How negative pressure is maintained in pleura?

discuss tHe clinical significance of pleural

pressure.

Hare Ram Karn 14-05-2011

Interpleural Pressure

• Pressure inside the pleural cavity i.e. between parital & visceral layer of pleura.

• Also k/a Intrathoracic pressure.

• Mainly exerted by ‘Suction Effect’ caused by recoiling of lung

• As the lungs recoils elastically pulling force in the plural cavity decreases pressure inside it.

Normal ValueDuring Inspiration : - 6 mmHg

During Expiration : -2 mmHg

Negative Plural Pressure

• Negative force is always required to keep the lungs expanded.

• Maintained by negative pressure in plural space.

• Negative plural pressure is basically caused by pumping out plural fluid

• Plural fluid is pumped out by lymphatics.

Lymphatic Drainage• Lymph from the parietal pleura is drained into the

lymph node of thoracic wall.

• Medistinum

• Lateral surfaces of the perital pleura

• Superior surface of Diaphragm

Lymph from visceral pleura is drained into deep pulmonary plexus

• Pressure at Artrial end of capillary 15-25mmHg

• Fluid diffused into interstital space is reabsorbed at venous end.

• Remainig fluid after venous reabsorption is carried away by lymphatics present over that area.

• Venous capillaries are more numerous & permeable than arterial capillaries.

• About nine tenth of fluid is absorbed at venous end .

• Only one tenth is drained by lymphatics into circulating blood.

• Normally when the fluid enters the terminal lymphatics capillaries, the lymph vessels walls automatically contract for a few seconds and pump the fluid into blood circulation.

The pleural membrane is a porous, mesenchymal, serous membrane through which small amount of interstital fluid transude continuously into plural space.This over all processes maintains the negative pleural pressure.

Clinical significance

Due to negative pressure thoracic region the larger veins and venacava becomes dilated.

The negative pressure acts as suction pump and pulls the blood from lower part of body against gravity. Hence, responsible for venous return and termed as respiratory pump.

Prevent collapsing tendency of the lungs caused by elastic recoiling of lung tissues.

• In some of the pathological conditions such as

Pneumothorax

Hydrothorax

Hemothorax

Pyothorax

Tends to be more positive.

-In Valsalva Maneuver pleural pressure becomes more positive.

-In reverse Valsalva/Mueller’s Maneuver pleural pressure becomes less positive.