fainting person instantly produces a deep, long-drawn breath. Certain
drugs, as opium, act to reduce the activity of this nerve center. Hence,
in opium poisoning, special attention should be paid to keeping up the
respiration. The condition of the lungs themselves is made known to the
breathing center, by messages sent along the branches of the great
pneumogastric nerve (page 276), leading from the lungs to the medulla
oblongata.

213. Effects of Respiration upon the Blood. The blood contains three
gases, partly dissolved in it and partly in chemical union with certain of
its constituents. These are oxygen, carbon dioxid, and nitrogen.
The latter need not be taken into account. The oxygen is the
nourishing material which the tissues require to carry on their work. The
carbon dioxid is a waste substance which the tissues produce by their
activity, and which the blood carries away from them.

As before shown, the blood as it flows through the tissues loses most of
its oxygen, and carbon dioxid takes its place. Now if the blood is to
maintain its efficiency in this respect, it must always be receiving new
supplies of oxygen, and also have some mode of throwing off its excess of
carbon dioxid. This, then, is the double function of the process of
respiration. Again, the blood sent out from the left side of the heart is
of a bright scarlet color. After its work is done, and the blood returns
to the right side of the heart, it is of a dark purple color. This change
in color takes place in the capillaries, and is due to the fact that there
the blood gives up most of its oxygen to the tissues and receives from
them a great deal of carbon dioxid.

In brief, while passing through the capillaries of the lungs the blood has
been changed from the venous to the arterial blood. That is to say, the
blood in its progress through the lungs has rid itself of its excess of
carbon dioxid and obtained a fresh supply of oxygen.[36]

214. Effects of Respiration upon the Air in the Lungs. It is well
known that if two different liquids be placed in a vessel in contact with
each other and left undisturbed, they do not remain separate, but
gradually mix, and in time will be perfectly combined. This is called
diffusion of liquids. The same thing occurs with gases, though the process
is not visible. This is known as the diffusion of gases. It is also true
that two liquids will mingle when separated from each other by a membrane
(sec. 129). In a