This is the sixth module in the series of Introductory Medical Physiology Modules. The modules in the series are designed to facilitate students to acquire relevant knowledge in basic medical physiology so that they can seamlessly embark on more applied medical science disciplines such as pathology, pathophysiology, pharmacology, cardiology, nephrology, neurology, etc. Each module is prepared with students’ active participation (self-study) in mind. Each concept in the modules is explained with appropriate diagrams and activities which the students should explore and investigate by themselves. They can check the answers of the activities with their friends and verify them with the answers found in booklets published separately.