In this fourth article in the Concepts in Anatomy series, Anne-Marie Ramsay joins John Clancy and Andrew McVicar in examining the upper and lower gastrointestinal tract and identifies some common homeostatic imbalances. Diagnostic tests for detecting abnormalities are described, highlighting the importance of the nurse's role in caring for the patient throughout these procedures. This series is based on Physiology and Anatomy, a homeostatic approach, 2nd ed, John Clancy and Andrew McVicar (eds), Edward Arnold, London, 1995, currently in print.