The first point where important changes occur is the lungs. Here the blood loses the purplish hue which it always has after being used in the body tissues, and takes on. a bright scarlet, - a phenomenon that is more easily understood when we understand the lung structure.

137. Object Of Breathing

Fig. 11. - Air tubes of lungs. (Gheeubh.)

Breathing is a matter of common experience. We all know how air is drawn into the lungs at regular intervals, and equivalent volumes of modified air being as regularly forced out. The mechanism of respiration (breathing) we will not discuss at length. It will aid us, however, if we know that the passage which the air follows to and from the lungs, the trachea (windpipe), divides into two branches, one to each lung, and these divide and subdivide until they branch into numerous fine tubes. Each of these tubes ends in an elongated dilation which is made up of air cells opening into a common cavity. These cells are so numerous in the lung tissues that only a very thin wall separates adjoining ones, and in this wall are carried the capillaries or fine divisions of the blood-vessels leading from the heart. This arrangement permits the (venous) blood, as it flows along, to take up oxygen from the respired air and transfer certain wastes into the lung cavities, and thus be made ready to go back to the body carrying a joint load of digested food and of oxygen that is held in combination with the hsematin. Of course the air that passes out of the lungs is less rich in oxygen than when it was taken in, and there have been added to it certain materials which we will notice later.

It is easily understood from these facts that respiration stands in a fundamental relation to nutrition. The lungs are to the body what the draft is to the furnace. Food can no more be used without the supply of oxygen through the lungs than can coal be burned without an access of air to the fuel box.