For external respiration, the air must be physically moved from the atmosphere into the lungs. This involves increasing and decreasing the size of the tho-racic cavity by movement of the chest wall and action of the respiratory muscles. This process is known as pulmonary ventilation, or breathing. The processof drawing air into the lungs is termed inspiration or inhalation, and the process of moving the airout ofthe lungs is referred to as expiration or exhalation.
The rate at which air flows is not only influenced by the pressure differences between the atmosphere and the thoracic cavity but also by the surface ten-sion in the alveoli, the compliance of the lungs (the ease with which the lungs expand), and the resistance offered by the airways.
Knowledge of basic physics will help you understand how air is moved in and out of the thoracic cavity. Ac-cording to Boyle’s law, the pressure inside a closed chamber is inversely related to the volume. Simply, if the volume in a closed container is reduced, the pres-sure of gas in the container increases. For example, if volume is reduced by half, the pressure would dou-ble. If volume is doubled, the pressure would be half of what it was originally. This is the principle behind movement of air in and out of the thoracic cavity.
The parietal pleura lines the inner wall of the tho-racic cavity and the visceral pleura lines the outside of the lungs. The pleural fluid keeps the two membranes in close contact by surface tension. Also, the pressure in the pleural cavity, which is less than that of the at-mosphere, a partial vacuum, keeps the two layers op-posed. When the thoracic cavity increases in volume by the action of muscles and movement of the tho-racic cage, it draws the pleura and, therefore, the lungs with it. This results in a drop in pressure inside the lungs (Remember, when the volume increases, the pressure decreases).Because the nasal cavity commu-nicates with the outside atmosphere, air flows into the lungs to equalize the pressure—inspiration.
Conversely, if the thoracic volume decreases, pres-sure inside the lungs increases (a decrease in volume increases the pressure), and the air flows out of the lungs through the nose to equalize the pressure with that of the atmosphere—expiration. Normal expira-tion is passive, and the change in thoracic volume is caused by the relaxation of the inspiratory muscles and the elastic recoil of the chest wall and the lungs that were stretched during inspiration.
The difference in the oxygen and carbon dioxide levels in the air and blood causes the gases to move by diffusion from the region of higher concentration to that of lower concentration across the alveoli. Thus, carbon dioxide diffuses from the blood to the air and oxygen from air to the blood. Respiration takes place using these physical principles.
The luminal surface of the alveoli has a layer of fluid. Because water molecules have a greater attraction be-tween each other than with gas molecules, an inward directed force is created. This inward force—surface tension—tries to draw the alveoli into the smallest possible diameter. Therefore, the alveoli tend to col-lapse during expiration. During inspiration, this force opposes entry of air into the alveoli. Surfactant, se-creted by the type II cells, reduces the surface tension and the resistance offered during inspiration.
Compliance is the change in volume for a unit changein pressure, and it reflects the ability of the lungs to stretch. If more air volume can be brought into the lung with smaller pressure differences between the lung and the exterior, the lung is considered more com-pliant. For example, lack of surfactant will make the lungs less compliant and make it harder to breathe in. Normal lungs have high compliance because they have elastic tissue that stretch easily. Also, the presence of surfactant reduces surface tension. Respiratory condi-tions that result in scar tissue formation and reduction of elastic fibers, fluid in the lungs, paralysis of respira-tory muscles, or reduced surfactant, lower compliance, increasing the work of breathing. In emphysema, the lungs become more compliant than normal. This, too, is not desirable because there would be a tendency for air to remain in the lungs even after expiration.
The resistance to air flow is largely determined by the caliber of the bronchi. By contracting and relaxing the smooth muscle of the bronchi, the resistance to airflow can be modified. When the sympathetic nerves are stimulated, the muscle wall relaxes, increasing the di-ameter of the bronchi and reducing resistance. In con-ditions such as asthma or obstructive pulmonary dis-ease, the airway resistance is increased, making it harder to breathe. Normally, the airways increase in width during inspiration and reduce during expiration.
Presence of mucus and edema in the airways can also affect resistance. In cystic fibrosis, as a result of the presence of a defective gene that carries instruc-tions for a transmembrane protein responsible for the active transport of chloride ions, the transport of salts and water is inefficient. Thick and viscous se-cretions result, with mucous plug formation, inflam-mation, predisposition to infection, and an increase in airway resistance.