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Pulmonary Mechanics

Bulk flow of air in between the external environment and the lungs is one important respiratory function of the respiratory system. This is brought about by coordinated, active movements of the thoracic wall and the diaphragm, causing the air to be sucked in (inspiration) and passive recoil forces of the lungs and the chest wall causing air to be pulled out (expiration).

Anatomy of the Thoracic Cavity

The lungs and the trachea are within the closed thoracic compartment. It is sealed at the top by connective tissue and muscles which are attached to the sternum, upper ribs and vertebral column, and is totally separated from the and abdomen by the upward dome of a thin sheet of skeletal muscle, the diaphragm. In the mid-line, membranes associated with the pericardium of the heart, large blood vessels (the aorta, the pulmonary artery, the superior vena cava and the pulmonary veins)and the oesophagus result in separation of the right and left compartments so that, should one lung collapse, the other can remain inflated and expand to compensate for the volume that is lost.

Mechanics of Inspiration and Expiration

The thoracic cavity, which is walled by the ribs and the intercostal muscles and lined interiorly by the parietal pleura, is a sealed cavity. The visceral pleura lines the outer surface of the lungs. Due to the continuity of the parietal and the visceral pleurae and due to the cohesion exerted by the thin layer of fluid in between the two linings, in health, the lungs do not separate from the chest wall. Contraction of the inspiratory muscles (the diaphragm, the external intercostal muscles and the accessory muscles of inspiration in the back and neck) causes the chest wall, and hence, the lungs to expand resulting in inspiration. Passive recoil of the lungs is entirely responsible for the expiration, resulting in the diaphragm to be pulled upwards.

How Inspiration is brought about….

The main inspiratory muscle at rest is the diaphragm. The diaphragm is a thin sheet of skeletal muscle separating the thorax and the abdomen and is innervated bilaterally by the phrenic nerves arising from cervical roots 3-5 of the spinal cord. When the diaphragm contracts, in response to the nerve impulses received via the phrenic nerves, it tends to depress. The diaphragm also elevates the lower ribs at their points of origin, causing widening of the thoracic cavity. Contractions of the external intercostal muscles also contribute by elevating the ribs anteriorly. This results in an increase in the intra-thoracic volume and therefore, at a static state when there is no airflow through the respiratory tract, it causes a reduction in the intra-thoracic pressure. During a strenuous inspiration, in addition to the diaphragm and the external intercostal muscles, the sternocleidomastoids and the scalene muscles aid in elevating the sternum and the pectoralis major, pectoralis minor and serratus anterior muscles aid in widening the rib cage.

How Expiration is brought about….

Expiration is a passive process during quite breathing. Passive elastic recoil of the lungs results in the doming of the diaphragm and returning of the thoracic wall to its original position. This decreases the intra-thoracic volume results in exhalation. However, if the exhalation has to become forceful, the active contraction of the abdominal muscles and the internal intercostal muscles result in a rapid increase in the thoracic pressure, leading to a rapid exhalation. Contractions of the abdominal muscles drive the diaphragm upwards into a highly domed shape expelling gas from the lungs. Hence, the abdominal muscles become more important as expiratory muscles.

The “Pump Handle” and the “Bucket Handle”…..

During inspiration, the sternum moves upwards and outwards in the antero-posterior plane and during expiration, the movement is reversed. This movement of the sternum is described as a “pump handle” moving upwards and downwards. Similarly, the lower ribs tend to move upwards and outwards in a lateral plane and this is reversed during expiration. Thus, the movement of the lower ribcage is described as a “bucket handle”. These examples of pump handle and bucket handle aids in understanding the mechanics of expansion and reduction of the intra-thoracic volume during a breathing cycle.

Duration of Expiration Determines the Rate of Respiration

During a quite breathing cycle, expiration is prolonged than inspiration. Following inspiration, there is a small pause prior to the commencement of expiration as some inspiratory diaphragmatic activity continues into the initial phase of expiration. In addition, contraction of the glottis during expiration exerts a braking effect. However, when the breathing becomes more rapid, the length of expiration shortens markedly with the early onset of the next inspiratory effort. Active contraction of the muscles of expiration and removal of the braking effect exerted by the glottis also aid in rapid exhalation.

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