CHANGES IN AIR AND BLOOD IN RESPIRATION. 673 



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 

 amount of a given gas dissolved by a given liquid varies, the temper- 

 ature remaining the same, directly with the pressure, that is, it in- 

 creases and decreases proportionally with the rise and fall of the 

 gas pressure. This is the law of Henry. On the other hand, 

 the amount of gas dissolved by a liquid varies inversely with the 

 temperature. It follows, also, from the same mechanical views 

 that in a mixture of gases each gas is dissolved in proportion 

 to the pressure that it exerts, and not in proportion to the pressure 

 of the mixture. Air consists, in round numbers, of 4 parts of N and 

 1 part of 0. Consequently, when a volume of water is exposed to 

 the air the oxygen is dissolved according to its "partial pressure," 

 43 



