The general aggregate diameter of the different parts of the vascular system varies greatly. The combined calibre of the branches of an artery exceeds that of the parent trunk, so that the aggregate sectional area of the arterial tree increases as one proceeds from the aorta toward the capillaries. After the muscular arterioles are passed the general diameter of the vascular system suddenly increases immensely, and in the capillaries it reaches its maximum, the aggregate sectional area of which is said to be several (5 to 8) hundred times as great as that of the aorta.
The aggregate sectional area of the veins diminishes as the tributaries unite to form main trunks, and reaches its minimum at the entrance of the vena cava into the right auricle.
Fig. 128. Diagram intended to give an idea of the aggregate sectional area of the different parts ot the vascular system.
(A) Aorta. (C) Capillaries. (V) Veins.
The transverse measurement of the shaded part may be taken as the width of the various kinds of vessels, supposing them fused together.
The capacity of the veins is, however, everywhere much greater than that of the corresponding arteries, the least difference being near the heart, where the calibre of the vense cavae is more than twice that of the aorta.
After this brief anatomical sketch, the most important properties of each part of the vascular system may be summarized thus: -
1. The structure of the walls of the large arteries shows them to be capable of sustaining considerable pressure, and of exerting powerful and continuous elastic recoil on the blood.
2. In the small arteries, as well as this elasticity, frequent variation in their calibre occurs, dependent on the contraction of their muscular coat which regulates the blood flow.
3. In the capillaries we find extreme thinness, elasticity, and permeability of their wall, which presents an immense surface, so as to allow free interchange between the blood and the surrounding textures.
4. The veins have yielding and distensible walls, capacity to accommodate a large quantity of blood, and valves to prevent its backward flow upon the capillaries.
5. The aggregate sectional area of the systemic capillaries is about three hundred times that of the great veins, and seven hundred times that of the aorta, so that the current of the blood must be proportionately slower in the capillary network.