All cells contain an intracellular fluid whose pH value is known as the intracellular pH (pHi). The pHi plays a critical role in the function of the cell, and close regulation is required for cells to survive. There are numerous mechanisms that can cause pHi to change, including metabolic acid production, leakage of acid across plasma and organelle membranes and membrane transport processes. The mechanisms that regulate pHi are usually considered to be plasma membrane transporters of which two main types exist — those independent of HCO3– and those requiring HCO3–. The normal extracellular pH is 7.4 +/- .1.{http://www.uptodate.com/contents/chapter-10c-intracellular-ph}
Intracellular pH is around 6.8 due to acid production. It contains low HCO3- level and thereby at pCO2 above 45 mmHg pH can not be 7.4 but much lower.pH = 7.3 in HEK293 cells