Physiology is the branch of biology that focuses on the functions and mechanisms occurring in living organisms. It explores how various systems, such as the nervous system and circulatory system, work together to maintain life and health. This field encompasses a wide range of topics, from the cellular processes that sustain life to the complex interactions between different organ systems. Understanding physiology is crucial for fields like medicine and sports science, as it helps explain how the body responds to various stimuli and maintains homeostasis.