e
cells, the oxygen passes twice through the blood--once as it goes toward
the cells and again as it passes from the cells to the exterior of the
body:

_Passage toward the Cells._--This is effected mainly through the hemoglobin
of the red corpuscles. At the lungs the oxygen and the hemoglobin form a
weak chemical compound that breaks up and liberates the oxygen when it
reaches the capillaries in the tissues. The separation of the oxygen from
the hemoglobin at the tissues appears to be due to two causes: first, to
the weakness of the chemical attraction between the atoms of oxygen and
the atoms that make up the hemoglobin molecule; and second, to a
difference in the so-called _oxygen pressure_ at the lungs and at the
tissues.(46)

The attraction of the oxygen and the hemoglobin is sufficient to cause
them to unite where the oxygen pressure is more than one half pound to the
square inch, but it is not sufficiently strong to cause them to unite or
to prevent their separation, if already united, where the oxygen pressure
is less than one half pound to the square inch. The oxygen pressure at the
lungs, which amounts to nearly three pounds to the square inch, easily
causes the oxygen and the hemoglobin to unite, while the almost complete
absence of any oxygen pressure at the tissues, permits their separation.
The blood in its circulation constantly flows from the place of high
oxygen pressure at the lungs to the place of low oxygen pressure at the
tissues and, in so doing, loads up with oxygen at one place and unloads it
at the other (Fig. 55).

_Passage from the Cells._--Since oxygen leaves the free state at the cells
and becomes a part of compounds, we are able to trace it from the body
only by following the course of these compounds. Three waste compounds of
importance are formed at the cells--carbon dioxide (CO2), water (H2O), and
urea (N2H4CO). The first is formed by the union of oxygen with carbon, the
second by its union with hydrogen, and the third by its union with
nitrogen, hydrogen, and carbon. These compounds are carried by the blood
to the organs of excretion, where they are removed from the body. The
water leaves the body chiefly as a liquid, the urea as a solid dissolved
in water, and the carbon dioxide as a gas. The passage of carbon dioxide
through the blood requires special consideration.

[Fig. 55]

Fig. 55--*Diagram illustrating movement, of oxygen and carbon diox