Anatomic human pulmonary model. Inspired gas enters the upper airway tree at the trachea and is delivered to the segmental compartments by a network of branching airway compartments. Each segmental compartment consists of a series of airway compartments leading to an alveolar compartment. The pulmonary circulation begins with the flow of deoxygenated blood from the heart; pulmonary arteries follow the path of the airway tree to the capillaries, where gas is exchanged with the alveolar regions. Oxygenated blood flows back to the heart via pulmonary veins.