Discuss the physiologcial factors that determine the work of breathing.

Outline:

·        Nonelastic work:

- viscous resistance

- airway resistance

·        Elastic work

Essay:

            Adequate ventilation depends on the ability of the lungs to expand normally. During inspiration, work is performed by the respiratory muscles in stretching the elastic tissues of the chest wall and lungs (elastic work), moving inelastic tissues (viscous resistance), and moving air through the respiratory passages.

            The lungs and thoracic cage are elastic structures. The elastic recoil of the normal lung at rest creates an inward pull while the elastic recoil of the chest wall tries to pull the chest wall outward. 65% of the work of breathing goes to overcome these elastic forces. The elastic forces present are the elastic forces of the lung tissue itself, due mainly to the elastin and collagen fibers interwoven among the lung parenchyma (one-third of total lung elasticity) and the elastic forces caused by surface tension of the fluid that lines the inside of the alveoli and other lung air spaces (two-thirds of total lung elasticity).

            7% of the work of breathing goes to overcoming viscous resistance of the inelastic tissues present in the lungs and chest wall structures and the rib cage itself. As the inelastic tissues are unable to stretch, work must be done by the respiratory muscles in overcoming their resistance when expanding the thoracic cavity. The mobility of the rib cage is important in changing the size of the thoracic cavity. Contraction of the intercostals swings the ribs upward and outward. Synovial joints between the heads and tubercles of the ribs and the vertebral body are responsible for these movements. Work is done to overcome frictional forces at the joints.

            Airway resistance work (28%) is required to overcome airway resistance during movement of air into the lungs. Nearly 90% of airway resistance are attributed to the trachea and bronchi, rigid structures supported by cartilage and bone so that their diameters normally do not change. The bronchioles normally do not contribute to air resistance as their total cross-sectional area is about 2000 times that of the trachea and hence they have the smallest resistance to air flow. But, because the bronchioles are collapsible tubes, a decrease in their diameter can suddenly turn them into a significant source of airway resistance.

            The work of breathing is greatly increased in diseases such as emphysema and asthma.