Mechanism for Increasing Exposed Surface of Fluid.— In the lung of the frog a much larger surface of blood can be exposed than in that of the newt, for the inner surface of the lung is thrown up into ridges, called septa. These again give rise to secondary and even ter tiary septa, as is shown in fig. i b. All these septa are richly sup plied with blood capillaries.
The lung of the warm blood animals is more complicated still. It may be likened, not to a grape, but to a bunch of grapes—in deed to several bunches of grapes. The unit corresponding to a single grape is called the infundibulum. That corresponding to a bunch the lobule. Each infundibulum is intermediate in structure between the whole lung of the newt and that of the frog. It con tains septa, but only primary septa (fig. lc). These divide the margin of the infundibulum into a number of chambers, the alveoli. The interior of the infundibulum is, therefore, a sort of honeycomb, the alveoli corresponding to the cells of the comb; indeed, they are often called the air cells. In microscopical sec tions of the lung the air cells are cut across in all sorts of quite irregular ways, but the general appearance much resembles that of a section of a rather broken honeycomb (fig. 2).
Before birth the whole lung is folded up, the opposing walls of the air cells are in contact with one another and there is, of course, no air in the lung. Such a lung will sink if thrown into water, in which respect it is in marked contrast to the normal or gan. It is one of the abiding mysteries of creation, that, when the new born child expands his lungs for the first time, the whole system of lobulae, infundibula and alveoli unfolds and fills with air. From that time onwards air is always passing into and out of the lung. The quantity taken in at each respiration is called the tidal air and is normally about 300-550 cu.cm. Of this about ISO cu.cm. never goes further than the respiratory passages; the remainder becomes mixed up with the air in the air cells (al veolar air) of which there is, perhaps, three litres in the lung. The following table gives the percentage composition of inspired and alveolar air:— There are several ways of measuring the composition of al veolar air. That of Haldane and Priestley consists of blowing with extreme suddenness and force down a rubber tube about 5 ft. in length and about i in. in diameter. The air from the respiratory passages passes first along the tube and is washed out by the air from the deeper parts of the lung. If the subject has emptied his lung to the maximum the tube, or at least, the portion next to his mouth will contain pure alveolar air. Immediately after the
expiration, the tube is closed with the tongue. To it, about r in. from the mouth, is fitted a vacuous sampling tube; by the open ing of a tap a sample of the air in the alveolar air tube can be taken into the sampling tube for analysis.
Residual air is the volume of air remaining in the chest after the most complete respiratory effort. It ranges from 1,600-2,ioo cu.cm.
Reserve or Supplemental Air is the volume of air which can be expelled from the chest after an ordinary quiet respiration—about 1,500 cu.cm.
Complemental air is the volume of air that can be forcibly in spired over and above what is taken in by normal inspiration and is 1,600-2,100 cu.cm.
Vital capacity is the quantity of air which can be expelled from the lungs by the deepest possible expiration, after the deepest possible inspiration. It obviously includes the complemental, tidal and reserve airs. The vital capacity of 73 Air Force pilots in the British army, tabu lated by Col. Flack, varied between 5,500 cu.cm. and 2,800 cu.cm. Considerable im portance is attached to the vital capacity as an index of the suitability of pilots for high flying. Vital capacity is measured by means of a spirometer, a graduated gasom eter into which air is blown from the lungs.
Lung Surface.—The whole surface pre sented by the walls of all the alveoli of a single human lung has been computed at about I,000 sq.f t.; over the whole of this there is a compact network of capillaries, spread like a close pattern on a carpet. The blood in this vessel is separated from the air in the alveoli only by a membrane of almost inconceivable thinness.
Minute Volume of Blood.—The quantity of blood which reaches the lung in man is variously computed as being from 3-7 litres per minute during rest, and may be increased probably to 20 or 30 litres per minute or even more during exercise and in athletic persons. This blood comes from the right side of the heart, along the pulmonary artery, and parts with about so cu.cm. of carbon dioxide per litre in its transit of the lung. Simultaneously it picks up about the same amount of oxygen or rather more.
Provision, therefore, is required for air to ventilate the lung in sufficient quantity to carry off about 25o cu.cm. of carbon dioxide per minute during rest, and to supply about 30o cu.cm. of oxygen. Moreover this oxygen must be contributed without so far deplet ing the air itself as seriously to reduce the rate of diffusion.
In practice, the level of carbon dioxide in air of the alveoli is not allowed to rise above 5.5%, nor the oxygen to sink below about 14% at the sea level.