Physiology is the branch of biology that deals with the functions and processes of living organisms, both animals and plants. It's biology in motion.
Physiology includes everything from how a single cell functions, to what makes your nerve receptors work, how your pancreas releases insulin, and what happens to your muscles when you exercise. Technology has made for great leaps in the science of physiology. The electron microscope, for instance, allows you to see down to even the molecules of the cell, and radioactive isotopes provide the means to track the movement of substances within an organism.