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Different types of pressures

1. Intrapleural pressure :

This is pressure within the pleural space (also called intrathoracic pressure) Oesophageal pressure measures intrapleural pressure. This pressure is always negative i.e. – 2-5 mm Hg because pleural cavity is a closed cavity and the recoil of lungs and thoracic cavity are in opposite direction.This increases the volume resulting in fall in pressure. It is more negative at apex (-6 mm Hg) than base(-2 mm Hg). It helps in :

  1. Preventing collapse of alveoli due to its –ve pressure (eg pneumothorax- lung collapse)
  2. Decreases work of breathing
  3. Some air is always present as it keeps alveoli semi inflated so gas exchange is continous.

During Valsalve maneuver, tension pneumothorax, coughing it can become as high as +100 mm Hg


2. Intrapulmonary pressure

This is the pressure in the airways                            

The transmural pressure are:

  1. Transpulmonary: The pressure difference between intrapleural and intrapulmonary pressure
  2. Transthoracic: The pressure difference between the intrapleural pressure and atmospheric pressure
  3. Also, one can talk of the pressure difference between the intrapulmonary pressure and atmospheric pressure



Inspiration and expiration

  1. Muscles involved in quiet respiration

Inspiration: Diaphragm is the main muscle; also external intercostal muscle

Expiration: No expiratory muscle (passive)

  1. Transpulmonary
  2. Transthoracic
  3. Pressure difference between intra pulmonary and atmospheric pressure (Trans respiratory) (Trans respiratory pressure)
  1. Muscles involved in forceful respiration

Inspiration: Scalene, sternocleidomastoid

Expiration: Internal intercostal muscles,Anterior abdominal muscle

  1. Pressure changes during respiration
  1. Intrapleural pressure:  At the beginning of quiet inspiration, it is – 2.5 mmHg subatmospheric i.e. 2.5 mmHg less than atmospheric pressure of 760 mmHg; at the end of inspiration, it becomes – 6.0 mmHg.
  2. Intra alveolar pressure : At the peak of inspiration, it is – 1 mmHg; at the peak of expiration, it is +1 mmHg. At the beginning and at the end of both inspiration and expiration, the interalveolar pressure is zero i.e. same as atmosphere pressure

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