Re: why do air sacs and capillaries have very thin walls?

Alex,

The walls of the air sacs (alveoli) and the pulmonary capillaries are 
collectively referred to as the respiratory membrane.  The walls of the 
alveoli and capillaries (pulmonary and systemic) in the body are made of 
simple squamous epithelium, which is a very thin tissue.  For instance, the 
thickness of the respiratory membrane averages only ½ of a micron (one 
micron is 1/1000 the thickness of a dime).

The alveoli and capillary walls are the points of exchange of gasses.  Most 
cells use oxygen to produce enough energy to carry on the basic functions 
of life.  As cells are using oxygen to produce energy they also produce 
carbon dioxide.  Carbon dioxide is harmful and must be eliminated from 
cells.  

Oxygen enters the blood through the respiratory membrane.  Blood transports 
the oxygen to the systemic capillaries.  There, the oxygen moves through 
the thin capillary walls into the cells.  Carbon dioxide passes through the 
capillary walls from the cells.  Blood transports the carbon dioxide to the 
lungs where it crosses the respiratory membrane and enters the alveoli.  
This exchange of gasses must be rapid enough so that trillions of cells can 
acquire the energy they need for survival.  The pulmonary and systemic 
capillary walls and the alveoli are very thin to allow for the rapid 
exchange of gasses.

Robert Houska
Mad Scientist