Part 32 (2/2)

491. The walls of the air-vesicles, and coats of the blood-vessels, are similar, in their mechanical arrangement, to the membranous bladder in the before described experiment. As the oxygen of the air has greater affinity for blood than for nitrogen, so it permeates the membranes that intervene between the air and blood more readily than the nitrogen. As the carbonic acid and water have a greater affinity for air than for the other elements of the blood, so they will also pa.s.s through the walls of the blood-vessels and air-cells more readily than the other elements of the dark-colored blood.

[Ill.u.s.tration: 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 pa.s.sing in the minute vessels over the air-cells, the oxygen pa.s.ses 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 pa.s.s 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.

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

_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 pa.s.sing through the bladder, and uniting with the blood, while the carbonic acid has escaped through the membrane.

[Ill.u.s.tration: 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 upon a cold, dry mirror for a few minutes, and it will be covered with moisture. This is condensed vapor from the lungs. In warm weather, this watery vapor is invisible in the expired air, but in a cold, dry morning in winter, the successive jets of vapor issuing from the mouth and nose are sufficiently obvious.

Give the experiment showing that oxygen changes the dark-colored blood to a bright red color. What is represented by fig. 99? 493. How can the presence of carbonic acid in the lungs be proved?

494. From the lungs are eliminated other impurities beside carbonic acid, the perceptible quality of which is various in different persons. The offensive breath of many persons may be caused by decayed teeth, or the particles of food that may be retained between them, but it often proceeds from the secretion, in the lungs, of certain substances which previously existed in the system.

_Ill.u.s.tration._ When spirituous liquors are taken into the stomach, they are absorbed by the veins and mixed with the dark-colored blood, in which they are carried to the lungs to be expelled from the body.

This will explain the fact, which is familiar to most persons, that the odor of different substances is perceptible in the breath, or expired air, long after the mouth is free from these substances.

How the watery vapor? 494. Are there other excretions from the lungs?

Give the ill.u.s.tration.

_Note._ Let the anatomy and physiology of the respiratory organs be reviewed from figs. 96, 97, and 99, or from anatomical outline plates Nos. 5 and 7.

CHAPTER XXV.

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