By calculating the SURFACE AREA TO VOLUME RATIO, the efficiency of each process can be determined. 

To increase the efficiency of substance exchange the surface area to volume ratio needs to be increased. 

 The body and other multicellularorganisms are adapted to allow these processes to proceed as efficiently as possible.  

The effectiveness of exchange surfaces is increased by having a large surface area.  

• The lungs have millions of alveoli creating a large surface area for rapid diffusion of oxygen into the red blood cells.

• The small intestine has millions of finger-like projections called villi creating a large surface area allowing rapid diffusion of digested food into the blood. 

• The kidneys have millions of nephrons creating a large surface area to allow the movement of water into and out of the blood to maintain the body’s water balance. 

The distance required for the process to take place is also important. Short distances make for greater efficiency.  

• Cell membranes are thin 
• The walls of capillaries are one cell thick 
• The epithelia of alveoli in the respiratory system and the villi in the small intestine are only one cell thick.