Human erythrocyte shape creates a capillary oxygen delivery advantage

Abstract

Textbooks still describe the “purpose” of the biconcave disk of the human erythrocyte (RBC) is “to maximize surface area for gas exchange.” This appears incorrect: 1) RBC surface area does not govern diffusion radius or limit the oxygen diffusion coefficient, and 2) quasi fractal mechanisms are almost universally employed for surface area advantages. Here, a mathematical model is used to explore oxygen delivery (DO2) advantage of the RBC shape, especially at high oxygen consumption (VO2) states (e.g. fight or flight, sepsis, etc). An axisymmetric model of a capillary and its service volume of target cells is simulated containing stacked RBCs, “parachute” RBCs, spherocytes, or bare Hb. Flow velocity, Hb saturation, and local VO2 are varied according to a Monte Carlo algrorithm. A no‐slip condition is approximated for laminar flow within the capillary. The model shows that the flow boundary layer of the capillary would tend to distribute bare hemoglobin to the axial center of the capillary. In contrast, RBC shape (normal and parachute, but not spherocyte) concentrates the Hb within a cylindrical manifold close to the capillary wall. Since target cell oxygen delivery depends on the square of the diffusion radius, even small reductions in radius convey a DO2 advantage in physiological stress. This model reveals constraints on design of artificial blood substitutes.