The respiration includes 2 other periods of (a) inspiration and (b) expiration, that are related to alternative increase and decline in the volume of thoracic cavity, respectively. During inspiration, the air is taken in (inhaled) and during expiration, the air is taken out (exhaled).

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.


During motivation the volume of thoracic cavity increases, which creates a negative intrathoracic pressure, therefore air is sucked into the lungs.

A rise in the capacity of thoracic cavity takes place vertically, anteroposteriorly, and transversely (i.e., side to side).

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

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The expiration is the passive process brought about by.

1. elastic recoil of the alveoli of the lungs,.

2. relaxation of the intercostal muscles and the diaphragm, and.

3. increase in the tone of the muscles of anterior abdominal wall.

Types Of Respiration (Breathing).

The respiration is classified into the following 3 types:.

A. Silent respiration.

B. Deep respiration.

C. 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.

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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.