42 ORGANISM AND ENVIRONMENT 



endothelial cells which line the capillary blood-vessels. 

 Here, then, we have another delicate regulation con- 

 nected with breathing. 



We must now turn to the respiratory regulation of 

 oxygen supply. Normally, as we have seen, it is the 

 CO2 pressure in the blood, and ultimately the reaction 

 of the blood, which seems to regulate the breathing. 

 Under normal conditions there is always a sufficient 

 reserve of oxygen in the alveolar air to saturate the 

 haemoglobin of the blood to about the full normal 

 extent, even if, from any cause, the oxygen percent- 

 age falls distinctly below normal. We can thus under- 

 stand how it is that even if the oxygen percentage in 

 the air breathed is reduced from 20.9 per cent, as in 

 pure air, to as little as 14 or 15 per cent, which 

 instantly extinguishes any ordinary flame, the breath- 

 ing is not sensibly affected at the time, and the alveolar 

 CO2 percentage is undisturbed although the alveolar 

 oxygen percentage has fallen from 14 to 7 or 8. When, 

 however, there is a further reduction in the oxygen 

 percentage the breathing begins to increase, and 

 the alveolar CO2 pressure consequently falls. The 

 face and lips also begin to have a bluish or lead- 

 coloured tinge, showing that the blood is not properly 

 oxygenated in the lungs ; and if such air is breathed for 

 a considerable time headache and nausea come on. If 

 there is only 6 or 8 per cent of oxygen in the air 

 breathed intense panting is at once produced, accom- 

 panied by rapidly increasing dizziness, mental fail- 

 ure, and other alarming symptoms, as well as marked 

 blueness or leaden colour of the face. 



