Question

Explain in detail how the movements of the diaphragm and ribs draw air into the lungs during the inhalation phase of the respiratory cycle during quiet breathing. Include in your answer the effects on intrapleural and intrapulmonary pressures and how these effects cause air to move into the lungs.

Answer 1

Muscles involved in quiet respiration:

Inspiration: Diaphragm is the main muscle (75%) and external intercostal muscle (25%).

Expiration: It is a passive process.

Pressure changes during respiration:

a) Intrapleural pressure: At the beginning of quiet inspiration, it is – 2.5 mm Hg or – 5 cm H2O (at the base of the lung), i.e. 2.5 mm Hg less than atmospheric pressure of 760 mm Hg; at the end of inspiration, it becomes – 6.0 mm Hg. Again, it returns to – 2.5 mm Hg at the end of expiration.

b) Intraalveolar pressure (intrapulmonary pressure): At the peak of inspiration, it is – 1 mm Hg; at the peak of expiration, it is + 1 mm Hg. At the beginning, at the end of both inspiration and expiration, the intraalveolar pressure is zero, i.e. same as atmospheric pressure.

Events involved in a normal Inspiration:

Inspiration is an active process.

• The brain initiates inspiratory effort. Nerves carry the inspiratory command to the inspiratory muscles.
• Diaphragm (and/or external intercostal muscles) contracts. Thoracic volume increases as the chest wall expands. Intrapleural pressure becomes more negative (normally it is – 2.5 mm Hg at the start of inspiration, decreases to about – 6 mm Hg).
• Alveolar transmural pressure gradient increases. Alveoli expand in response to the increased transmural pressure gradient. This increases alveolar elastic recoil.
• Alveolar pressure falls below atmospheric pressure as the alveolar volume increases, thus establishing a pressure gradient for airflow (at the peak of inspiration, it is – 1 mm Hg).
• Air flows into the alveoli until alveolar pressure equilibrates with atmospheric pressure.

Events involved in a normal expiration:

It is a Passive process.

• Brain ceases inspiratory command. Inspiratory muscles relax.
• Thoracic volume decreases, causing intrapleural pressure to become less negative and decreasing the alveolar transmural pressure gradient.
• Decreased alveolar transmural pressure gradient allows the increased alveolar elastic recoil to return the alveoli to their pre-inspiratory volumes.
• Decreased alveolar volume increases alveolar pressure above atmospheric pressure, thus establishing a pressure gradient for airflow.
• Air flows out of the alveoli until alveolar pressure equilibrates the atmospheric pressure.