The arterial blood, while flowing through the capillaries of the systemic circulation and supplying the tissues with nutriment, undergoes changes which are called internal or tissue respiration, and which may be shortly defined to be the converse of pulmonary or external respiration. In the external respiration the blood is changed from venous to arterial; whereas in internal respiration the blood is again rendered venous.
There can now be no doubt that these chemical changes take place in the tissues themselves, and not in the blood as it flows through the vessels. The amount of oxidation that takes place in the blood itself is indeed very small. The tissues, however, along with the substances for their nutrition, extract a certain part of the O from the blood. In the chemical changes which take place in the tissues, they use up the oxygen, which rapidly disappears, the tension of that gas becoming very low; at the same time other chemical changes are indicated by the appearance of C02. The disappearance of the O and the manufacture of C02 do not exactly correspond in amount, and they, doubtless, often vary in different parts and under different circumstances. Of the intermediate steps in the tissue chemistry we are ignorant. We do not know the way in which the oxygen is induced by the tissues to leave the haemoglobin; we can only say that the tissues have a greater affinity for O than the haemoglobin has, and they at once convert the O into more stable compounds than oxyhaemoglobin, and ultimately manufacture C02, which exists in the tissues and fluids of the body at a higher tension than even in the venous blood.