similar manner two gases, especially if of different
densities, may mingle even when separated from each other by a membrane.

In a general way this explains the respiratory changes that occur in the
blood in the lungs. Blood containing oxygen and carbon dioxid is flowing
in countless tiny streams through the walls of the air cells of the lungs.
The air cells themselves contain a mixture of the same two gases. A thin,
moist membrane, well adapted to allow gaseous diffusion, separates the
blood from the air. This membrane is the delicate wall of the capillaries
and the epithelium of the air cells. By experiment it has been found that
the pressure of oxygen in the blood is less than that in the air cells,
and that the pressure of carbon dioxid gas in the blood is greater than
that in the air cells. As a result, a diffusion of gases ensues. The
blood gains oxygen and loses carbon dioxid, while the air cells lose
oxygen and gain the latter gas.

[Illustration: Fig. 92.--Capillary Network of the Air Cells and Origin of
the Pulmonary Veins.

A, small branch of pulmonary artery;
B, twigs of the pulmonary artery anastomosing to form peripheral network
of the primitive air cells;
C, capillary network around the walls of the air sacs;
D, branches of network converging for form the veinlets of the pulmonary
veins.
]

The blood thus becomes purified and reinvigorated, and at the same time is
changed in color from purple to scarlet, from venous to arterial. It is
now evident that if this interchange is to continue, the air in the cells
must be constantly renewed, its oxygen restored, and its excess of carbon
dioxid removed. Otherwise the process just described would be reversed,
making the blood still more unfit to nourish the tissues, and more
poisonous to them than before.

215. Change in the Air in Breathing. The air which we exhale during
respiration differs in several important particulars from the air we
inhale. Both contain chiefly the three gases, though in different
quantities, as the following table shows.

Oxygen. Nitrogen. Carbon Dioxid.
Inspired air contains 20.81 79.15 .04
Expired air contains 16.03 79.58 4.38

That is, expired air contains about five per cent less oxygen and five per
cent more carbon dioxid than inspired air.

The temperature of expired air is variable, but generally is higher than
that of inspired