RESPIRATION 



53 



rjjfcrjT" Copper sulphate 



FlG. 27. Showing osmosis (Zuppke). 



It is a well-known fact that gases, liquids, and even 

 solids that are perfectly dissolved can pass through 

 delicate, living membranes. That is just what happens 

 in the lungs. The oxy- 

 gen passes through the 

 walls of the air sacs and 

 capillaries and enters 

 the blood, where it is 

 picked up by the red 

 corpuscles, which then 

 carry it to the tissues all 

 over the body. The 

 carbon dioxid and other 

 wastes that are found in 

 the blood enter the air sacs of the lungs in a very 

 similar way and are exhaled. This exchange of sub- 

 stances through a membrane is called osmosis. 



1. Take a fresh hen's egg and remove the shell from the larger 

 end so as to expose the delicate membrane beneath. Immerse the 



egg, large end up, in a glass of water and set it aside for a few hours. 

 The water will slowly pass through the delicate membrane, causing 

 it to bulge out. The water soaks through the thin animal mem- 

 brane just as the gases do in the air sacs of the lungs. 



2. Tie a piece of bladder tightly over the bottom of a small bottle 

 from which the bottom 'has been removed. Fill the bottle with a 

 strong solution of copper sulphate. Fit a cork with a piece of glass 

 tubing to resemble the thistle tube shown in the figure. Immerse 

 the bottle in a glass of water. The next day the water will have 

 risen in the glass tube, and some of the copper sulphate will be found 

 in the glass, showing that there has been an interchange of sub- 

 stances through the membrane. An Argand lamp chimney may be 

 used instead of the bottle. Alum, sugar, or common salt may be 

 used instead of copper sulphate. 



