These are the exact reverse of those exhibited by the breathed air -what the air gains the blood loses, and vice versa. The blood loses heat, and water, and carbon dioxide in the pulmonary capillaries, and gains oxygen. These gains and losses are accompanied by a change of color from the dark purple which the blood exhibits in the pulmonary artery, to the bright scarlet it possesses in the pulmonary veins.

Why The Blood Changes Its Color As It Flows Through 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 Hæmoglobin (p. 204), and Hæmoglobin, as we have learned, is a substance which has the power of absorbing oxygen and forming a bright scarlet compound called oxyHæmoglobin. This oxyHæmoglobin very easily gives up its oxygen when it is placed under conditions where that gas is scarce: the Hæmoglobin left behind has a dark purple color. The blood leaving the lungs by the pulmonary veins is bright red because all its Hæmoglobin has been turned into oxy-Hæmoglobin. 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-Hæmoglobin is broken up, the oxygen taken away, and dark purple-red Hæmoglobin 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 becomes bright-red oxyHæmoglobin. The red corpuscles of the blood are so many little boxes in which oxygen is packed away in the lungs for conveyance to distant parts of the body.

What are stoves apt to do? Point out a good way of supplying fresh air to a room warmed by a stove? What are especially important in a room heated by "direct radiation"? Why is it fortunate that doors and windows do not fit air-tight?

What is the relationship between the gains and losses of blood in the pulmonary circulation, and the losses and gains of the breathed air? What does the blood lose as it flows through the lungs? What does it gain? What change in the color of the blood accompanies these gains and losses?

Why must the change in color of the blood during its pulmonary circuit be due to a change in its red corpuscles? What is the chief constituent of the red blood-corpuscles? What property does Hæmoglobin possess with reference to oxygen? What is the color of oxyHæmoglobin?

When does oxyHæmoglobin give off its oxygen? What is the color of Hæmoglobin? 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 oxyHæmoglobin 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?

What is the function of the red blood-corpuscles?