The lower limit of pleura is 10th interspace posteriorly and 6th anteriorly.
Hence gravity facilitates accumulation of fluid in the
gutter
posteriorly first.
With most etiologies (except negative pressure induced pleural effusion) once the
fluid accumulates in pleural space the negative pressure decreases and
eventually becomes positive.
Loss of negative pressure in pleural space results in higher resting
position of hemithorax.
The lung relaxes and becomes smaller since there is no negative
pressure to hold it close to chest wall.
Once the pressure becomes positive the mediastinum and diaphragm get pushed.
The diaphragm eventually can become concave upwards.
Fluid is subpulmonic to start with the lung relaxes and floats attached to hilum.
As the lung retracts towards hilum, fluid tracks up between visceral and parietal pleura.
Fluid has a broad base and thin apex along the chest circumference. The meniscus
appearance of the fluid is a visual illusion, thickness of fluid level is higher along
sides compared to the middle.
Fluid moves freely and shifts with position.
As the pleural pressure increases with more fluid formation i.e. massive pleural
effusion the lung becomes completely atelectatic. Airways are patent. Fluid is a
good conductor of sound.