With regard to the physiology of respiration, we entirely discard the unlikely "theory of secretion" (by means of the epithelium of the lung), and explain the interchange of gases between the blood and the air in the lungs, in accordance with many physiologists, by purely chemical and physical conditions.

The blood contains gases to the extent of 70 per cent. of its volume. At 0° C, and with the barometer at 760 mm., we have, according to Bohr: -

In arterial blood 20 per cent. oxygen. 43.6 per cent. C02. 1.2 per cent. nitrogen.

In venous blood 12 per cent. oxygen. 50 per cent. C02. 1.2 nitrogen.

The oxygen is to a large extent loosely combined with haemoglobin in the red blood corpuscles (according to Bohr in the form of "haemo-chrome "); only a comparatively small amount is found in the plasma, and this in simple solution. The C02 in the blood is combined mainly as carbonates, and to a less extent as phosphates and protein compounds. In addition, as much as will go into simple solution naturally does so. The small quantity of nitrogen is dissolved in the plasma.

In the venous blood in the lungs the pressure of oxygen is 22 mm. Hg. and the pressure of C02 is 41 mm.* In the atmospheric air the pressure of oxygen is about 152 mm., that of C02 0.3 mm. The pressure of gases in the alveoli of the lungs is not known, but from there in the whole length of the air passages to the opening at the nose or mouth the pressure of C02 diminishes; oxygen diminishes in the whole length in the opposite direction. As every gas goes from the place of higher to that of lower pressure, the oxygen, for this and other reasons, goes from the nostrils to the alveoli, C02 travelling in the opposite direction.

According to Speck, during rest we take in 0.461 to 0.601 grm. oxygen every hour for every kilo of body weight, and give off during the same time 0.535 to 0.717 grm. CO,. This takes place by means of the surface area of the lung, which for a person weighing 70 kilos amounts to 80 to 00 square metres. The oxygen taken in combines loosely with the haemoglobin (forming oxyhaemoglobin) in the red corpuscles. One grm. haemoglobin contains 1.6 to 1.8 c.cm. oxygen, and the red corpuscles in the capillaries of the lungs represent a large area for absorption. Their surface area for the quantity of blood, reckoned at about 4 litres in the whole body, is said to amount to 2,560 sq. metres, or over 1/4 hectar.

* In arterial blood the pressure of oxygen is given by Pfluger as 30 mm., by Bohr as 20 ram.; that of Co2 is generally taken as about 20 mm.

The "internal, parenchymatous" respiration, or the act by means of which in the capillaries we take C02 from, and give off oxygen (from the oxygen-laden red corpuscles) to the tissues, is the real and final object of the whole process of respiration. It is the "internal" respiration, which by means of the oxidising of carbohydrates, fat, and in all probability, under certain conditions, also of protein, produces the energy we require.

The production of C02 does not depend upon the ventilation of the lung, but really upon our muscular work.

During energetic muscle action, with simultaneous good ventilation of the lung, the blood takes up so much oxygen that it becomes richer in this than under normal conditions (L. Hermann); in other words, the blood in the veins becomes less venous, the blood in the arteries "hyperarterial."

Internal respiration constitutes a large part of the metabolism of the organism, but, as one now understands, not the whole of metabolism. For this consists not only of oxidation processes, but also of processes of disintegration, and probably also of synthetic processes.

Oxygen easily passes by way of the lymph from the capillaries, where its pressure is great, to the tissues, where it is least or (in the muscles) nil. C02 passes easily from the tissues, where its pressure is great, to the capillaries, where it is only 20 mm., and where C02is taken up principally by the carbonates and to some extent by the haemoglobin and other proteins and the phosphates.

As to the question of the seat of internal respiration and of the whole of metabolism, there are two chief currents of opinion.

The theory of Pfluger states that internal respiration takes place chiefly in the tissues. Probably, too, most physiologists hold the opinion that the cells of the organism are continually burning away their own component parts, and giving off products of combustion to the blood, from which also they continually take up and assimilate the food substances found there in order to replace those used up.

Voit and his followers consider the blood to be the seat of actual combustion and consider that the tissues take the second place. To a certain extent the theory of Voit coincides with that of Pfluger. For the blood is certainly a "moving tissue," and its cells, the blood corpuscles, undoubtedly perform in their own bodies some part of oxidation and of metabolism as a whole.

It is the C02 in the expired air and the nitrogen in the urine which enable us to estimate metabolism, C02 as regards the three organic food substances (protein, fat, carbohydrate), and the nitrogen of the urine as regards protein.

We may consider it certain that the combustion of the sugars in the blood to C02 and water, which according to many authors is the only way, according to others one of the most important ways, of producing energy in the organism, goes on in all the tissues, quite vigorously in the working glands, but more actively in the working muscles than in any other tissue. Interchange of gases in the working muscle may be over twenty times as great as in the muscle at rest (Chauveau and Kauffmann).

The muscle itself does not contain free oxygen, but only Co2 and some nitrogen.

During work the muscle changes its own chemical composition and that of the blood by using up glycogen, sugar, and fat; diminishes the organic combinations of phosphorus and increases mono-phosphates; increases the formation of sulphates by means of oxidation of the sulphur of the proteins to sulphuric acid; increases the oxidation of protein, at least under certain conditions, and with it urea, and changes its amphoteric reaction to an acid reaction - probably by the formation of lactic acid.

Many people think that the production of C02 in this way is not directly the result of oxidation, but of the disintegration of substances themselves formed by oxidation. Frogs give off nearly as much C02 in an atmosphere free from oxygen as in the air. But one thing is certain - that oxygen is necessary for the working muscle; without it the muscle becomes suffocated.