The study of the structure of the body and the of the relationship of its constituent parts to each other, is called:

The branch of biology that studies how the body and its organs, systems and cell's function is called Physiology.
The branch of medicine that studies the causes, nature and effects of diseases by examining tissues, organs, body fluids is called Pathology.
The branch of science that studies drugs, their actions, effects and how they interact with the body is called Pharmacology.