298 HUMAN PHYSIOLOGY 



carbon dioxid from something like 6 per cent, to half as 

 much permits a corresponding reduction in the carbon 

 dioxid of the blood. 



It seems to be established that a certain concentration 

 of carbon dioxid in the blood and tissues is necessary to 

 normal reaction. We may think of the gas as having 

 a mild stimulating effect upon the nerve centers and as- 

 sume that the want of it results in a paralysis of some of 

 these cell-groups. The case of the respiratory center 

 seems especially clear. We have seen that this center 

 is stimulated to increased activity when the local con- 

 centration of carbon dioxid is raised. Conversely it may 

 cease to act for a time if the carbon dioxid in its vicinity 

 is cut down. Thus acapnia produces a suspension of 

 breathing apnea, as we call it. When the breathing 

 is at a standstill an increase of carbon dioxid is to be 

 expected and this will lead to a resumption of breath- 

 ing within a short time. Yet it is possible that the 

 stimulus to resume breathing may not be furnished 

 until the shortage of oxygen has wrought irreparable 

 damage. 



High Altitudes. The barometric pressure is lowered 

 progressively as one ascends from the sea-level. At a 

 height of 15,000 feet, corresponding to that of the Alps 

 and the Rockies, it is reduced to about one-half the coast 

 standard. This reduction may be thought of as affect- 

 ing both the nitrogen and the oxygen gases. (The share 

 of the total pressure which can be referred to any gas 

 in a mixture is said to be the " partial pressure" of that 

 gas.) No distinct results follow the gradual diminution 

 of the partial pressure of the nitrogen as one goes up a 

 mountain but the lessening of the oxygen pressure makes 

 itself felt. 



If the oxygen in the blood were taken up according to 

 the laws of physical solution, halving the pressure 

 would also halve the quantity of the gas absorbed. But 

 we have seen that the union of oxygen with hemoglobin 

 is a chemical one and it is rapidly accomplished even 



