December 30, 2012 | David F. Coppedge

Red Blood Cells Are Frisbees, Tanks and Wheels

Cells as commonplace as red blood cells still keep researchers wondering how they perform their job so well.

Red blood cells (RBCs), also called erythrocytes, are unique for their biconcave shape, lack of a nucleus, and high concentration of hemoglobin.  Their main task is to deliver nutrients and oxygen (picked up in the lungs) to all the cells of the body.  To accomplish this task, they need to negotiate all the blood vessels, from the superhighways of main arteries to the single-file capillaries.  Their unique biconcave shape enables them to do this – but it’s more than shape.

In a new paper in the Proceedings of the National Academy of Sciences (PNAS, Dec. 3 issue), Dupire, Socol and Viallat said that the deformation state of the RBC is not known, nor is its stability.  They observed the dynamics of RBCs in a dextrose solution with video cameras at 25 frames per second to watch that happens.  Individual cells transition from a rolling state to a Frisbee-like spinning state to a “tank-treading” state in free flow, depending on the shear rate.  The cells transition into these modes that are more efficient than “flipping” or tumbling against the current.

In all cases, the cells have “shape memory” that pops the cell back into its biconcave shape when a deforming force is removed.  This appears to be the most natural, low-energy state.  The biconcave shape is more deformable than a sphere.  The shape memory depends on “rheological properties” of the cell, including the viscosity of the interior fluid, the membrane, and the internal cytoskeleton.  In free flowing blood, RBCs can sometimes stack like coins.  Their dynamical behavior affects the viscosity of flowing blood.

Even with their observations and videos, the researchers realize there is still much to learn:

Our results raise new questions about the behavior of viscoelastic particles in a viscous fluid at very low Reynolds number. Future models and simulations on RBCs should consider that the axis of revolution of the cell does not necessarily lie in the shear plane. Whether the observed orientational behavior can be explained by the minimum energy dissipation, as speculated by Jeffery, is still an open question, and we hope our work will stimulate new theoretical numerical studies. We also hope it will generate works on the strain energy of tank-treading buckled shapes of elastic capsules, starting from a quasi-spherical spheroid stress-free shape. Finally, the high stability of the biconcave RBC shape makes analytical shape preserving models (AFV, SS) very attractive.

Their work thus represents an incremental step in a complex subject regarding fluid dynamics, shape dynamics and shape memory.

In an old (c. 1962) Moody Institute of Science film about blood called Red River of Life, Dr. Irwin Moon used slide rules, calculators and an early IBM computer model to show that the optimum shape to balance volume against surface area was a biconcave shape exactly like a red blood cell (see the episode on YouTube).  In a later Moody Institute of Science film series called Wonders of God’s Creation (initially released singly as Distinctively Human in 1987), blood was likened to a system of canals in a city, bringing free goods, food and tools to every home.  We should certainly be thankful for this river of life inside us.  Here we are in 2013, many years after those films were made, and there is still more to learn.  The closer one looks at life, the more amazing it becomes.  As Dr. Moon said in that early film, “To me, the only adequate explanation is intelligent design.”


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