Human physiology is the study of the functioning of living human beings. It originally referred to the study of nature but came to focus on how humans and other living things function. Today, human physiology includes the study of molecular, cellular, endocrine, cardiovascular and respiratory systems. Applied fields of human physiology also examine how physiology relates to areas like space, sports, exercise, altitude and clinical medicine. Human physiology is now understood as the study of dynamic interactions between humans and their total external and internal environments over time.