Lyases are enzymes that catatyze the cleavage of C-C, C-O, C-N bonds by other means than by hydrolysis or oxidation. These bonds are cleaved by the process of elimination and the resulting product is the formation of a double bond or a new ring. An enzyme present in red blood cells, carbonic anhydrase, aids in the conversion of carbon dioxide to carbonic acid and bicarbonate ions. When red blood cells reach the lungs, the same enzyme helps to convert the bicarbonate ions back to carbon dioxide, which we breathe out.