Thoracic Movements And The Respiratory Pressures

Inspiration causes the enlargement of thoracic cage. The size of the thoracic cage is increased in all diameters by the combination of respiratory muscles, ribs and the respiratory pressures exerted in the lungs and the thoracic cavity.

The five units combine together and changes the size of thoracic cavity in all diameters.
  • Respiratory pressures
  • Respiratory muscles
  • Thoracic lid
  • Upper coastal series
  • lower coastal series 
Increase of anterior-posterior and transverse diameters occurs due to the elevation of the ribs.
The vertical diameter of thoracic cage is increased by the descent of diaphragm.

Respiratory Pressures

Respiratory pressures are exerted in the thoracic cavity and the lungs during the process of respiration. These are of two types- 

1. Intrapleural pressure or intrathoracic pressure

It is the pressure existing in pleural cavity, in between visceral and parietal layers of pleura.
This pressure is exerted by the suction of the of the fluids that lines the pleural cavity.

The normal value of intrapleural pressure-
  • During inspiration = -6 mm Hg (760-6=754)
  • During expiration = -2 mm Hg (760-2= 758) 

2. Intraalveolar pressure or intrapulmonary pressure

This is the pressure existing in the alveoli of the lungs.

The normal value of intraalveolar pressure-
  • During inspiration= -4 mmHg(760-4=756)
  • During expiration= +4 mmHg(760+4=764)

Respiratory muscles

  • Primary inspiratory muscles
  • Accessory inspiratory muscles
  • Primary expiratory muscles
  • Accessory expiratory muscles
Read more about Respiratory Muscles

Thoracic lid

The thoracic lid is formed by the manubrium sterni and the first pair of ribs. This is the junction which moves the thoracic cavity upwards and in anteroposterior diameter of thoracic cage.

Upper coastal series 

The upper coastal series is formed by the 2nd to 6th pair of ribs. This series moves the thoracic cavity upwards and forwards and this movement is called pump handle movement.
Pump handle movement increases the anteroposterior diameter of the thoracic cage.

Lower coastal series 

The lower coastal series is formed by the 7th to 10th pairs of ribs. This series moves the thoracic cage in horizontal direction upward and outwards, increases the transverse diameter of the thoracic cage.


The ability of the lungs and thorax to expand or the expansibility of lungs and thorax is called compliance.
It is defined as the change in volume per unit change in the pressure of thoracic cavity.

  • Compliance in intraalveolar pressure-

This is the volume increases in lungs per unit increase in the intraalveolar pressure.

  • Compliance in intrapleural pressure-

This is the volume increase in lungs per unit decrease in the intrapleural pressure .

Factors affecting the compliance

There are so many factors that affects the expansibility of the thorax and the lungs.
  1. Deformities of thorax- kyphosis and scoliosis
  2. Fibrotic pleurisy
  3. Paralysis of respiratory muscles
  4. Abnormal thorax- hydrothorax, pneumothorax, pyothorax.
Any question regarding this topic, ask me in comment section. 
Want to know about Respiratory function and stages of Respiration, click on link.

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