668 PHYSIOLOGY OF RESPIRATION. 



the capillaries 5.5 vols, of oxygen are still used by the tissues it is 

 apparent that the blood will enter the veins with practically all of 

 its hemoglobin in the form of reduced hemoglobin; there is com- 

 plete or nearly complete oxygen-unsaturation. In some inter- 

 esting experiments, upon the gases in human blood it has been 

 shown that the local temperature of the tissues greatly influences 

 the degree of unsaturation of the hemoglobin. When the arm, 

 for example, is exposed to cold the skin, as everyone knows, be- 

 comes cyanotic; when it is warmed the skin takes on a vivid 

 pink color. Direct observations* show that in the blood of the 

 superficial veins under the influence of cold the oxygen-unsatura- 

 tion may be complete, the hemoglobin has lost all of its oxygen, 

 while in the warmed skin, on the contrary, the venous blood 

 may contain almost as much oxygen as the arterial blood (94 as 

 compared with 96 per cent, saturation). These great fluctua- 

 tions with temperature are to be explained presumably by the 

 effects of the heat and cold upon the rapidity of the circulation 

 through the capillary region. Much work has been done in 

 physiolog}^ to determine the condition in which the gases are 

 held in the blood. The results obtained show that they are held 

 in small part in solution, but mainly in chemical combination. 

 To understand the part played by each factor and the conditions 

 that control the exchange of gases in the lungs and tissues it is 

 necessary to recall some facts regarding the physical and chem- 

 ical properties of gases. 



The Pressure of Gases and the Terms Expressing These 

 Pressures. — The air around us exists under a pressure of one 

 atmosphere and this pressure is expressed usually in terms of the 

 height of a column of mercury that it will support, namely, a 

 column of 760 mms. Hg, which is known as the normal barometric 

 pressure at sea-level. Air is a mixture of gaseSj and according to the 

 mechanical theory of gas-pressure each constituent exerts a pressure 

 corresponding to the proportion of that gas present. In atmospheric 

 air, therefore, the oxygen, being present to the extent of 20 per 

 cent., exerts a pressure of ^ of an atmosphere or ;^X 760 = 152 

 mms. Hg. When we speak of one atmosphere of gas pressure, 

 therefore, we mean a pressure equivalent to 760 mms. Hg, and in 

 any given mixture the pressure exerted by any constituent may 

 be expressed in percentages or fractions of an atmosphere, or in the 

 equivalent height of the mercury column which it will support. 



Absorption of Gases in Liquids. — When a gas is brought into 

 contact with a liquid with w^hich it does not react chemically a 

 certain number of the moving gaseous molecules penetrate the 



*Meakins and Davies, "The Journal of Pathology and Bacteriology," 

 23, 451, 1920. 



