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and is an index of an individual's power of breathing in disease and prolonged severe exercise. The combined amounts of the tidal, the complemental, and reserve air, 230 cubic inches, represents the vital capacity of an individual 5 feet 7 inches in height. The vital capacity varies chiefly with stature. It is increased 8 cubic inches for every inch in height above this standard, and diminishes 8 cubic inches for each inch below it.

The Tidal Volume of air is carried only into the trachea and larger bronchial tubes by the inspiratory movements. It reaches the deeper portions of the lungs in obedience to the law of diffusion of gases, which is inversely proportionate to the square root of their densities.

The ciliary action of the columnar cells lining the bronchial tubes also assists in the interchange of air and carbonic acid.

The entire volume of air passing in and out of the thorax in 24 hours is subject to great variation, but can be readily estimated from the tidal volume and the number of respirations per minute. Assuming that an individual takes into the chest 20 cubic inches at each inspiration, and breathes 18 times per minute, in 24 hours there would pass in and out of the lungs 518,400 cubic inches, or 300 cubic feet.

Chemistry of Respiration.-As the inspired air undergoes a change in composition during its stay in the lungs which renders it unfit for further respiration, it becomes requisite, for the correct understanding of respiration, to ascertain the composition of both inspired and expired air.

Composition of Air.-Chemical analysis has shown that every 100 vols. of air contains 20.81 vols. of oxygen, and 70.19 vols. of nitrogen, and 0.03 vol. of carbonic acid. Aqueous vapor is also present, though the quantity is variable. The higher the temperature the greater the amount. The changes in the air effected by respiration are—

Loss of oxygen, to the extent of 5 cubic inches per 100 of air, or I in

20.

Gain of carbonic acid, to the extent of 4.66 cubic inches per 100 of air, or .93 inch in 20.

Increase of water vapor and organic matter.

Elevation of temperature.

Increase and at times decrease of nitrogen.

Gain of ammonia.

The total quantity of oxygen withdrawn from the air and consumed by the body in 24 hours amounts to 15 cubic feet, and can be readily estimated from the amount consumed at each respiration. Assuming that one inch of oxygen remains in the lungs at each respiration, in one hour there

are consumed 1080 inches, and in 24 hours, 25,920 cubic inches or 15 cubic feet, weighing 18 oz. To obtain this quantity, 300 cubic feet of air are necessary.

The quantity of oxygen consumed daily is subject to considerable variations. It is increased by exercise, digestion, and lowered temperature, and decreased by the opposite conditions.

The quantity of carbonic acid exhaled in 24 hours varies greatly. It can be estimated in the same way. Assuming that an individual exhales .93+ cubic inch at each respiration, in one hour there are eliminated 1008 cubic inches, and in 24 hours, 24.192 cubic inches or 14 cubic feet, containing 7 ozs. of pure carbon.

The exhalation of carbonic acid is increased by muscular exercise, nitrogenous food, tea, coffee, and rice, age, and by muscular development; decreased by a lowering of temperature, repose, gin and brandy, and a dry condition of the air.

As there is always more oxygen consumed than carbonic acid exhaled, and as oxygen unites with carbon to form an equal volume of carbonic acid, it is evident that a certain quantity of oxygen disappears within the body. In all probability it unites with the sulphur hydrogen of the food to form water.

The amount of watery vapor which passes out of the body with the expired air is estimated at from one to two pounds.

The organic matter, though slight in amount, gives the odor to the breath. In a room with defective ventilation, the organic matter accumulates and gives rise to headache, nausea, drowsiness, etc. Long-continued breathing of such air produces general ill health. It is not so much the presence of CO, in increased amount as the presence of organic matter which necessitates thorough ventilation.

Condition of the Gases in the Blood.

Oxygen is absorbed from the lungs into the arterial blood by the coloring matter, hemoglobin, with which it exists in a state of loose combination, and is disengaged during the process of nutrition.

Carbonic acid, arising in the tissues, is absorbed into the blood, in consequence of its alkalinity, where it exists in a state of simple solution and also in a state of feeble combination with the carbonates, soda and potassa, forming the bicarbonates.

Nitrogen is simply held in solution in the plasma.

Exchange of Gases in the Air Cells.-From the difference in tension of the oxygen in the air cells (27.44 mm. of Hg), and of the oxygen

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in the venous blood (22 mm. Hg), and of the difference of the carbonic acid tension in the venous blood (41 mm. Hg), and in the air cells (27 mm. Hg), it might be concluded that the passage of the gases might be due solely to pressure. The absorption of oxygen, however, does not follow absolutely the law of pressures; that chemical processes are involved is shown by the union of oxygen with the hemoglobin of the blood corpuscles. The exhalation of CO, is also partly a chemical process, as it has been shown that the quantity excreted is greatly increased when oxygen is simultaneously absorbed. Oxygen not only favors the exhalation of loosely combined CO2, but favors the expulsion of that which can only be excreted by the addition of acids to the blood.

Changes in the Blood during Respiration.

As the blood passes through the lungs it is changed in color, from the dark purple hue of venous blood to the bright red scarlet of arterial blood.

The heterogeneous composition of venous blood is exchanged for the uniform composition of the arterial.

It gains oxygen and loses carbonic acid.

Its coagulability is increased. Temperature is diminished.

Asphyxia. If the supply of oxygen to the lungs be diminished and the carbonic acid retained in the blood, the normal respiratory movements cease, the condition of asphyxia ensues, which soon terminates in death.

The phenomena of asphyxia are, violent spasmodic action of the respiratory muscles, attended by convulsions of the muscles of the extremities, engorgement of the venous system, lividity of the skin, abolition of sensibility and reflex action, and death.

The cause of death is a paralysis of the heart, from over distention by blood. The passage of the blood through the capillaries is prevented by contraction of the smaller arteries, from irritation of the vasomotor center. The heart is enfeebled by a want of oxygen and inhibited in its action by the inhibitory centers.

ANIMAL HEAT.

The Functional Activity of all the organs and tissues of the body is attended by the evolution of heat, which is independent, for the most part, of external conditions. Heat is a necessary condition for the due performance of all vital actions; though the body constantly loses heat by radiation and evaporation, it possesses the capability of renewing it and main

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