red corpuscles are exceedingly numerous, there being as many as five
millions in a small drop (one cubic millimeter) of healthy blood. But the
number varies somewhat and is greatly diminished during certain forms of
disease.

[Fig. 9]


Fig. 9—Red corpuscles from various animals. Those from mammals are without
nuclei, while those from birds and cold-blooded animals have nuclei.


It is the _function_ of the red corpuscles to serve as _oxygen carriers_
for the cells. They take up oxygen at the lungs and release it at the
cells in the different tissues.(8) The performance of this function
depends upon the hemoglobin.

*Hemoglobin.*—This substance has the remarkable property of forming, under
certain conditions, a weak chemical union with oxygen and, when the
conditions are reversed, of separating from it. It forms about nine tenths
of the solid matter of the red corpuscles and to it is due the colors of
the blood. When united with the oxygen it forms a compound, called
_oxyhemoglobin_, which has a bright red color; the hemoglobin alone has a
dark red color. These colors are the same as those of the blood as it
takes on and gives off oxygen. The stroma, which forms only about one
tenth of the solid matter of the corpuscles, serves as a contrivance for
holding the hemoglobin. The conditions which cause the hemoglobin to unite
with oxygen in the lungs and to separate from it in the tissues, will be
considered later (Chapter VIII).