Page 18, Turbulence & Rheology, Dr. D. Penney

Circulatory Design (continued..)

What is the best size for the packets, i.e. the erythrocytes? The answer to this question has to do with surface area to volume relationships. The smaller the packet the larger is the ratio of surface area to volume, and thus the greater the diffusional capacity. Related to this question is the question of the best shape for the packets.

A sphere has the smallest surface area per volume of any structure. By not using a sphere, the surface area will be greater. Another major disadvantage of a sphere is that it is unable to deform unless the surface membrane stretches, which red blood cell membranes cannot do. A more complex geometric shape such as the biconcave discoid provides a large surface area and the ability of the shape to deform without the necessity for stretching the boundary membrane.

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