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 
CO, 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 
CO, 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 CO, 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. 
