128 HUMAN BIOLOGY 



walls of the air sacs and of the capillaries. From the capil- 

 laries of the lungs, the blood finally collects into veins that 

 convey the blood to the left auricle. 



182. The function of red corpuscles. — In 7 we called 

 attention to the structure of the red corpuscles of the blood. 

 Like other cells red corpuscles are composed of protoplasm. 

 Chemical analysis shows that the most important ingredient 

 is a protein substance called hemoglobin, a compound that 

 contains iron. Hemoglobin gives the red color to the blood 

 and has a remarkable power of combining with oxygen when 

 that element is abundant, and of giving it up wherever it is 

 needed in the various parts of the body. We may, therefore, 

 compare the blood corpuscles to countless little boats, float- 

 ing in a stream of plasma ; they take on their cargo of oxygen 

 from the air in the lungs and discharge it in the cells of the 

 tissues. 



183. Change in the color of the blood after mixing with 

 oxygen. — When the blood passes through the lungs, as 

 already stated, it absorbs oxygen. The resulting change 

 in color is seen from the following experiment. Pour into a 

 glass bottle a small amount of blood that has been prevented 

 from clotting by stirring it vigorously with a bunch of 

 twigs, and stopper tightly. When the bottle is shaken 

 violently, the blood is mixed with the oxygen in the bottle, 

 and the dark maroon color changes almost instantly to a 

 bright scarlet. The pupil will doubtless have observed that 

 the blood in the veins on the back of the hand, for instance, 

 is blue, but that whenever blood flows from any of these 

 veins because of a slight cut, the color is always bright red 

 after the blood comes in contact with the oxygen of the air. 



184. Hygiene of the red corpuscles. — Since supplying 

 oxygen to the various tissues is the function of the red cor- 



