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OBJECTIVES
The reader knows the structure, function, distribution, and control of the blood supply of the lung.
Compares and contrasts the bronchial circulation and the pulmonary circulation.
Describes the anatomy of the pulmonary circulation, and explains its physiologic consequences.
Compares and contrasts the pulmonary circulation and the systemic circulation.
Describes and explains the effects of lung volume on pulmonary vascular resistance.
Describes and explains the effects of elevated intravascular pressures on pulmonary vascular resistance.
Lists the neural and humoral factors that influence pulmonary vascular resistance.
Describes the effect of gravity on pulmonary blood flow.
Describes the interrelationships of alveolar pressure, pulmonary arterial pressure, and pulmonary venous pressure, as well as their effects on the regional distribution of pulmonary blood flow.
Predicts the effects of alterations in alveolar pressure, pulmonary arterial and venous pressure, and body position on the regional distribution of pulmonary blood flow.
Describes hypoxic pulmonary vasoconstriction and discusses its role in localized and widespread alveolar hypoxia.
Describes and explains the effects of mechanical positive-pressure ventilation on pulmonary blood flow.
Describes the causes and consequences of pulmonary edema.
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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 supplies part of the tracheobronchial tree with systemic arterial blood. Pulmonary blood flow (PBF) constitutes the entire output of the right ventricle and 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. That is, PBF is equal to the cardiac output—normally about 3.5 L/min/m2 of body surface area at rest.
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There is about 250 to 300 mL of blood per square meter of body surface area in the pulmonary circulation. About 60 to 70 mL/m2 of this blood is located in the pulmonary capillaries. It takes a red blood cell about 4 to 5 seconds to travel through the pulmonary circulation at resting cardiac outputs; about 0.75 of a second of this time is spent in pulmonary capillaries. Pulmonary capillaries have average diameters of around 6 mm; that is, they are slightly smaller than the average erythrocyte, which has a diameter of about 8 mm. Erythrocytes must therefore change their shape slightly as they pass through the pulmonary capillaries. An erythrocyte passes through a number of pulmonary capillaries as it travels through the lung. Gas exchange starts to take place 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. In most cases in this book, pulmonary capillaries refer to functional pulmonary capillaries rather than to anatomic capillaries.
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