Y. Henderson and H. W. Haggard B hy 
blood’ by influencing the respiratory center controls the CO, 
(H.CO3). On the other hand in the low and high levels produced 
by disturbances of breathing the respiratory center by controll- 
ing the CO, exerts a powerful influence upon the alkali (NaHCOs). 
If objection be taken to our saying that C, may be “normal”’ 
at various levels of CO., we would point out that people living 
5,000 or 10,000, or even 15,000 feet above sea level, or again 
below sea level, have as much right to consider themselves nor- 
mal as have we who live by the sea. But their levels of CO, are 
widely different from ours.® 
Method. 
All experiments were performed on healthy dogs. The femoral 
artery was exposed under local anesthesia (cocaine 0.5 cc. of 2 
per cent solution) and a cannula inserted. Samples of blood, 3 
to 6 cc. each, were withdrawn at regular intervals with proper 
precautions to avoid getting stagnant blood. Clotting was pre- 
vented by a small amount of powdered potassium oxalate. 1 cc. 
of the whole blood was analyzed to determine its CO, content. 
The remainder of the sample was then brought into equilibrium 
at 20°C. with air containing 5.4 to 5.7 per cent CO, (determined 
by analysis). 1 cc. of this whole blood was then analyzed. This 
gave the CO, capacity. The methods were those described by 
Henderson and Morriss.6 The figures for CO: capacity are pro- 
portional and closely approximating but not absolutely identical 
with those which would be computed by the Van Slyke method. 
We prefer the expression ‘“‘CO; capacity,” to the still rather hypo- 
thetical expression ‘‘alkaline reserve.”’ The capacity of the pro- — 
teins to combine with CO, may be considerable (Buckmaster’). 
By CO, capacity we mean the total amount of CO, (reduced to 
0° and 760 mm.) in the blood whether in solution or combined 
with alkali or protein at the tension of the CO» in the air with 
which the sample of blood is brought into equilibrium. . 
The alveolar air was determined with a mask and bag by the 
Higgins-Plesch method. 
5 Fitzgerald, M. P., Phil. Tr. Roy. Soc., Series B, 1918, ecili, 351; Proc. 
Roy. Soc., Series B, 1914-15, Ixxxviu, 248. 
6 Henderson and Morriss, J. Biol. Chem., 1917, xxxi, 217. 
7 Buckmaster, G. A., J. Physiol., 1917, li, 164. 
