Malpighian tubule absorbs water, potassium urate and CO2 from the blood. Inside the tubules, water, CO2 and potassium urate react together to produce potassium bicarbonate and uric acid. In these, potassium bicarbonate and water are reabsorbed and returned to the blood. Uric acid, on the other hand, passes into the rectum. Excess water from uric acid is absorbed in the colon and pure uric acid is excreted out of the body with stool.