Lung Mechanics MCQs

Surfactant is synthesized, secreted, and recycled by alveolar type II cells Reference: Ganong, 25th edition page no.630

Airway resistance decreases as lung volume increases because the increasingly subatmospheric intrapleural pressure increases the radial traction on the airways, thereby increasing airway diameter and decreasing airway resistance. During a forced expiration or at the residual volume, the intrathoracic pressure is positive, compressing the airways and increasing their resistance. The vagus nerve constricts airway smooth muscle. Resistances in parallel add as reciprocals. Thus, the large number of small, peripheral airways increases the number of airways arranged in parallel, and lowers the total resistance of the peripheral airways compared to the total cross-section of the central airways. Reference: Levitzky, pp 35-41.

There are two major forces that oppose lung distensibility (Compliance). They are, Elastic forces of the lung tissues Opposing force caused by surface tension Reference: Ganong, 25th edition page no.629

Conditions causing increase in compliance As a part of normal ageing-Compliance of lung increases in old age Emphysema (COPD) In emphysema there is loss of elastic recoil of lung Emphysematous lung is easy to inflate (overdistension) but because of the loss of elastic recoil, additional effort must be given to force air out of lang Flail chest Sternotomy Conditions causing decrease in compliance "Any condition that cause lung stiffness or that opposes lung expansion always decreases lung compliance" Pulmonary fibrosis In pulmonary fibrosis, elastic fibers of lung are replaced by stiff collagen fibers Consolidation Pulmonary edema Pneumothorax Obesity Kyphoscoliosis Ankylosing spondylosis Reference: Ganong, 25th edition page no.629

C i.e. Increase in alveolar surface area by the surfactant

Pumping of fluid from the pleural space by the lymphatics is an important cause for negative Intrapleural pressure Reference: Ganong, 25th edition page no.629

An increased velocity of airflow will increase turbulence, as predicted by Reynold's number. Resistance to turbulent airflow exceeds that for laminar airflow, and thus the pressure gradient required for airflow increases when flow is turbulent. Because the velocity of airflow is greatest in the trachea and large airways, the predisposition to turbulent airflow is greater in the central than in the peripheral airways. Airway resistance varies inversely with the fourth power of airway radius, according to Poiseuille's law. Reference: Ganong, pp 582-585. Levitzky, pp 32-35.

Coughing is protective reflex & example of valsalva maneuver where 1 st there will be deep inspiration followed by forceful expiration against closed glottis. Then glottis will open with removal of air or harmful substances