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).
Intrapleural pressure
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.
Lung volumes
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.
