carbon dioxide is a necessary component of the carbonic acid-bicarbonate buffering system of the body. this is regulated through the lungs, kidneys and bone. carbon dioxide exists as either a volatile gas in the form of carbonic acid which is excreted through the lung, or as a nonvolatile acid which easily combines with the end products of the metabolism of protein, carbohydrate and fat to form organic acids. these organic acids are normally eliminated through the renal tubules. when the failing kidney can no longer excrete those organic acids, they build up in the body creating a situation called metabolic acidosis. the lungs will attempt to compensate by blowing off more volatile co2 gas.
one of the jobs of the renal tubules is to regulate carbonic acid loss, a source of carbon dioxide. but, if the kidneys are failing, this mechanism is lost as the kidney's ability to regulate acid-base balance is snuffed out. any available co2 is being used up in the buffering of the end products of metabolism which the kidney can no longer excrete. this is all part of the process of the buffering systems of the body attempting to normalize the ph of the body. for this reason you will see decreased co2 levels in renal failure because (1) it is being eliminated by the lungs, and (2) it is being bound chemically into building up organic acids that the kidneys cannot excrete in an attempt to keep the body ph into a normal range.
here is lab information on serum co2: http://www.labtestsonline.org/unders.../co2/test.html