Physiology is the science of the functioning of living organisms. Like other animals, humans move, respond to environmental stimuli, breathe, eat and digest food, excrete wastes, reproduce, and grow. Multiple organ systems in the body perform these various functions. Each organ system is characterized by a specialized function and thus, division of labor is a basic principle underlying physiology. Another striking feature is the complex organization of an organ system. Several structurally and functionally distinct organs make up an organ system, and each organ contributes to the specialized function of the system as a whole. An organ is further made up of two or more related tissues that determine the specific function of the organ. Based on function, tissues in the human body have been classified into four primary types: epithelial, connective, muscle, and nervous tissues. A tissue is composed of functionally similar cells. Cells are...