5.7 Malpighian tubules transport uric acid to the digestive tract. Conversion of nitrogenous wastes to uric acid conserves water but is energetically expensive.

• 马氏管把尿酸传入消化道内。把含氮代谢废物转换成尿酸保存了水分，但它是一种耗能反应(但因此付出的能源是昂贵的)。