The lung receives blood flow via both the bronchial circulation and the pulmonary circulation. Bronchial blood flow constitutes a very small portion of the output of the left ventricle, and it supplies part of the tracheobronchial tree with systemic arterial blood. Pulmonary blood flow (PBF) constitutes the entire output of the right ventricle, and it supplies the lung with the mixed venous blood draining all the tissues of the body. It is this blood that undergoes gas exchange with the alveolar air in the pulmonary capillaries. Because the right and left ventricles are arranged in series in normal adults, PBF is approximately equal to 100% of the output of the left ventricle.
The bronchial arteries arise variably, either directly from the aorta or by branching from the intercostal arteries. The bronchial blood flow constitutes about 2% of the output of the left ventricle. Blood pressure in the bronchial arteries is the same as that in the other systemic arteries.
Although some of the bronchial venous drainage is via the azygous and hemiazygous veins, a substantial portion of bronchial venous blood directly enters the pulmonary veins, and thus contributes to the normal anatomic right-to-left shunt. Anastomoses, or connections, between some bronchial and pulmonary capillaries and between bronchial arteries and branches of the pulmonary artery have also been demonstrated. These connections are probably not open in a normal healthy person but may open if either bronchial or PBF to a portion of lung is occluded, as by a pulmonary embolus.
In the normal adult the outputs of the two ventricles are approximately equal and are about 3.5 l/min/m2 body surface area. At any moment, the pulmonary circulation contains about 250 to 300 ml of blood per m2 of body surface area, about 60 to 70 ml/m2 of which is in the pulmonary capillaries. It takes a red blood cell an average of 4 to 5 s to travel through the pulmonary circulation at resting cardiac outputs; about 0.75 to 1.2 s of that time is spent in pulmonary capillaries. In traveling through the lung, an erythrocyte passes through a number of successive pulmonary capillaries. Gas exchange starts to occur in smaller pulmonary arterial vessels, which are not truly capillaries by histologic standards. These arterial segments and successive capillaries may be thought of as functional pulmonary capillaries. Usually, when we refer to pulmonary capillaries, we mean functional pulmonary capillaries rather than only anatomic capillaries.
Each alveolus is completely enveloped by pulmonary capillaries. Estimates of the total number of alveoli in the lungs of an adult are on the order of 300 million; the total number of anatomic capillaries is estimated to be 280 billion. The capillaries are so close to each other that some investigators have described pulmonary capillary blood flow as resembling ...