During normal cellular metabolism, both carbon dioxide and metabolic acids are produced. Carbon dioxide (co2) combines with water (h2o) to form carbonic acid (h2co3). Both carbonic and metabolic acids must be excreted to maintain acid-base homeostasis. Buffers are chemicals (a weak acid plus its base) that prevent large changes in ph by releasing or taking up hydrogen ions (h+). The bicarbonate buffer system is the most important buffer in the extracellular fluid. The normal ratio of bicarbonate to carbonic acid is 20:1. Any deviation from this ratio alters the ph of the blood. Rate and depth of respiration normally are adjusted by chemoreceptors in response to acid-base and oxygen status. Increased ventilation (hyperventilation) decreases the amount of carbon dioxide in blood and thus reduces the amount of carbonic acid. Decreased ventilation (hypoventilation) allows carbon dioxide to accumulate and thus increases the amount of carbonic acid in the blood.