Describe and explain the changes in volume, flow rates and pressures in the lungs and thorax during normal respiration in the adult.

Outline:

·        Description of the breathing apparatus

·        Mechanics of pulmonary ventilation

·        Initiation of breathing: change in volume of lungs and thorax

·        Flow of air as a result of pressure difference

·        Respiratory cycle: inspiration and expiration

Essay:

            Breathing is an active process that uses muscle contraction to create pressure gradients. The primary muscles involved during normal respiration are the diaphragm, the intercostals and the scalenes. The lung and the chest wall are elastic structures, separated by a thin layer of fluid, the pleural fluid. 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. The two pleural membranes are held together by the pleural fluid bond, so the elastic lungs are forced to stretch to conform to the larger volume of the thoracic cavity. In this way, alterations in volume of the thoracic cavity will result in a change in the volume of the lungs. This generates a pressure difference between it and the external environment. Air flows into the lungs from a higher to lower pressure and it moves out when the pressure in the lungs is higher.

            The lungs can be expanded or contracted in two ways: by the downward and upward movement of the diaphragm to lengthen or shorten the chest cavity and by elevation and depression of the ribs to increase and decrease the anteroposterior diameter of the chest cavity. A single respiratory cycle consists of an inspiration followed by an expiration.

            At the start of an inspiration, intrapulmonary pressure (pressure in the air spaces of the lungs) is equal to atmospheric pressure and there is no air flow. As inspiration begins, contraction of the diaphragm, external and scalene muscles increases the size of the thoracic cavity. With the increase in volume (of both lungs and thorax), intrapulmonary pressure falls gradually to about 1 mmHg below atmospheric pressure. Air flows into the lungs, increasing the pressure until it is equal to the atmospheric pressure. At this point, which is the end of inspiration, the volume of air in the lungs is at a maximum; flow rate is zero as there is no pressure difference.

            During expiration, the respiratory muscles relax, decreasing the volume of the thorax and lungs. Air pressure in the lungs increases, reaching about 1 mmHg above atmospheric pressure at its maximum. Intrapulmonary pressure is now higher than atmospheric pressure, so air flow reverses and air moves out of the lungs, decreasing the intrapulmonary pressure until it is again equal to the atmospheric pressure. Air flow ceases at this point. As air exits from the lungs, its volume decreases gradually to its original volume. With the cessation of expiration, the respiratory cycle has ended and is ready to begin again with the next breath.