Domestic science (Johnson, for schools and homes)

United with oxygen, the combination performs the functions of animal life. Respiration, mechanically considered, consists of the compound act of inhaling and exhaling air; commonly called breathing. The mission of respiration is to "purify ihe blood"; that is, to burn up its waste matter. 104 DOMESTIC SCIENCE From all parts of the body the blood comes pouring in dark red floods into the heart, which acts as a double force* pump, driving the blood to the lungs and over the body with each rythmic dilation and contraction. When the dark red blood, sent to the heart by vessels called veins, is pumped to the lungs, it does not freely en- ter the tiny air cells which compose the lungs; but it is forced into the net-work of hair-like vessels, called capillaries, which Fig. 31. The Lungs. A. Trachea (windpipe). C. D. Bronchial Tubes. lie on the mucuous membrane lining the air cells. The walls of the capillaries are so thin that the gases pass through them readily; the oxygen of the air in the air cells enter- ing the capillaries, and combining with the carbon in the APPENDIX 105 blood, and, in this act, releasing a gas from the carbon, (carbonic acid gas), which enters the air in the air cells, and mx yxx L.A. Left auricle. L.V. Left ventricle. Ao. Aorta. H.V. Hepatic vein. V.C.L Inferior vena ca- va. P.A. Pulmonary ar- tery. Lg. Ly. R.A. R.V. H.A. V.P. Lung. Lymphatic. Right auricle. Right ventricle. Hepatic artery. Vena portae. V.C.S. Superior vena cava. P.V. Let. Th.D. A.I. Pulmonary vein. Lacteals. Thoracic Duct. Arteries to the upper part of the body. A.2. Arteries to the lower part of the body. V.L Veins of the up- per part of the body. V.2. Veins of the low- er part of the body. The arrows indicate the course of the flow of blood, lymph, and chyle. Chart of the System of Blood Circulation. is exhaled. This action of the oxygen upon the blood, changes the color of the blood from the dark red of the im- 106 DOMESTIC SCIENCE pure, or venous blood, to the bright scarlet of the pure blood. The pure blood is now returned to the heart, whence it is pumped through a different class of vessels, called arteries, and is thus sent all over the body to carry oxygen, and pure building material to the tissues. The air, on entering the lungs, if fresh, is composed of about one-third oxygen and two-thirds nitrogen and moisture, with a trace of carbon; after it has performed its work in the lungs, it is exhaled, containing less oxygen, more watery va- por and more carbonic acid gas; the oxygen, in part, having been taken up and carried throughout the body by the blood vessels, and the watery vapor and carbonic acid gas having been given out from the blood after the ''burning", or union of the carbon with the oxygen. The temperature of the ex- haled air is also higher than that of the inhaled air, and germs flying off the mucuous lining of the trachea and lung cells may be present in exhaled air also, in greater propor- tion than in the inhaled air. The fitness of this exhaled air to be inhaled again, depends upon its proper dilution with fresh out-door air.