60 MOVEMENTS OF MATERIALS INTO AND OUT OF THE CELL 



the cell are the same in concentration as the salt in the water outside. 

 Finally, if we place some red blood cells in a 5 per cent salt solution, 

 the water will pass out more rapidly and the cells will shrivel and shrink. 

 We can restore the cells to a normal state by placing them in the 0.87 

 per cent salt solution. 



Thus we see how cells respond to variations in the concentrations of 

 solutions around them. All cells of the body are not so fragile as red 

 blood cells, and most of them do not burst when placed in plain water, 

 but they will swell. Everyone has noticed how the skin of the hands 

 will become soft and swollen when the hands are kept in water for a 

 long period of time. This can be prevented if some salt is placed in 

 the water to prevent the cells of the skin from absorbing water through 

 osmosis. Physicians often advise patients to soak a sprained ankle in 

 hot water containing some Epsom salts. This prevents the undesirable 



Isotonic Hypotonic Hypertonic 



Fig. 4.5. Reaction of red blood cells to solutions of different concentration. The cell 

 remains normal in the isotonic solution, it swells and bursts in the hypotonic solution, 



and it shrinks in the hypertonic solution. 



swelling of the cells of the skin and will help in reducing a swollen area 

 by removal of water through osmosis. This effect of Epsom salts is 

 especially noticeable when it is taken internally as a cathartic. The ions 

 of Epsom salts are not readily diffused through the cells of the intestine, 

 and they raise the concentration within the intestine to a point where 

 water is withdrawn from the body by osmosis ; this keeps the contents 

 of the intestine in a highly liquid state. 



A solution which has the same concentration of dissolved substances 

 as is found in the protoplasm of certain living cells is said to be isotonic 

 to those cells. Most drugs which are prepared for injection into the 

 human body contain 0.87 per cent sodium chloride to make them iso- 

 tonic with the cells of the body. Plain water injected into the blood 

 stream would cause the bursting of many red blood cells before it mixed 

 with the blood plasma. Also, nose drops are prepared in an isotonic 

 solution to prevent irritation to the delicate cells lining the nasal cav- 



