256 A MANUAL OF PHYSIOLOGY. 



oxygen, alkalies pass out of the serum into the corpuscles, which 

 at the same time lose water and shrink in volume, while the mole- 

 cular concentration of the serum is diminished. Hamburger has 

 extended these observations to the circulating blood, and has shown 

 that the plasma of venous blood has a higher percentage of alkali, 

 protein, sugar, and fat, than the plasma of arterial blood, and that 

 the corpuscles have a greater volume, though not a greater diameter. 

 We may therefore suppose that in the pulmonary capillaries, under 

 the influence of oxygen, water passes into the plasma from the cor- 

 puscles. In the systemic capillaries the blood becomes loaded with 

 carbon dioxide, and therefore the corpuscles take up water from 

 the plasma, which accordingly has a more concentrated supply of 

 food-substances to offer to the tissues than the plasma of arterial 

 blood itself. Some writers see in this interchange an automatic 

 arrangement by which oxidation is favoured. Whatever may be 

 thought of this view and objections to it are not wanting the 

 current theory, that the corpuscles are simply passive carriers of 

 oxygen, and exercise no further influence on the plasma, breaks 

 down in face of the facts. We must admit that an active and 

 many-sided commerce exists between them and the liquid in which 

 they float. 



The nitrogen of the blood is simply absorbed. 



The Tension of the Blood-gases. If the gases of the blood 

 existed in simple solution, their tension or partial pressure could 

 be deduced from the amount dissolved and the co-efficient of 

 absorption. Since they are chemically combined, it is necessary 

 to determine it directly. 



This has been done by means of an apparatus called the aerotono- 

 meter. The blood is made to pass directly from the vessel to glass 

 tubes, which it traverses at the same time, the stream being divided 

 between them ; it then passes out again. The tubes are warmed 

 by means of a water-jacket to the body temperature. Some of them 

 are filled with gaseous mixtures having a greater, and the others with 

 mixtures having a smaller, partial pressure, say of carbon dioxide, 

 than is expected to be found in the blood. As the latter runs in a 

 thin sheet over the walls of the tubes, it loses carbon dioxide to some 

 of them and takes up carbon dioxide from others. From the altera- 

 tion in the proportion of the carbon dioxide in the tubes, it is 

 easy to calculate the partial pressure of that gas in the blood ; that 

 is, the partial pressure which it would be necessary to have in the 

 tubes in order that the blood might pass through them without 

 losing or gaining carbon dioxide (p. 248). 



The pressure of oxygen in arterial blood was given by 

 Strassburg as about 30 mm. of mercury in the dog (corresponding 

 to the partial pressure of oxygen in a gaseous mixture at atmo- 

 spheric pressure when 4 per cent, of it is present), and in venous 

 blood as something like 20 mm. If we were to accept the 

 experiments of Bohr, made by means of a special form of aero- 

 tonometer constructed and worked much in the same way as 

 Ludwig's stromuhr (p. 112), and inserted into the course of a 

 bloodvessel, it would be necessary to treble or quadruple these 

 numbers. 



