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Structure of the Respiratory System
III
Chapter
36
Structure of the Respiratory System: Lower Respiratory Tract Christopher A. D’Angelis, Jacqueline J. Coalson, and Rita M. Ryan
PEARLS • L ungs increase in volume from about 250 mL at birth to 6000 mL in the adult. • A t birth, the pulmonary artery and aorta are comparable in medial thickness and configuration and are the same size; by age 2 years, elastic tissue decreases in the pulmonary artery and its thickness is only about 60% that of the aorta. • E ach bronchopulmonary segment is divided by connective tissue septa that define the smallest surgically resectable portions of the lung. • T he airway branching pattern in the lung undergoes multiple generations, yielding a total of 27 or 28 divisions when counting begins from the primary bronchus. • T he bronchial mucosa contains several epithelial cell types, with the ciliated cell comprising more than 90% of the epithelial cell population in the conducting airways, but the proportion and number of cilia per cell decrease from the proximal to distal airways. • T he acinus, which is approximately spherical in shape and has a diameter of about 7 mm and a length of 0.5 to 1 cm, is the gas exchange portion of the lung. • A lthough there is disparity concerning the time alveolarization is completed, alveoli in a normal adult number from 300 to 500 million and have a diameter of 150 to 200 μm. • T he two epithelial cells of the alveolus are the gas-exchanging type I cell and the surfactant-producing type II cell that is responsible for epithelial repair. • T he alveolar-capillary unit is comprised of three major constituents: the epithelial lining of the alveolus, capillary endothelial cells, and a mixture of cellular and extracellular interstitial components. • T he large pulmonary arteries traverse with the cartilaginous airways and extend from the hilum to nearly halfway in the bronchial tree. • S maller pulmonary arteries measure between 100 and 1000 μm in diameter, branch with the bronchial tree, and lie close to bronchi and bronchioles. • T he pulmonary veins do not course with the bronchial tree and instead are seen within the interlobular septa. • T he diaphragm, the principal muscle of respiration, is essential during deep anesthesia because other muscles of respiration become inactive.
490
Lower Respiratory System Lungs
Lung weights of children from birth to age 12 years have been published.1-13 At birth, the lungs weigh about 40 g and double in weight by 6 months. Mature respiratory alveoli appear at approximately 36 weeks of gestation and continue to develop until about 2 years of age. By age 2 years, when most of the alveolarization process is completed, total lung weight is approximately 170 g. In the normal adult, the lungs weigh approximately 1000 g.9 Lung volume increases from about 250 mL at birth to 6000 mL in the adult. The height of a normal adult lung is 27 cm at total lung capacity, but in the range of normal breathing it is approximately 24 cm in height. Externally, the lungs are paired structures that, with the mediastinum, fill the thoracic cavity. Normally the right lung is comprised of three lobes and the left lung consists of two lobes and the lingula, which arises from the left upper lobe. The lobes are separated by fissures and have hili that receive a primary lobar bronchus, pulmonary artery and veins, bronchial arteries and veins, lymphatics, and nerves (Figure 36-1).14 The lobes are further subdivided into 19 bronchopulmonary segments that receive a primary segmental bronchi and a tertiary pulmonary artery branch and are drained by pulmonary veins. The pulmonary veins do not course with the airway and pulmonary artery dyad; instead, they course midway between the dyads and can be readily identified in the septa that run within the intersegmental septa. The connective tissue septa that demarcate each bronchopulmonary segment define the smallest surgically resectable portions of the lung.
Airways and Bronchus-Associated Lymphoid Tissue The airway branching pattern in the lung undergoes multiple generations, yielding a total of 27 or 28 divisions when counting begins from the primary bronchus. The bronchi are the larger intrinsic cartilaginous airways and comprise nine to 12 generations starting with the primary bronchus and terminating in bronchi having a diameter of approximately 1 mm. Bronchioles, sometimes called membranous bronchioles or distal noncartilaginous airways, are the last of the conducting system. They comprise an additional 12 generations before ending as terminal bronchioles, the last purely conducting
Chapter
36
Structure of the Respiratory System: Lower Respiratory Tract Christopher A. D’Angelis, Jacqueline J. Coalson, and Rita M. Ryan
PEARLS • L ungs increase in volume from about 250 mL at birth to 6000 mL in the adult. • A t birth, the pulmonary artery and aorta are comparable in medial thickness and configuration and are the same size; by age 2 years, elastic tissue decreases in the pulmonary artery and its thickness is only about 60% that of the aorta. • E ach bronchopulmonary segment is divided by connective tissue septa that define the smallest surgically resectable portions of the lung. • T he airway branching pattern in the lung undergoes multiple generations, yielding a total of 27 or 28 divisions when counting begins from the primary bronchus. • T he bronchial mucosa contains several epithelial cell types, with the ciliated cell comprising more than 90% of the epithelial cell population in the conducting airways, but the proportion and number of cilia per cell decrease from the proximal to distal airways. • T he acinus, which is approximately spherical in shape and has a diameter of about 7 mm and a length of 0.5 to 1 cm, is the gas exchange portion of the lung. • A lthough there is disparity concerning the time alveolarization is completed, alveoli in a normal adult number from 300 to 500 million and have a diameter of 150 to 200 μm. • T he two epithelial cells of the alveolus are the gas-exchanging type I cell and the surfactant-producing type II cell that is responsible for epithelial repair. • T he alveolar-capillary unit is comprised of three major constituents: the epithelial lining of the alveolus, capillary endothelial cells, and a mixture of cellular and extracellular interstitial components. • T he large pulmonary arteries traverse with the cartilaginous airways and extend from the hilum to nearly halfway in the bronchial tree. • S maller pulmonary arteries measure between 100 and 1000 μm in diameter, branch with the bronchial tree, and lie close to bronchi and bronchioles. • T he pulmonary veins do not course with the bronchial tree and instead are seen within the interlobular septa. • T he diaphragm, the principal muscle of respiration, is essential during deep anesthesia because other muscles of respiration become inactive.
490
Lower Respiratory System Lungs
Lung weights of children from birth to age 12 years have been published.1-13 At birth, the lungs weigh about 40 g and double in weight by 6 months. Mature respiratory alveoli appear at approximately 36 weeks of gestation and continue to develop until about 2 years of age. By age 2 years, when most of the alveolarization process is completed, total lung weight is approximately 170 g. In the normal adult, the lungs weigh approximately 1000 g.9 Lung volume increases from about 250 mL at birth to 6000 mL in the adult. The height of a normal adult lung is 27 cm at total lung capacity, but in the range of normal breathing it is approximately 24 cm in height. Externally, the lungs are paired structures that, with the mediastinum, fill the thoracic cavity. Normally the right lung is comprised of three lobes and the left lung consists of two lobes and the lingula, which arises from the left upper lobe. The lobes are separated by fissures and have hili that receive a primary lobar bronchus, pulmonary artery and veins, bronchial arteries and veins, lymphatics, and nerves (Figure 36-1).14 The lobes are further subdivided into 19 bronchopulmonary segments that receive a primary segmental bronchi and a tertiary pulmonary artery branch and are drained by pulmonary veins. The pulmonary veins do not course with the airway and pulmonary artery dyad; instead, they course midway between the dyads and can be readily identified in the septa that run within the intersegmental septa. The connective tissue septa that demarcate each bronchopulmonary segment define the smallest surgically resectable portions of the lung.
Airways and Bronchus-Associated Lymphoid Tissue The airway branching pattern in the lung undergoes multiple generations, yielding a total of 27 or 28 divisions when counting begins from the primary bronchus. The bronchi are the larger intrinsic cartilaginous airways and comprise nine to 12 generations starting with the primary bronchus and terminating in bronchi having a diameter of approximately 1 mm. Bronchioles, sometimes called membranous bronchioles or distal noncartilaginous airways, are the last of the conducting system. They comprise an additional 12 generations before ending as terminal bronchioles, the last purely conducting