258 THE HUMAN BODY. 



the Pulmonary Capillaries. The color of the blood depends 

 on its red corpuscles, since pure blood-plasma or blood- 

 serum is colorless or at most a very faint straw yellow. 

 Hence the color change which the blood experiences in 

 circulating through the lungs must be due to some change 

 in its red corpuscles. These consist chiefly of hcemogloMn 

 (p. 178), and haemoglobin, as we have learned, is a sub- 

 stance which has the power of absorbing oxygen and form- 

 ing a bright scarlet compound called oxylicemoglo'bin. 

 This oxyhaemoglobin very easily gives up its oxygen when 

 it is placed under conditions where that gas is scarce: the 

 haemoglobin left behind has a dark purple color. The 

 blood leaving the lungs by the pulmonary veins is bright 

 red because all its haemoglobin has been turned into oxy- 

 haemoglobin. From the left side of the heart it is conveyed 

 by the branches of the aorta to all the organs of the body. 

 These are constantly using oxygen, which is therefore very 

 scarce in them, and as the blood flows through its oxy- 

 haemoglobin is broken up, the oxygen taken away, and 

 dark purple-red haemoglobin left to be conveyed by the 

 veins to the right auricle of the heart. From there it 

 passes to the right ventricle and thence by the pulmonary 

 artery to the lungs, where it again picks up oxygen and 



Why must the change in color of the blood during its pulmonary 

 circuit be due to a change iu its red corpuscles? What is the chief 

 constituent of the red blood-corpuscles? What property does haemo- 

 globin possess with reference to oxygen? What is the color of oxy- 

 hsernoglobin? 



When does oxyhaemoglobin give off its oxygen? What is the 

 color of haemoglobin? Why is the blood in the pulmonary veins 

 bright red? Where is it conveyed? Why is oxygen scanty in most 

 organs of the body? What results as regards the oxyhaemoglobin of 

 the blood? What is the color of the blood sent to the right auricle 

 of the heart? What is the subsequent course of this blood until it 

 reaches the lungs? What does it receive in the lungs? How is its 

 color altered in 'those organs? 



