Oxygen transport and exchange provide necessary oxygen to the body's cells through the process of respiration. Oxygen enters the respiratory system through the mouth and nose, and travels through the pharynx, larynx, trachea, and bronchi during inspiration (inhalation) to reach the lungs. The bronchi enter into the lungs and branch to form bronchioles. The bronchioles further divide to form alveolar ducts. These ducts lead to tiny sacs called alveoli that are surrounded by pulmonary capillaries where oxygen exchange takes place. There are approximately 300 million alveoli that greatly increase the surface area of the lungs. Oxygen exchange occurs by the process of diffusion across the alveolar-capillary wall. During diffusion, oxygen travels down its concentration gradient into the blood and is exchanged for carbon dioxide. The partial pressure of oxygen within the body is denoted by pO₂ and is measured in millimeters of mercury...