Again, nerves of sensation on the surface of the body convey influences to this nerve center and lead to its stimulation, resulting in a vigorous breathing movement.  Thus a dash of cold water on the face or neck of a fainting person instantly produces a deep, long-drawn breath.  Certain drugs, as opium, act to reduce the activity of this nerve center.  Hence, in opium poisoning, special attention should be paid to keeping up the respiration.  The condition of the lungs themselves is made known to the breathing center, by messages sent along the branches of the great pneumogastric nerve (page 276), leading from the lungs to the medulla oblongata.

213.  Effects of Respiration upon the Blood.  The blood contains three gases, partly dissolved in it and partly in chemical union with certain of its constituents.  These are oxygen, carbon dioxid, and nitrogen.  The latter need not be taken into account.  The oxygen is the nourishing material which the tissues require to carry on their work.  The carbon dioxid is a waste substance which the tissues produce by their activity, and which the blood carries away from them.

As before shown, the blood as it flows through the tissues loses most of its oxygen, and carbon dioxid takes its place.  Now if the blood is to maintain its efficiency in this respect, it must always be receiving new supplies of oxygen, and also have some mode of throwing off its excess of carbon dioxid.  This, then, is the double function of the process of respiration.  Again, the blood sent out from the left side of the heart is of a bright scarlet color.  After its work is done, and the blood returns to the right side of the heart, it is of a dark purple color.  This change in color takes place in the capillaries, and is due to the fact that there the blood gives up most of its oxygen to the tissues and receives from them a great deal of carbon dioxid.

In brief, while passing through the capillaries of the lungs the blood has been changed from the venous to the arterial blood.  That is to say, the blood in its progress through the lungs has rid itself of its excess of carbon dioxid and obtained a fresh supply of oxygen.[36]

214.  Effects of Respiration upon the Air in the Lungs.  It is well known that if two different liquids be placed in a vessel in contact with each other and left undisturbed, they do not remain separate, but gradually mix, and in time will be perfectly combined.  This is called diffusion of liquids.  The same thing occurs with gases, though the process is not visible.  This is known as the diffusion of gases.  It is also true that two liquids will mingle when separated from each other by a membrane (sec. 129).  In a similar manner two gases, especially if of different densities, may mingle even when separated from each other by a membrane.