Porth's Essentials of Pathophysiology, 4e

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Control of Respiratory Function

C h a p t e r 2 1

Macrophage

Basal lamina

Inhaled particle

Erythrocyte

Type I alveolar cell

Alveolar lumen

Type II alveolar cell

Nucleus

Lamellar inclusion body

Capillary lumen

Mitochondria

of urban dwellers, as well as smokers, usually show many alveolar macrophages filled with carbon and other polluting particles from the environment. The alveolar macrophages also phagocytose insoluble infec- tious agents such as Mycobacterium tuberculosis. The activated macrophages then aggregate to form a fibrin- encapsulated granuloma, called a tubercle, which serves to contain the infection (see Chapter 23). Pulmonary and Bronchial Circulations The lungs are provided with a dual blood supply: the pulmonary and bronchial circulations. The pulmonary circulation arises from the pulmonary artery and pro- vides for the gas exchange function of the lungs (see Fig. 21-7). Deoxygenated blood leaves the right heart through the pulmonary artery, which divides into a left pulmonary artery that enters the left lung and a right pulmonary artery that enters the right lung. Return of oxygenated blood to the heart occurs by way of the pulmonary veins, which empty into the left atrium. It is important to note that this is the only part of the circu- lation in which arteries carry deoxygenated blood and veins carry oxygenated blood. The pulmonary circulation serves several important functions in addition to gas exchange. It removes throm- boemboli (blood clots) from the circulation, functions as a metabolic organ, and serves as a blood reservoir for the left side of the heart. Small pulmonary vessels trap thromboemboli, and endothelial cells lining the vessels release fibrolytic substances that help dissolve them. Vasoactive hormones are metabolized in the pulmonary circulation. Angiotensin I is activated and converted to FIGURE 21-8. Schematic illustration of type I and type II alveolar cells and their relationship to the alveoli and pulmonary capillaries.Type I alveolar cells comprise most of the alveolar surface.The type II alveolar cells, which produce surfactant, are located at the corners between adjacent alveoli. Also shown are the endothelial cells, which line the pulmonary capillaries, and an alveolar macrophage.

Surfactant

Endothelial cells

angiotensin II by the angiotensin-converting enzyme (ACE) located on the surface of the pulmonary capil- lary endothelial cells. Pulmonary endothelial cells also inactivate bradykinin, serotonin, and some of the pros- taglandins. As a blood reservoir, the pulmonary circula- tion contains approximately 500 mL of the total blood volume. During a hemorrhagic event, some of this blood can be mobilized to improve cardiac output. The bronchial circulation provides the blood supply for the conducting airways and the supporting struc- tures of the lung. It also has a secondary function of warming and humidifying incoming air as it moves through the conducting airways. The bronchial arter- ies arise from the thoracic aorta and enter the lungs with the major bronchi, dividing and subdividing along with the bronchi as they move out into the lung, sup- plying them and other lung structures with oxygen. The blood from the capillaries in the bronchial circulation drains into the bronchial veins, with the blood from the larger bronchial veins emptying into the vena cava and blood from the smaller bronchial veins draining into the pulmonary veins. Because the bronchial circu- lation does not participate in gas exchange, this blood is deoxygenated. As a result, it dilutes the oxygenated blood returning to the left side of the heart by way of the pulmonary veins. The bronchial blood vessels are the only ones that can undergo angiogenesis (formation of new vessels) and develop collateral circulation when vessels in the pulmonary circulation are obstructed, as in pulmonary embolism. The development of new blood vessels helps to keep lung tissue alive until the pulmonary circulation can be restored.