Lung Capacity.—The quantity of the air we breathe is also important. We may eat too much food, even though it be absolutely pure and wholesome, but we cannot consume too much pure air. The larger the lung space, therefore, the better for health and strength.
The full lung capacity of the average adult is about 330 cubic inches, but an ordinary inspiration does not take in more than one-eleventh part of that volume. The value of full, deep breathing, and of large lung capacity becomes at once apparent. The larger the quantity of air consumed, the greater the amount of life-giving oxygen conveyed through the blood to all parts of the body. No form of physical exercise, therefore, can exceed in value the breathing exercises described in another chapter.
Rate of Breathing.—It is estimated that we breathe once during every four beats of the heart, or about eighteen times a minute. The relation between the heart and lungs is so close that whatever modifies the pulse affects the breathing. When the heart action is hurried, more blood is sent to the lungs, requiring more rapid action on their part. About every fifth breath the inspiration is longer and fuller, the effect being to change more completely the air of the lungs.
Holding the Breath.—While respiration is, for the most part, involuntary, we may arrest the breathing for the space of twenty to thirty seconds. If we first fortify the lungs by taking several deep inspirations and expelling the impure air as fully as possible, we may hold the breath for a minute or two. This power will prove of advantage if we have occasion to pass through a room or hallway filled with smoke, or to remain under water for a brief time. The pearl-fishers, as a result of training, remain under water from three to four minutes.
Importance of Pure Air.—Pure air means pure blood. The air of the mountain tops or by the sea fills us with life, while that of narrow streets, crowded rooms, unventilated dwellings, schools, churches, and theatres is depressing, weakening, and death-dealing.
So far from the aristocracy having a monopoly of blue blood, it flows through the veins of high and low alike. It goes out from the lungs bright and rich with oxygen; it comes back to the heart dark with the waste and poisonous matters which it has gathered in its course.
Atmospheric air is composed of several gases, the principal elements being oxygen, nitrogen, and watery vapor. All animal life requires oxygen for the combustion of the material supplied through the blood. The blood makes its circuit through the body three times a minute. It comes to the lungs laden with poisonous matter. Nearly one-third of the excretions of the body are eliminated through the lungs. The average adult contaminates about five thousand cubic inches of air with every breath. The importance of having an abundant supply of pure air at all times is obvious.
In ordinary respiration an adult abstracts sixteen cubic feet of oxygen from the atmosphere every twenty-four hours, and adds to it fourteen cubic feet of carbonic acid in the same time. If the individual were confined in a close apartment, in which the air could not mingle with the atmosphere without, the processes of life could not long be maintained.
History furnishes many instances of the direful effects of crowding a number of human beings into a limited space without ventilation. One hundred and fifty passengers were confined in the small cabin of a steamship one stormy night, and when morning came only eighty were found alive. Three hundred prisoners, after the battle of Austerlitz, were crowded into a close prison, and within a few hours two hundred and sixty of them had died.