The study of human physiology emphasizes the interrelationship between large populations of cells, tissues, organs, and organ systems of the body. It encompasses various aspects, including nerve cells and synapses, muscle function, reflexes, the autonomic nervous system, blood and the cardiovascular system, respiration, digestion, the urinary system and body fluids, the endocrine system, the reproductive system, the central nervous system, special senses, energy metabolism, and temperature regulation