The intestines, esophagus, and other soft-walled tubes in the human body can contract and push the contents in the right direction. Rigid-walled tubes such as the bronchi have cilia that move contents such as phlegm to clear the lungs.
Due to the peristaltic movements of the walls of the digestive tract. Actively pushes content, in a word. Well, cutting off the reverse movement by any sphincters between separate "compartments" works.