Page 5, Architecture of the Circulation, Dr. D. Penney

Circulation Symmetry:

Examining the dimensions and numbers of components of the circulation at each level (Table 7.01), points out many symmetries and leads to several inescapable conclusions regarding hemodynamics and function. Vessel diameter declines progress-ively from the large aorta tothe tiny capillaries, then increases again, ending with the aorta-sized vena cavae. Whereas we begin with one aorta and end with one vena cava (actually there are two in the whole body), in between there are an increasing numbers of smaller vessels as the vessels arborize to smaller and smaller sizes.

As the result of this pattern, total cross-sectional area increases progressively from aorta to capillary (See Figure 7.03). This dictates that blood flow velocity must decrease sharply between the aorta and the capillaries, since all the components are in series and the entire blood volume must pass through all the components. This also dictates that blood flow velocity must increase again in flowing from the capillaries to the vena cava. In fact, flow velocity in the vena cava is about 75% that in the aorta. Thus, there is a reciprocal relationship in the circulation between total cross-sectional area and flow velocity.