Of course, we see immediately one serious defect of this arrangement of the lung surface; every one of the individual sacs is full of air and so the blood vessels which line its wall have exposure to air, but between the individual lung sac and the outside atmosphere is, first, the very tiny bronchial tube with which the sac connects, then the somewhat larger one into which that opens, then a still larger one, and so on until we come by way of the chief bronchial tube and windpipe up to the throat, and so through the mouth or nose to the outside. It is quite evident that this system of passages does not permit of a very free movement of air. We must realize also that the blood which flows through the walls of the lung sacs must constantly take up oxygen from the air within the sacs, if it is to meet the needs of the body tissues. Simple diffusion through the narrow bronchial tubes could not possibly bring oxygen into the lung sacs fast enough to supply the requirements of the blood flowing through their walls. The situation is met by active lung ventilation; that is, by forcibly changing the air in the lungs at frequent intervals. The way in which this is done is, as we all know, by breathing. Breathing is nothing but a bellows movement of the chest by which air is alternately expelled and allowed to enter. It does not require a very active lung ventilation to keep the air in the lung sacs sufficiently supplied with oxygen under conditions of bodily quiet. Our ordinary breathing movements are gentle, less than a quart of air is breathed in and out again in every breath, and we breathe only fifteen or sixteen times a minute. Of course, when there is vigorous functional metabolism, as in brisk muscular exercise, the oxidation processes in the tissues go on at a very much more rapid rate, and correspondingly larger amounts of oxygen must be carried by the blood to meet the demand. Under these circumstances there is an improvement in lung ventilation, the movements of the chest are greater and also happen more times in a minute.