Normal Lung II

Pleura
The normal visceral pleura is a thin translucent sheet of mesothelial tissue. It is contiguous with the parietal pleura at the hilum, the parietal pleura being the surface covering of the chest wall. Pleural spaces are filled with a minimal amount of fluid ranging from 1 to 20 mL. The movement of fluid into and out of the pleural space depends on the combined effects of hydrostatic, colloid osmotic, and tissue pressures in the parietal and visceral pleura. The parietal pleura contains lymphatics that drain into the internal mammary artery, periaortic arteries, and diaphragmatic lymph nodes. Pleural fluid is thought to arise primarily from the capillaries lining the parietal pleura. This fluid circulates back across the parietal pleura, where it is cleared by lymphatics. Tracer studies have suggested that the parietal pleura accounts for most fluid movement into and out of the pleural spaces under normal conditions. Visceral pleural capillaries and visceral pleural lymphatics do not normally play a major role in fluid fluxes through the pleural space.
In total, the driving force withdrawing fluid from pleural spaces is greater than the net force moving fluid out of the pleural capillaries and into the pleural spaces. This results in the pleural space remaining relatively dry. Fluid does not normally accumulate in the pleural space unless hydrostatic pressure is elevated in the pulmonary capillary bed or an inflammatory condition of the pleura causes protein leakage into perivascular and pleural spaces, decreasing the oncotic pressure gradient and thereby the major force favoring extraction of fluid from the pleural spaces.
Anatomically, the pleura is made up of mesothelium. Mesothelial cells are characterized by their long microvilli, up to 2 µm in length. These cells contain desmosomal intracellular attachments (macular adherens) and also intermediate filaments in their cytoplasm (cytokeratin). Mesothelial cells have a well-developed endoplasmic reticulum, which suggests that they are metabolically active. Beneath the mesothelial cells is a thin, loose connective tissue structure containing both capillaries and lymphatics. There is also a deeper layer of elastic fibers between the relatively thin visceral pleura and the immediately subjacent alveolar septal tissues. The parietal pleura has a similar architecture, except that the underlying connective tissue layer is substantially thicker and overlies intercostal muscle, fat, and vascular structures.
Lung Lymphatics
Tissue fluids in the lung move centrally toward the hilum. In alveolar tissue, alveolar septal junctions create spaces through which fluid is thought to move until it reaches the walls of an airway or vascular structure in which lymphatic structures are present. These intrapulmonary lymphatics, termed deep lymphatics, drain the bronchovascular bundles toward the lung hilum. The superficial pleural lymphatics carry fluid along the pleural surfaces to the point at the hilum where the visceral pleura reflects into the parietal pleura. These superficial lymphatics also follow interlobular septa and thereby interconnect with the deep pulmonary lymphatic system. The deep pulmonary lymphatic system can be clearly identified anatomically beginning at about the level of respiratory bronchioles.
Lymph nodes are abundant in the pulmonary hilum and along the trachea and extrapulmonary bronchi. Lymphatic fluid drains through anastomosing channels that connect these lymph nodes and moves upward along the trachea. The lymphatics on the right side re-enter the systemic circulation through the subclavian vein near its junction with the jugular vein. Pulmonary lymphatics on the left side return to the systemic circulation through the thoracic duct or by directly emptying into the left subclavian vein.
Four major groups of lymph nodes exist in the lung. These include intrapulmonary nodes adjacent to lobar, segmental, and smaller bronchi and small nodes (1 to 3 mm) located in subpleural regions, often at junctions with interlobular septa. Extrapulmonary nodes are situated in the subcarinal region near the bifurcation of the main bronchi. They are also found along the walls of the trachea and main bronchi. The intrapulmonary nodes, which are part of either the pleura or small intrapulmonary airways, are termed N1 nodes. Extrapulmonary nodes along the main bronchi may also be termed N1. Subcarinal and ipsilateral tracheal nodes are termed N2, whereas contralateral hilar, tracheal, or bronchial nodes are termed N3.
The lung also can contain aggregates of lymphoid tissue along all levels of large and small airways. This tissue is called bronchus-associated lymphoid tissue (BALT). BALT contains lymphoid follicles with germinal centers but does not have the fibrous capsule and capsular sinus characteristic of lymph nodes. The question has been raised as to whether BALT occurs normally in humans or rather develops only after stimulation. Its specific role in immune regulation is not yet well defined.