The stationary air mingles with the tidal air at every breath. If you want to ventilate a room, you are not obliged to take a pair of bellows and drive out every bit of the old air in the room, and supply its place with new air: it will be enough if you open a window or a door and let in a draught of pure air across one corner, say, of the room. That current of pure air flowing across the corner will mingle with all the rest of the air until the whole air in the room becomes pure; and the mingling will take place very quickly. So it is in the lungs. The tidal air comes in with each inspiration as pure air from without; but before it comes out at the next expiration it gives up some of its oxygen to the stationary air, and robs the stationary air of some of its carbonic acid. For each breath of tidal air the stationary air is so much the better, having lost some of its carbonic acid and gained some fresh oxygen. The tidal air rapidly purifies the stationary air, and the stationary air purifies the blood.
Thus it comes to pass that the tidal air, which at each pull of the diaphragm and push of the sternum goes into the chest as pure air with twenty-one parts oxygen to seventy-nine parts nitrogen in every hundred parts, comes out, when the diaphragm goes up and the sternum falls back, as impure air with only sixteen parts oxygen, but with five parts carbonic acid to seventy-nine of nitrogen. That lost oxygen is carried through the stationary air to the blood in the capillaries, and the gained carbonic acid came through the stationary air from the blood in the capillaries. So each breath helps to purify the blood, and the pumping of air in and out of the chest changes the impure, hurtful, venous, to pure, refreshing, arterial blood; the blood breathes air in the lungs, that all the body may in turn breathe blood.
HOW THE BLOOD IS CHANGED BY FOOD: DIGESTION. § VII.
[45.] The blood is not only purified by air, it is also renewed and made good by food. The food we eat becomes blood. But our food, though frequently moist, is for the most part solid. We cut it into small pieces on the plate, and with our teeth we crush and tear it into still smaller morsels in our mouth. Still, however well chewed, a great deal of it, most of it in fact, is swallowed solid. In order to become blood it must first be dissolved. It is dissolved in the alimentary canal, and we call the dissolving digestion. Let us see how digestion is carried on.
Your skin, though sometimes quite moist with perspiration, is as frequently quite dry. The inside of your mouth is always moist—very frequently quite filled with fluid; and even when you speak of it as being dry, it is still very moist. Why is this? The inside of your mouth is also very much redder than your skin. The redness and the moisture go together.
In speaking of the capillaries, I said that almost all parts of the body were completely riddled with them, but not quite all. A certain part of the skin, for instance, has no capillaries or blood-vessels at all. You know that where your skin is thick, you can shave off pieces of skin without “fetching blood;” if your
a, horny epidermis; b, softer layer; c, dermis; d, lowermost vertical layer of epidermic cells; e, cells lining the sweat duct continuous with epidermic cells; h, corkscrew canal of sweat duct. To the right of the sweat duct the dermis is raised into a papilla, in which the small artery, f, breaks up into capillaries, ultimately forming the veins, g.