Figure 17-30 A, Schematic diagram of the concept of a continuous connective tissue (CT) sheath-alveolar septum interstitial space. The entry of the main stem bronchi and pulmonary artery into the lung parenchyma invaginates the pleura at the hilum and forms a surrounding connective tissue sheath. The connective tissue sheath ends at the level of the bronchioles. The space between the pulmonary arteries and bronchi and the interstitial space is continuous with the alveolar septum interstitial space. The alveolar septum interstitial space is contained within the endothelial and epithelial basement membranes of the capillaries and alveoli, respectively. B, Schematic diagram showing how interstitial fluid moves from the alveolar septum interstitial space (no lymphatics) to the connective tissue interstitial space (lymphatic capillaries first appear). The mechanisms are a negative-pressure gradient (sump), the presence of one-way valves in the lymphatics, and the massaging action of arterial pulsations. (Redrawn with modification from Benumof JL: Anesthesia for Thoracic Surgery, 2nd ed. Philadelphia, WB Saunders, 1995, Chapter 8.)