620 PHYSIOLOGY OF RESPIRATION. 



It will be seen that under similar conditions there is much less 

 oxygen used and carbon dioxid formed in the brain than in the 

 limbs (muscles). In the former organ the physiological oxidations 

 must either be small compared with those of the muscles, or the 

 brain tissues receive a relatively ample supply of blood, so that the 

 tissue metabolism has less effect upon the blood composition. The 

 venous blood as it comes to the lungs is a mixture of bloods from 

 different organs, and its composition in gases will be constant only 

 when the conditions of the body are kept uniform. Much work 

 has been done in physiology to determine the condition in which 

 these various gases are held in the blood. The results obtained 

 show that they are held partly in solution and partly 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 chemical 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 gases, 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 which it does not react chemically a 

 certain number of the moving gaseous molecules penetrate the 

 liquid and become dissolved. Some of these dissolved molecules 

 escape from the water from time to time, again becoming 

 gaseous. It is evident, however, that if a liquid, water, is brought 

 into contact with a gas under definite pressure, that is, containing 

 a definite number of molecules to a unit volume, an equilibrium 

 will be established. As many molecules will penetrate the liquid 

 in a given time as escape from it, and the liquid will hold a definite 

 number of the gas molecules in solution; it will be saturated for 

 that pressure of gas. If the pressure of the gas is increased, how- 

 ever, an equilibrium will be established at a higher level and more 

 molecules of gas will be dissolved in the liquid. Experiments have 

 shown, in accordance with this mechanical conception, that the 



