the bicarb is acting as a buffer and is being used up by the over abundance of acidic waste products that are now not as readily excreted by the kidneys (urea is the main by product)
The waste is not being excreted by the kidneys so the acidic waste is building up in the blood causing the blood Ph to drop and become acidotic. The body is dumping its bicarb faster than it can produce it causing the bicarb in the blood to drop. The body then will move to blowing off carbon dioxide in order to try and compensate further for the acidosis. So this is a case of metabolic acidosis with respiratory compensation.
