Explain how respiration may be affected by:

(a)        the entry of a large volume of air into the pleural space (pneumothorax)

(b)        left ventricular failure

(c)        surfactant deficiency

Essay:

(a)        The lung is enclosed by a two-layered membrane known as the pleura. The outer or parietal layer, lines the thoracic wall, while the inner or visceral layer envelops the lungs. The space between the two layers of pleura is called the pleural space. There is a thin layer of fluid present in this space which holds the two pleural membranes together and to the chest wall. The combination of the outward pull of the thoracic cage and inward recoil of the elastic lungs creates an intrapleural pressure of about –3 mmHg. This negative intrapleural pressure is essential tot keep the alveolar sacs open and prevent their collapse while at the same time preventing pleural effusion of fluid into the alveoli.

            When air is admitted into the pleural space, through either a rupture in the lung or a hole in the chest wall, the fluid bond holding the lungs to the chest wall is broken and the lung on the affected side collapses to an unstretched state because of its elastic recoil. Inspiration becomes impaired due to the difficulty in inflating the collapsed alveoli, and hypoxia develops, stimulating an increase in ventilation. Respiratory distress range from mild to severe depending on the size of the pneumothorax.

(b)        In left ventricular heart failure, the left ventricle is unable to pump blood efficiently out into the aorta and blood accumulates in the left ventricle and atrium. This leads to the build-up of blood pressure in the pulmonary capillaries, the exudation of fluid into the interstitial spaces. A more than 50 per cent increase in the interstitial fluid volume of the lungs will rupture the alveolar epithelial membranes and the filling of the alveoli with large amounts of free fluid, resulting in pulmonary edema. The presence of fluid in the alveoli increases the distance through which oxygen must diffuse to reach the pulmonary capillaries. Therefore, the rate of diffusion of oxygen into the blood is decreased and respiration is impaired. In severe pulmonary edema, little oxygen is absorbed by the pulmonary capillaries and hypoxia results leading eventually to death.

(c)        The presence of fluid lining the alveoli creates surface tension that increases the resistance of the lung to stretch and the work needed to expand the alveoli with each breath. Surfactants are secreted by type II alveolar epithelial cells. It is composed chiefly of dipalmitoylphosphatidylcholine, surfactant apoproteins and calcium ions. The surfactant molecules disrupt the cohesive forces between the water molecules and decrease the surface tension of the fluid lining the alveoli, thereby preventing small alveoli from collapsing. When surfactant is deficient, the increased surface tension of the fluid lining the alveoli increases the amount of energy required to expand the alveoli, which may become collapsed. Respiration is severely impaired and may leads to respiratory distress and eventual death.