The red blood cell (RBC) is described as a biconcave disc. If you squeeze a soft ball until the opposite sides are close, you have a good approximation of the shape. Some real cells are shown in the scanning electron microscope image below:
In human cells the diameter of the disk is about 7μm. The shape is important: when blood cells pass through small capillaries, surprisingly the cell starts in with the disk across the vessel, the thin part of the disk then goes ahead with the shape becoming like a bullet with a greatly reduced diameter - explaining how the cells can go through capillaries only 1μm in diameter.
The shape also means the cells can swell through osmosis until they become spherical without any stress being applied to the membrane. But once they are spherical, additional water entry stretches the membrane and it becomes leaky. Two things are then apparent:
This process of leaking haemoglobin is called haemolysis.
Note that the cells in the NaCl have carried all the haemoglobin down to the pellet, indicating that they are intact. The ghosts on the right have left haemoglobin behind. You can probably see the pellet of ghosts in this enlargement:
Obviously a pure water environment is not a healthy one for red cells, and osmosis is not a process to be trifled with.
Department of Physiology,
University of Sydney
Last updated 5 May 2010