Figure 2.

Schematic representation of the pulmonary circulation in normoxia (A) and when one bronchus is obstructed and the respective alveoli are hypoventilated (B). During normoxia in the healthy state, deoxygenated blood from the pulmonary artery flows through the capillary bed surrounding the alveoli, where it takes up oxygen and, fully oxygenated, returns through the pulmonary vein to the left atrium of the heart. Local hypoventilation of an area of the lungs causes vasoconstriction of the pulmonary arteries in the same area; thus, less blood flows through the hypoventilated area and relatively more through other, better ventilated areas, resulting in a minimal reduction of the oxygenation status of the blood returning into the systemic circulation through the pulmonary vein (Euler-Liljestrand mechanism). (C) In global hypoxia, hypoxic pulmonary vasoconstriction occurs throughout the lung. This obviously does not improve the oxygenation status of the blood, but it causes a major increase in total pulmonary vascular resistance. When this situation is maintained for longer time periods, pulmonary hypertension may occur, resulting in right ventricular hypertrophy and failure.