Dynamics of subatmospheric pressure in the pulmonary interstitial fluid

Systems analyses are presented for several aspects of pulmonary fluid dynamics, especially those related to (1) transport of fluid between the pulmonary interstitial spaces and the alveoli, (2) the possibility of a mechanism for concentrating protein in the lymph vessels, and (3) the effects of very high resistance to fluid flow in the alveolar septal wall. The analysis of fluid transport between the interstitial space and the alveoli, assuming that there is no active secretory or active absorptive process, shows that the interstitial fluid pressure in the normal lung cannot be more positive than the fluid pressure in the alveoli. Since the surface tension of this fluid causes it to have a subatmospheric pressure, the calculated maximum pressure for interstitial fluid in the normal lung is about -2 mmHg (-0.266 kPa). At any pressure more positive than this the alveoli will fill with fluid. The systems analyses for concentrating protein in the pulmonary lymphatics and for the effects of high resistance to fluid flow in the alveolar septal wall offer possible explanations for very negative pressures of pulmonary interstitial fluid, even though calculations of the interstitial fluid pressure based on the assumption that the colloid osmotic pressure of pulmonary interstitial fluid is equal to the osmotic pressure of pulmonary lymph give estimated pressures of pulmonary interstitial fluid approaching 0 mmHg.