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Definition of Pinocytosis
Medically reviewed by Min Clinic Staff | Updated: January 2026
Pinocytosis: A cellular process that permits the active transport of fluid from outside the cell through the membrane surrounding the cell into the inside of the cell.
In pinocytosis, tiny incuppings called caveolae (little caves) in the surface of the cell close and then pinch off to form pinosomes, little fluid-filled bubbles, that are free within the cytoplasm of the cell.