A Medical Cannula Latin meaning 'little reed'; plural cannulae or cannulas) is a tube that can be inserted into the body, often for the delivery or removal of fluid or for the gathering of samples. In simple terms, a cannula can surround the inner or outer surfaces of a trocar needle thus extending the effective needle length by at least half the length of the original needle. Its size mainly ranges from 14 to 24 gauge. The different-sized cannula has different colours as coded. All these intravenous cannulas have a specific purpose and are used for separate treatments.