Pressures And Volumes During Normal Breathing
Functional residual capacity
The volume left in the lungs at the end of a normal breath is known as the functional residual capacity (FRC). At FRC, the respiratory muscles are relaxed and its volume is determined by the elastic properties of the lungs and chest wall.
The lungs are elastic bodies whose resting volume when removed from the body is very small. The natural resting position of the chest wall, seen when the chest is opened surgically, is about 1 L larger than at the end of a normal breath.
In the living respiratory system, the lungs are sealed within the chest wall. Between these two elastic structures is the intrapleural space, which contains only a few millilitres of fluid When the respiratory muscles are relaxed, the lungs and chest wall recoil in opposite directions, creating a subatmospheric ('negative') pressure in the space between them, and this tends to oppose the recoil of both the lungs and the chest wall. FRC occurs when the outward recoil of the chest wall exactly balances the inward recoil of the lungs (Fig. 3a). When the chest is opened, air enters the intrapleural space, the pressure be- comes atmospheric and nothing opposes the recoil of the lungs and chest wall. The lungs shrink to a small volume and the chest wall springs out.
If the elastic recoil of either the lungs or the chest wall is abnormally large or small, FRC will be abnormal. In lung fibrosis the lungs are stiff and have increased elastic recoil, so the balance point, and hence FRC, occurs at a small lung volume. In emphysema, there is loss of alveolar tissue and with it, loss of elastic recoil. When the respiratory muscles are relaxed, the reduced elastic recoil of the lungs offers less opposition to the outward recoil of the chest wall and FRC is increased (the barrel chest of emphysema). Increased FRC can also occur because of 'air trapping' (see Chapter 7).
The space between the visceral pleura lining the lungs and the parietal pleura lining the chest wall is so small that measuring intrapleural pressure with a needle risks puncturing the lung. Intrapleural pressure can be indirectly assessed from oesophageal pressure (Fig. 3b). The oesophagus is normally closed at the top and bottom except during swallowing and in the upright subject the oesophageal pressure is the same as in the neighbouring intrapleural space. The subject swallows either a miniaturized pressure transducer or a balloon containing a little air connected by a tube to an external manometer. Gravity affects the fluid-line intrapleural space, and at FRC in an upright subject, the intrapleural pressure at the apex of the lungs is about 0.5 kPa ( 5 cmH20) and about 0.2 kPa ( 2 cmH20) at the bottom.
Pressures, ﬂow and volume during a normal breathing cycle
During inspiration, the chest wall is expanded and intrapleural pressure falls. This increases the pressure gradient between the intrapleural space and alveoli (Fig. 3c), stretching the lungs. The alveoli expand and alveolar pressure falls, creating a pressure gradient between the mouth and alveoli, causing air to f ow into the lungs. The airflow profil (Fig. 3d) closely follows that of alveolar pressure. During expiration, both intrapleural pressure and alveolar pressure rise. In quiet breathing, intrapleural pressure remains negative for the whole respiratory cycle, whereas alveolar pressure is negative during inspiration and positive during expiration. Alveolar pressure is always higher than intrapleural because of the recoil of the lung. It is zero at the end of both inspiration and expiration, and airflow ceases momentarily. When ventilation is increased, the changes of intrapleural and alveolar pressure are greater and in expiration intrapleural pressure may rise above atmospheric pressure. In forced expiration, coughing or sneezing, intrapleural pressure may rise to +8 kPa (+60 mmHg) or more.
If a subject breathes in and out of a simple water-ﬁlled spirometer (Fig. 3e(i)), the drum falls and rises and the pen, attached by a pulley system, produces a trace (Fig. 3e(ii)) which illustrates the important lung volumes. Conventionally, volumes composed of two or more volumes are known as 'capacities', whereas those that cannot be subdivided are known as 'volumes'. The volume breathed in (or out) is known as the tidal volume, and the trace shows several resting tidal volumes, which are typically about 500 mL. At the end of a normal quiet inspiration, the subject could breathe in more and this is the inspiratory reserve volume (IRV). Similarly, the volume that he or she could exhale after a normal expiration is the expiratory reserve volume (ERV). For the fourth breath, the subject breathes in and out as fully as possible. This maximum tidal volume is the vital capacity (VC= VT +IRV+ERV). At the end of a maximal breath out, the volume remaining in the lungs is the residual volume. FRC and total lung capacity are the volumes in the lungs at the end of a normal expiration and after a maximal breath in, respectively. Possible values for a man are given in Table 1. Although a zero volume line is shown (Fig. 3e(ii)), it is not possible to know where this actually is on a trace, because the subject cannot empty the lungs into the drum. For this reason, although illustrated in Fig. 3e(ii), volumes shown in red in Table 1 cannot be measured from a simple spirometer trace. They can be measured using helium dilution or body plethysmography (Chapter 20). The range of normal lung volumes is large and an individual's volumes must be assessed with the aid of nomograms that give the predicted value of for the subject's age, sex and height.