blood flow rate in the veins is weaker than in the arteries. Thicker artery
walls allow them to withstand more pressure. There are valves in large veins so that
blood cannot flow in the opposite direction. In addition, the movement of blood in the
veins is helped by the surrounding muscles, which, contracting, squeeze the veins.

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