**3. Shape of RBC**

158 Blood Cell – An Overview of Studies in Hematology

**Figure 3.** Space-Fill Representation of ATP[14]

In immobile state, the normal human RBC is shaped as a biconcave disc. The disc shape is important to erythrocyte function. The ratio of surface to volume is optimized so that oxygen transfer is possible. Also the biconcave disc is more deformable than a sphere and undergoes the change in shape necessary for optimal movement in microvasculature.

The four possible forces to maintain the shape described are (1) elastic forces within the membrane, (2) surface tension, (3) electrical forces on the membrane surface, and (4) osmotic or hydrostatic pressures. The maintenance of RBC shape is dependent on the structure of the cell as well as in the external environment. If these are changed, the cell may become spherical.

When RBC's are suspended in hypotonic solutions andosmotic swelling occurs. This can make the cell spherical. These changes are associated with an increase in volume while the cell surface area remains the same or changes only slightly. When spherical shape is attained, the cell diameter decreases, and this shows the elastic properties of the membrane.

Discocyte-echinocyte transformation takes place when ATP is depleted, when intracellular calcium is increased, when the cell is exposed to plasma, anionic detergents, high pH, lysolecithin or fatty acids.
