The act of respiration consists in (a) absorbing oxygen into blood, and (b) giving out carbon dioxide (shortly called C02) from the blood. Each act of respiration includes one act of inspiration (or drawing air from outside into our lungs) and another of expiration, (i.e., giving out of impure or C02 laden air from the lungs into the outer air). In health, in a minute, we take about 18 respirations. The act of respirations is partly mechanical and partly (reflexly) nervous. Elastic recoil of muscles like the diaphragm and the intercostals, (i.e., those muscles that are attached to the ribs), the tilted condition of the ribs themselves, the lungs being placed inside the practically closed cavity called thorax—all these help to mechanically carry on the act of respiration, But the most important factor in carrying on the act, is the stimulus to the nervous system, caused by accumulation in the blood of excessive carbonic acid gas. The moment this gas accumulates in excess in blood, a strong impulse is sent down, by the nervous system, to all the muscles concerned in carrying on the respiratory act, to act vigorously and quickly—and respiration is the result. Normally, in health, such an impulse is being continually sent out by the nervous system in response to the accumulation of C02 in blood and this is what automatically carries on the act and keeps us alive.
In this connection, it will be well to become acquainted with certain technical terms. The amount of air constantly going into and coming out of the lungs, is called tidal air. The quantity of air that can be drawn into the lungs by the deepest act of inspiration is called the complemental air, that quantity of air which can be forced out by the strongest expiratory effort, is called supplemental air. Vital capacity is the sum total of tidal, complemental and supplemental air (230 cub. inch of air). Asphyxia is a condition of stoppage of breathing; dyspnoea is difficult or embarrassed breathing. Orthopnoea is a condition of difficult breathing eased by sitting down.