Are the shapes of RBCs really important? 

People often say it’s to maximize surface area to volume ratio and thus the efficiency of oxygen diffusion from the blood plasma to all the hemoglobin in the RBC. I used to write that myself. I don’t believe it anymore, on at least two grounds:

(1) The RBCs of birds have to be just as efficient as ours in transporting oxygen, if not more so because of the demands of flight. But bird RBCs are football-shaped with a big nucleus, so I don’t think RBC shape has anything to do with gas transport efficiency.

Bird erythrocytes:

(2) The only places that our RBCs load or unload oxygen is in the blood capillaries of the lungs and systemic tissues, and in both of these places, our RBCs don’t have that discoidal, biconcave shape. To squeeze through the capillaries, they become rounded, teardrop-shaped, or even folded over like a soft deflated air pillow folded double. The biconcave shape exists only in the larger vessels where gas exchange isn’t occurring.

I feel that advances in biophysics and fluid dynamics, and innovations in expensive microfluidic instrumentation, are producing an entirely new perspective on blood flow (hemodynamics) and the reason for RBC shape. (Implicit in this is one answer to people who ask me, “Why do you have to revise a human anatomy textbook every 3 years? The anatomy of the body doesn’t change, does it?”)

So why do mammalian RBCs like ours have the biconcave shape? Based on what I read in recent research journals, I changed this discussion in the blood chapter of my main textbook to this:

“There has been appreciable, unresolved debate over whether the biconcave shape of the RBC has any functional advantage. Some suggest that it maximizes the ratio of cell surface area to volume and thereby promotes the quick diffusion of oxygen to all of the hemoglobin in the cell. This is hard to reconcile with the fact that the only place RBCs load oxygen is in the capillaries, and while squeezing through the tiny capillaries, they generally are not biconcave but compressed into ovoid or teardrop shapes. They spring back to the biconcave shape when reentering larger blood vessels, but no oxygen pickup occurs here. Another hypothesis is that the biconcave shape enables the dense slurry of RBCs to flow through the larger blood vessels with a smooth laminar flow that minimizes turbulence. It has also been argued that it is simply the easiest, most stable shape for the cell and its cytoskeleton to relax into when the nucleus is removed, and it may have no physiological function at all.” (Anatomy & Physiology: The Unity of Form and Function, 8th ed., McGraw-Hill, 2018)

Going slightly deeper into it than my limited textbook space allows—think of how an ice skater spins at high angular velocity if she tucks her arms in, and slows down if she throw her arms out to the sides. One research article posited that by having a thin sunken center and almost all the cytoplasm distributed to the periphery of an RBC, mammalian RBCs are like the ice skater with her arms extended. This distribution of mass reduces the spin of the RBC, and reducing spin reduces turbulence in the blood flow.

Compare human RBCs:

Blood flow turbulence favors the development of arterial disease (atherosclerosis) by its mechanical effect on the endothelial lining of the blood vessels. Thus, one can understand why it would be advantageous to health and survival to minimize it, and this is achieved through the “design” (natural selection) of RBC shape.

Credits: Prof Ken Saladin