Respiratory physiology is the branch of human physiology concerned with the study of respiration. The human respiratory system is a biological system consisting of a series of organs responsible for taking in oxygen and releasing carbon dioxide to the environment. The lungs are the primary organs of respiration, which takes in oxygen and helps breathe out carbon dioxide. The lungs are a part of the lower respiratory tract that begins at the trachea and branches into the bronchi and bronchioles. These divide until air reaches microscopic alveoli and the process of gas exchange takes place.