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Mechanism of Respiration

Respiration or pulmonary ventilation, is the process that exchanges air between the atmosphere and the alveoli of the lungs. Air moves into and out of the lungs along an air pressure gradient-from regions of higher pressure to regions of lower pressure. There are three pressures that are important in breathing:

  1. Atmospheric pressure is the pressure of the air that surrounds the earth. Atmospheric pressure at

    sea level is 760 mm Hg, but at higher elevations it decreases because there is less air at higher elevations.

  2. Intra-alveolar (intrapulmonary) pressure is the air pressure within the lungs. As we breathe in and out, this pressure fluctuates between being lower than atmospheric pressure and higher than atmospheric pressure.
  3. Intrapleural pressure is the pressure within the pleural cavity. It is about 2 to 6 mm Hg below the atmospheric pressure during various phases of breathing. This lower intrapleural pressure is often described as “negative pressure,” and it keeps the lungs stuck to the internal walls of the thoracic cage and helps expand the lungs, even as the thoracic cage expands and contracts during breathing. If the intrapleural pressure were to equal atmospheric pressure, the lungs would collapse and be nonfunctional.


Rate Of Respiration

The typical rate of respiration is 18 per minute in normal resting state of an adult. It’s more rapid in children and slower in the aged.


The process of moving air into the lungs is called Inspiration or inhalation. When the lungs are at rest, the air pressure in the lungs is the same as the atmospheric pressure. In order for air to flow into the lungs, the intra-alveolar pressure must be decreased to below atmospheric pressure. This change allows for air to flow from the higher air pressure in the atmosphere towards the lower air pressure within the lungs. The contraction of the diaphragm and the external intercostals during inspiration causes an increase in lung volume, which results in a decrease in intra-alveolar pressure.

The dome-shaped diaphragm is a thin sheet of skeletal muscle separating the thoracic and abdominal cavities. When it contracts, the diaphragm pulls inferiorly and becomes flattened, which increases the volume of the thoracic cavity. At the same time, contraction of the external intercostals elevates and protracts the ribs and pushes the sternum anteriorly, which further increases the volume of the thoracic cavity.

Because the negative intrapleural pressure and the surface tension of the pleural fluid keep the visceral pleura stuck to the parietal pleura, the lungs are pulled along when the thoracic cage expands. Therefore, the expansion of the thoracic cavity increases the volume of the lungs, which decreases the intra-alveolar pressure. Then, the higher atmospheric pressure forces air through the air passageways into the lungs until intra-alveolar and atmospheric pressures are equal. Quiet inspiration requires the contraction of the diaphragm and the external inter-costals only. Forceful inspiration requires the involvement of additional muscles in the neck and chest, such as the sternocleidomastoid, scalenes, serratus anterior, and pectoralis minor. The contraction of these muscles elevates and protracts the ribs to a greater extent, leading to a greater increase in the volume of the thoracic cavity. Through this further increase in thoracic volume, intra-alveolar pressure decreases to a greater extent, which results in greater airflow into the lungs.

Vertical Diameter

Theoretically, the vertical diameter of the thoracic cavity can raise, if the roof of the thoracic cavity is lifted or its floor lowered or both. The roof of thoracic cavity is composed by rough suprapleural membrane, that is repaired, therefore can not move up and down. But, the floor of thoracic cavity is composed by the freely movable diaphragm. So when the diaphragm contracts, its central tendon descends, and its domes are flattened. Because of this, there’s a rise in the vertical diameter of the thoracic cavity.

Anteroposterior Diameter

A rise in anteroposterior diameter of the thoracic cavity happens when sternum moves forwards and upwards.

Every rib acts a lever, the fulcrum of which is located just lateral to the tubercle of the rib. Hence 2 arms of lever are considerably disproportional, example, posterior arm is really short and anterior arm is really long. Hence small movement in the vertebral end of the rib is significantly magnified in the anterior end of the rib.

Since anterior ends of the ribs are simply at a lower level than their posterior ends, during elevation of the ribs, when their anterior ends move upwards and forwards, they take with them the sternum. (This movement takes place mainly in vertebrosternal ribs.) Therefore, the anteroposterior diameter of the thoracic cavity is raised. This movementis named pump-handle movement because sternum moves up and down like a handle of pump during respiration.

Transverse Diameter

The middle of the shaft of the ribs is located at the lower level in relation to the plane going through its 2 ends (anterior and posterior). This arrangement resembles a pail handle. Accordingly, during elevation of the ribs, the shafts ofthe ribs move outwards like the pail handle– pail handle movement. This causes increase in the transverse diameter of the thoracic cavity. The axis of movement enters from the tubercle of the rib to the middle of the sternum.

The bucket-handle movement is generated by vertebrochondral ribs.

Variables responsible for the increase in different diameters of the thoracic cavity during motivation

DiameterFactors responsible for increase
VerticalDescent (contraction) of the diaphragm
AnteroposteriorPump-handle movement of the sternum (brought about by the elevation of vertebrosternal ribs)
TransverseBucket-handle movement of the vertebrochondral ribs


The expiration is the passive process brought about by:

  • Elastic recoil of the alveoli of the lungs
  • Relaxation of the intercostal muscles and the diaphragm
  • Increase in the tone of the muscles of anterior abdominal wall

Expiration or exhalation, occurs when the diaphragm and external intercostals relax, allowing the thoracic cage and lungs to return to their original size. This results in a decrease in the volume of the thoracic cavity and lungs. The decrease in lung volume increases intra-alveolar pressure to a level higher than atmospheric pressure. The higher intra-alveolar pressure forces air out of the lungs until intra-alveolar and atmospheric pressures are equal.

Expiration during quiet breathing is a rather passive process because the abundant elastic connective tissue in the lungs and thoracic wall causes them to return to their original size as soon as the muscles of inspiration relax. However, a forceful expiration is possible by contraction of the internal intercostals, which depresses and retracts the ribs, and by the muscles of the abdominal wall, which move the abdominal viscera and diaphragm superiorly. These contractions further decrease the volume of the thoracic cavity and lungs, which increases the intraalveolar pressure, causing more air to flow out of the lungs.

Types of Respiration (Breathing)

The respiration is classified into the following 3 types:

  • Silent respiration
  • Deep respiration
  • Compelled respiration

In quiet respiration, the movements are normal as described above.

In deep respiration, movements described for quiet respiration are raised. The 1st rib is elevated by scalene and sternocleidomastoid muscles.

In forced respiration, all movements are exaggerated. The scapula is mended and elevated by trapezius, levator scapulae, rhomboideus major, and rhomboideus small muscles, in order that pectoral muscles and serratus anterior can raise the ribs.

Muscles acting during distinct types of respiration

Type of respirationInspiration (elevation of ribs)Expiration (depression of ribs)
Quiet respirationExternal intercostal musclesPassive
 DiaphragmNo muscles
Deep respirationExternal intercostal musclesPassive
 Scalene musclesNo muscles
 Levatores costarum 
 Serratus posterior superior 
Forced respirationAll the muscles involved in deep inspiration (vide supra)Quadratus lumborum
 Levator scapulaeInternal intercostal muscles
 TrapeziusTransverse thoracis
 RhomboidsSerratus posterior inferior
 Pectoral muscles 
 Serratus anterior

Clinical Significance

Pose Of Patient During Asthmatic Episode

During asthmatic episode (defined by breathlessness/ trouble in breathing), the patient is the most comfortable on sitting up, leaning forwards and fixing the arms on the bed/table. This is because in the sitting position, the diaphragm is at its lowest level, enabling maximum breathing. Fixation of arms fixes the scapulae, so the pectoral muscles and serratus anterior may act on the ribs that they elevate.

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