In a container containing water and oxygen gas, some of the oxygen dissolves in the water. Both gases dissolve equally in the water. The carbon dioxide gas dissolves in the water to a greater extent than the oxygen gas. The oxygen gas dissolves in the water to a greater extent than the carbon dioxide gas. Internal respiration occurs at the systemic capillaries within the tissues of the body.
External respiration occurs at the pulmonary capillaries within the lungs b.
In internal respiration oxygen diffuses from the systemic capillaries into the cells, and carbon dioxide diffuses from the cells into the systemic capillaries. The surface area and structure of the respiratory membrane. The partial pressure gradients between the alveoli and capillaries. As air travels through the respiratory passageways to the alveoli it is humidified, picking up water molecules. The PO2 of the alveolar air is 104 mm Hg.