Читать книгу Respiratory Medicine - Stephen J. Bourke - Страница 14

The lungs receive a blood supply from both the pulmonary circulation and the systemic circulation, via bronchial arteries. The purpose of the pulmonary circulation is to take the entire circulating volume of (deoxygenated) blood through the lungs in order to pick up oxygen and offload carbon dioxide. The bronchial arteries carry oxygenated blood from the systemic circulation to supply the tissues of the lung.

Figure 1.1 Diagram of bronchopulmonary segments. LING, lingula; LL, lower lobe; ML, middle lobe; UL, upper lobe.

Figure 1.2 Surface anatomy. (a) Anterior view of the lungs. (b) Lateral view of the right side of the chest at resting end‐expiratory position. LLL, left lower lobe; LUL, left upper lobe; RLL, right lower lobe; RML, right middle lobe; RUL, right upper lobe.

The pulmonary artery arises from the right ventricle and divides into left and right pulmonary arteries, which further divide into branches accompanying the bronchial tree. The pulmonary capillary network in the alveolar walls is very dense and provides a very large surface area for gas exchange. The pulmonary venules drain laterally to the periphery of lung lobules and then pass centrally into the interlobular and intersegmental septa, ultimately joining together to form the four main pulmonary veins, which empty into the left atrium.

Several small bronchial arteries usually arise from the descending aorta and travel in the outer layers of the bronchi and bronchioles, supplying the tissues of the airways down to the level of the respiratory bronchiole. Most of the blood drains into radicles of the pulmonary vein, contributing a small amount of desaturated blood, which accounts for part of the ‘physiological shunt’ (blood passing through the lungs without being oxygenated) observed in normal individuals. The bronchial arteries may undergo hypertrophy when there is chronic pulmonary inflammation, and major haemoptysis in diseases such as bronchiectasis or aspergilloma usually arises from the bronchial rather than the pulmonary arteries and may be treated by therapeutic bronchial artery embolisation. The pulmonary circulation normally offers a much lower resistance and operates at a lower perfusion pressure than the systemic circulation. The pulmonary capillaries may be compressed as they pass through the alveolar walls if alveolar pressure rises above capillary pressure.

Figure 1.3 Structure of the alveolar wall as revealed by electron microscopy. la, type I pneumocyte; lb, flattened extension of type I pneumocyte covering most of the internal surface of the alveolus; II, type II pneumocyte with lamellar inclusion bodies, which are probably the site of surfactant formation; IS, interstitial space; RBC, red blood corpuscle. Pneumocytes and endothelial cells rest upon thin continuous basement membranes, which are not shown.