The document describes the human respiratory system. It includes diagrams of the major structures like the nose, mouth, pharynx, larynx, trachea, bronchi, bronchioles and lungs. It defines respiration as the process of gas exchange between oxygen and carbon dioxide in the blood and alveoli. Breathing is controlled by the medulla oblongata which monitors carbon dioxide levels and signals the diaphragm to contract and expand the lungs during inhalation. Oxygen diffuses into blood capillaries surrounding each alveolus while carbon dioxide diffuses out, allowing for gas exchange.