Movements of the Thorax During Ventilation

Each rib has its own pattern of movement, but generalizations can be made. The ribs attach anteriorly to the sternum (except ribs 11 and 12) and posteriorly to the vertebral bodies, disks, and transverse processes, making a closed kinematic chain. The thorax enlarges in all three planes of movement during inspiration.
Increase in the AP dimension. There is a forward and upward movement of the sternum and upper ribs, described as a pump-handle motion. The thoracic spine extends (straightens), enabling greater excursion of the sternum.

Increase in the transverse (lateral) dimension. There is an elevation and outward turning of the lateral (midshaft) portions of the ribs, described as a bucket handle motion. The lower ribs (8-10), which are not attached directly to the sternum, also flare or open outward, increasing the subcostal angle. This is described as caliper motion. The angle at the costochondral junction also increases, making the rib segments longer during inspiration.
Increase in vertical dimension. The central tendon of the diaphragm descends as the muscle contracts. This is described as a piston action. Elevation of the ribs increases the vertical dimension of the thorax and improves the effectiveness of the diaphragm. At the end of inspiration, the muscles relax and elastic recoil causes the diaphragm to move superiorly. The ribs return to their resting position.

Movement of Air

As noted previously, ventilation is the mass exchange of gases to and from the body. During inspiration, as the thorax enlarges, the pressure inside the lungs (alveolar pressure) becomes lower than the atmospheric pressure, and air rushes into the lungs. At the end of inspiration, the muscles relax, and the elastic recoil of the lungs pushes the air out, resulting in expiration.

Compliance
Compliance refers to the distensibility of tissue or how easily the lungs inflate during inspiration. With regard to ventilation, it relates to how easily the lungs inflate or the chest wall expands during inspiration. Normal lungs are highly distensible (compliant), but compliance changes with age and the presence of disease. During the normal aging process lung tissue becomes more compliant. Diseases of the pulmonary system that, for example, cause fibrosis of tissues (alveolar or pleural) make the lungs rigid (i.e., less compliant), whereas emphysema, one of the chronic obstructive diseases, makes lung tissue more compliant to pressures.

Airway Resistance
The amount of resistance to the flow of air through the airways depends on a number of factors. The bifurcation and branching of airways is a source of airway resistance. The size (diameter) of the lumen of each airway also influences resistance. The diameter of the lumen can be decreased by mucus or edema in the airways, contraction of smooth muscles, and the degree of elasticity or distensibility of the lung parenchyma.

Normally, the airways widen during inspiration and narrow during expiration. As the diameter of the airway decreases, the resistance to airflow increases. With diseases that cause bronchospasm (asthma) or increased mucus production (chronic bronchitis), airway resistance is even greater than normal, particularly during expiration.
 

Flow Rates
Flow rates indicate measurements of the amount of air moved in or out of the airways over a period of time. Flow rates, which are related to airflow resistance, reflect the ease with which ventilation occurs.

Expiratory flow rate is determined by the volume of air exhaled divided by the amount of time it takes for the volume of gas to be exhaled.

Flow rates are altered as the result of diseases that affect the respiratory tree and chest wall. For example, with chronic obstructive pulmonary disease, the expiratory flow rate is decreased in comparison to normal. That is, it takes a longer than the normal amount of time to exhale a specific volume of air.

Buy the Book that holds this excerpt: Therapeutic Exercise: Foundations and Techniques (Therapeutic Exercise: Foundations & Techniques)

Related Articles