Buffer systems in the rumen of sheep. I. pH and bicarbonate concentration in relationship to pCO₂.

Abstract

Experiments are described which emphasize the importance of avoiding loss of carbon dioxide when estimating the pH or bicarbonate concentration of ruminal fluid. The high pCO₂ of ruminal fluid is stressed; this may be 10 times or more as great as that of arterial blood. The relationship between pCO₂, pH, and [HCO₃-] was examined in terms of the Henderson-Hasselbalch equation over a wide range of pCO₂. From this, the pK₁' of the carbonic acid system in four ruminal fluids was determined as 6.21-6.28, mean 6.25. The higher pH of saliva-free samples of ruminal fluid withdrawn by suction through a tube passed down the oesophagus, as compared with that of the bulk fluid obtained through a ruminal fistula, is considered to be due to loss of carbon dioxide during collection. A better estimate of intraruminal pH is obtained, even when salivary contamination occurs, if such samples are equilibrated with a sample of the animal's ruminal gas; if this is not practicable, an arbitrary gas mixture of high pCO₂, e.g. 50 per cent. carbon dioxide and nitrogen, may be used.