DO YOU EVER stop to think about your cells’ needs? Every one of the trillions of cells making up your body requires energy and is fed by blood. Armies of red blood cells continually parade through the heart, to the furthest backwaters of your body, and back again.
To get to every part of your body, the capillaries that conduct these nutrient-carrying cells to their destinations must be tiny. Blood cells are forced to march single file through severely confining micro-veins. In fact, the blood cells are actually 25 per cent larger than the smallest capillaries they travel through. How can this be?
Alison Harman is a graduate student in the Department of Physics at the University of Ottawa. As a physicist, she is more interested in the mechanics of cells than anything else. She doesn’t worry about the fact that the cell is alive. Instead, Harman uses simplified computer models to simulate the physical properties of cells.
These virtual cells are still complicated, but they are simple enough that Harman can extract information about cellular membranes without worrying about the intricacies of life.
Harman models the blood cells, simulating each of the lipids that make up the cell membrane, but not the contents of the cell. Each of the lipid molecules are made up of a head that likes water and a tail that avoids water.
To keep everyone happy, the lipids organize themselves into a bi-layer with tails facing in and heads facing out, which results in an empty vesicle.
Vesicles are most comfortable as spheres, but they are very deformable. When they are carried through fairly large capillaries, the flow pulls them out into a flat, parachute-like shape. Faster flow stretches the vesicle longer.
Harman’s simulations show this stretching happens at the edges of the parachute. At very high flow rates—about 100 times faster than blood actually flows in the body—the vesicles eventually break.
Harman sees a different picture in the smallest capillaries. The vesicles are so crammed they must fill the entire tube of the capillary and deform into a pill-like shape. As the flow rate increases, vesicles stretch until they can’t stretch anymore. They usually break along their side where the membrane is closest to the wall, fitting into the capillaries.