where oxygen Is absent, or present only in small quantity. Hence, as the blood flows through the lungs, which are constantly supplied with fresh air, its corpuscles take up oxygen, which, as it flows on, is carried by them to distant parts of the body where oxygen is deficient, and there given up to the tissues. This oxygen-carrying is the function of the red corpuscles.
Hemoglobin, as the coloring-matter of the blood is called, is dark purplish-red in color; combined with oxygen it is bright "scarlet red." Accordingly, the blood which flows to the lungs after giving up its oxygen is dark red in color, its dark color being due to the impurities it contains; and that which, having received a fresh supply of oxygen, flows away from the lungs is bright scarlet having been cleansed of its impurities. The bright red blood is called arterial, and the dark red venous.
The work assigned to the blood in the economy of the human system is: first, to pick up nutriment in its course through the walls of the alimentary canal, and oxygen, as it flows through the lungs, and convey these to all other parts of the body. Second, to act as a sort of sewage stream that drains off waste matter, and to carry this to the organs of excretion by which waste is expelled from the body.
