The thickness of the pleural liquid

Abstract

Cat chests were quick-frozen and cores drilled out. These were cut and observed through a reflex light microscope in a special cryostat. Since the pleural liquid reflects less light than the surrounding tissues it appears as a dark band. The thickness of the pleural liquid at FRC is 5–10μ both in the superior and dependent parts. When the lung volume is 3 times that at FRC the thickness is halved. If 5 ml of saline are introduced into the superior part of the pleural space its thickness increases only in the dependent part. The finding of an equal thickness in the superior and dependent parts of the normal cat agrees with previous findings showing that pleural liquid pressure is more subatmospheric than pleural surface pressure because other forces besides the lung recoil oppose reduction of pleural liquid below a certain limit. If pleural liquid and pleural surface pressure were the same, the thickness of the pleural liquid in the dependent part would be markedly greater than in the superior part, but this was observed only when the volume of pleural liquid was increased.