The blood undergoes a series of modifications, and is constantly being altered as it passes from one part or organ to another.

It has already been seen that a quantity of nutrient material is taken up by the blood on its way through the capillaries of the alimentary tract, and a stream of lymph and chyle is poured into it when it reaches the great venous trunks; so that from two sources the blood is obviously increased in quantity. The most essential change that takes place in the circulatory fluid is the respiratory, and the addition it most urgently demands is that which it receives in the capillaries of the lungs. All the blood passes through these organs in order to ensure the elimination of the carbonic acid acquired in the general systemic capillaries, and the recharging of the red corpuscles with oxygen.

These gas interchanges will form the subject matter of the present chapter; and the more especial modifications which the blood undergoes in the ductless glands, the spleen, the liver, etc., as well as in the kidneys and other excretory glands, will be considered subsequently.

As has already been pointed out (Chapter V (Food)), an animal during its life may be said to use the substances supplied to it in food as fuel, and thus to acquire the energy which is bound up in them; for the activities of the various tissues are really combustions, being invariably associated with oxidation of some of the carbon compounds, so as to produce carbon dioxide and water. In order that the structures may be able to undergo this change they must have a ready supply of oxygen constantly at hand, and, moreover, the carbon dioxide which is formed in the process must be removed. The regular income of oxygen and the regular discharge of carbon dioxide are the first essentials to life; hence we find in almost all animals special arrangements known as the respiratory apparatus, by means of which these gases can find their way to and from the tissues and external air respectively.

Here, as in the case of the nutritive materials, the blood acts as the carrier. The pulmonary half of the circulation is devoted to the gas interchange between the blood and the atmosphere, and is sometimes spoken of as external respiration. The gas interchange between the blood and the tissues takes place in the general systemic capillaries, and has, therefore, been spoken of as the internal or tissue respiration.

In mammalia the pulmonary apparatus is so far perfected that all the necessary gas interchange can be carried on by the lungs, and the respiratory influence of the external skin or the mucous passages may be regarded as insignificant. But it should be remembered that, whenever the blood is in close relation to oxygen, as in the case of swallowed air, the oxygen is soon absorbed by the blood.

In some of the lower animals the cutaneous surface aids very materially in respiration; for example, frogs can live by this cutaneous respiration alone for an almost indefinite time.

The change in the lungs consists in (1) oxygen being taken from the atmospheric air* by the blood and (2) carbon dioxide being given off from the blood to the air. In the capillaries, on the other hand, the blood takes the carbon dioxide from the tissues, and yields to them a great portion of its oxygen.