LYMPHATIC SYSTEM

Abstract

Lymphatic networks in the lung are found in close association with blood vessels and bronchi, and in the pleura. These networks anastomose at the lung surface and interlobular septa, and drain via the hilar region into the mediastinal and tracheobronchial lymphatic system, and thence to the right lymphatic duct or thoracic duct. Interstitial fluid from the alveolar walls drains into the parenchyma of the alveolar ducts, where it enters blind-ended lymphatic capillaries consisting of simple discontinuous but overlapping endothelial cells. These in turn drain into collecting lymphatic vessels that contain smooth muscle and one-way valves to help pump the lymph centrally along the network, significantly aided by respiratory and vascular movement. This drainage maintains normal tissue hydration, but is overcome in cardiovascular and lung disease resulting in fluid accumulation at the alveolar level. The lymphatics also provide a route of removal of inflammatory and pathological material, including tumor cells, which often end up in lymph nodes. Factors that can compromise lymphatic drainage from the lungs include reduced lymphatic pumping by compromised respiratory, vascular, or body movement; inhibition of lymphatic pumping by inflammatory cytokines or cells; constriction of lymphatic vessels by external forces; obstruction of the lymphatics by tumor; or increased central venous pressure. Pleural fluid drainage is via intercellular gaps called stomata in the pleural mesothelial lining, which directly connect the pleural cavities to lymphatics that drain the dependent regions of the parietal pleura.