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Blood vessels are the structural conduits of the circulatory system which allow for the metabolic needs of our bodily tissues to be met. The circulatory system has two divisions: systemic and pulmonary (supplying the lungs) (Figure 1-1). From the heart, oxygenated blood travels through the aorta, arteries, and arterioles and ends in the capillaries with the uptake of oxygen into the tissues and the transfer of deoxygenated blood and metabolic waste out. Deoxygenated blood is then taken up into venules and carried into veins and the venae cavae to return to the heart to again be oxygenated in the lungs.1


Schematic of the circulatory system and its components as a percentage of total blood.

Different parts of the circulatory system have distinct macro and micro anatomy to support their roles. Conductance or large vessels are capable of pulsating and propagating blood flow, but do not directly contribute to the regulation of blood pressure. Generally, the wall of a vessel is multilayered and is frequently surrounded by perivascular adipose tissue (PVAT). This can be seen in Figure 1-2. Larger arteries have a lumen higher than 300 µm in diameter, their tunica media, or middle, layer is thicker and more elastic than small arteries, and consequently, they accommodate higher pressures. On the other hand, small arteries present a less elastic, but more muscular tunica media layer. As described in detail in the section “Basic Principles of Circulatory Function,” blood flow varies inversely with the total cross-sectional area of the blood vessels (Table 1-1); consequently, small arteries, which present a greater total cross-sectional area compared to larger arteries, present slowest blood flow and lower blood pressure. However, each small artery, arteriole, and capillary are extremely small compared to conductance vessels. Therefore, each small artery and arteriole offer, individually, higher resistance to the flow. As a result, these arteries constrict and dilate to exert local blood flow and, directly, control blood pressure. Taken together, although the resistance of an individual small artery is higher when compared to a larger artery, the flow and blood pressure are lower in small arteries and arterioles because of the higher total cross-sectional area.


Representative schematic of a blood vessel with labelled layers and the surrounding perivascular adipose tissue.

TABLE 1-1Breakdown of Approximate Collective Cross-Sectional Area in the Whole Body (70-kg Male)

The aorta, the largest blood vessel in the body, is ...

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