As the carbonic acid and water have a greater affinity
for air than for the other elements of the blood, so they will also
pass through the walls of the blood-vessels and air-cells more readily
than the other elements of the dark-colored blood.

[Illustration: Fig. 98. 1, A bronchial tube divided into three branches.
2, 2, 2, Air-cells. 3, Branches of the pulmonary artery, that spread over
the air-cells. Through the pulmonary artery the dark, impure blood is
carried to the air-cells of the lungs. 4, Branches of the pulmonary vein,
that commence at the minute terminations of the pulmonary artery. Through
the pulmonary vein the red blood is returned to the heart.]

492. As the impure blood is passing in the minute vessels over the
air-cells, the oxygen passes through the thin coats of the air-cells
and blood-vessels, and unites with the blood. At the same time, the
carbonic acid and water leave the blood, and pass through the coats of
the blood-vessels and air-cells, and mix with the air in the cells.
These are thrown out of the system every time we breathe. This
interchange of products produces the change in the color of the
blood.

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Explain fig. 98. 492. How and where is the blood changed?

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_Experiment._ Fill a bladder with dark blood drawn from any animal.
Tie the bladder closely, and suspend it in the air. In a few hours,
the blood next to the membrane will have become of a bright red color.
This is owing to the oxygen from the air passing through the bladder,
and uniting with the blood, while the carbonic acid has escaped
through the membrane.

[Illustration: Fig. 99. An ideal view of the pulmonary circulation. 1, 1,
The right lung. 2, 2, The left lung. 3, The trachea. 4, The right
bronchial tube. 5, The left bronchial tube. 6, 6, 6, 6, Air-cells. 7, The
right auricle. 8, The right ventricle. 9, The tricuspid valves. 10, The
pulmonary artery. 11, The branch to the right lung. 12, The branch to the
left lung. 13, The right pulmonary vein. 14, The left pulmonary vein. 15,
The left auricle. 16, The left ventricle. 17, The mitral valves.]

493. The presence of carbonic acid and watery vapor in the expired
air, can be proved by the following experiments: 1st. Breathe into
lime-water, and in a few minutes it will become of a milk-white color.
This is owing to the carbonic acid of the breath uniting with the
lime, forming the _carbonate of lime_. 2d. Breathe u