Department of Physiology

Basic Cell Physiology

Haemolysis of Red Blood Cells

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:

1. The cell suspension changes from cloudy to clear. This is because the cells normally have a high concentration of haemoglobin inside, giving the cytoplasm a very high refractive index, so each cell behaves like a tiny lens and together they scatter the light going through the suspension. It is the scattered light that makes for the cloudiness. When the cell membrane is stretched the haemoglobin leaks out down the concentration gradient leaving the intracellular concentration low. The cells cease to behave like lenses and no longer scatter light so the suspension is clear. In practice the remaining cell membrane bag is difficult to see even microscopically, and the cells are called ghosts.

   This process of leaking haemoglobin is called haemolysis.

2. If the suspension is put in a centrifuge and the cells spun down, the resulting supernatant is red coloured due to the haemoglobin in solution, and the cell pellet at the bottom of the tube is very pale. The picture below shows a comparison of spinning down a suspension of cells in 0.9%NaCl and some suspended in water.

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.

©D.F. Davey, Department of Physiology, University of Sydney
Last updated 5 May 2010