There are three aspects of pulmonary function: perfusion, diffusion, and ventilation. Perfusion relates to blood flow through pulmonary vessels; diffusion refers to movement of oxygen and carbon dioxide across alveolar capillary membranes; and ventilation relates to air exchange between alveolar spaces and the atmosphere.

During breathing, the lung–thorax system acts as bellows to provide air to the alveoli for adequate gas exchange to take place. Like a spring or rubber band, the lung tissue also possesses the property of elasticity. When the inspiratory muscles contract, the thorax and lungs expand; when the same muscles relax and the force is removed, the thorax and lungs return to their resting position. Also, when the thorax and lungs expand, the alveolar pressure is lowered below atmospheric pressure. This permits air to flow into the trachea, bronchi, bronchioles, and alveoli. Expiration is mainly passive. It occurs because the thorax and lungs recoil to their resting position. The alveolar pressure increases above atmospheric pressure, and air flows out through the respiratory tract. The major function of the lungs is to provide adequate ventilation to meet the metabolic demands of the body during rest and during exercise. The primary purpose of pulmonary blood flow is to conduct mixed venous blood through the capillaries of the alveoli so that oxygen (O₂) can be taken up by the blood and carbon dioxide (CO₂) can be removed from the blood.