Human Anatomy is often defined as the study of structures in the human body. Anatomy focuses on the description of form, or how body structures at different levels look. Gross anatomy studies macroscopic structures (for example, the body, organs, and organ systems), and histology studies microscopic structures (for example, tissues, cells, and organelles). Human Physiology studies the “nature” of the human body, nature in the sense of how structures at different levels work. Physiology focuses on function, or how structures at different levels work.
Anatomy and physiology are intimately related. A hand is able to grab things (function) because the length, shape, and mobility of the fingers (form) determine what things a hand can grab (function). A muscle contracts and brings bones together (function) due to the arrangement of muscles and bones, and the arrangement of organelles inside of muscle cells (form) determines how much and for how long a muscle can contract (function).
Body structure functions depend on their form. The way structures work depends on the way they are organized. So, understanding Physiology requires an understanding of Anatomy, and vice versa.