Form and function of the diaphragm
The
diaphragm is made up of a central tendinous part, extended
via the diaphragmatic muscle between rib cage and spinal column.
At the insertion to the rib the costophrenic angle is sharp
(left panel of figure). This dome-shaped form of the diaphragm
is crucial to its function. When the muscle contracts it pulls
the tendinous part down, and the diaphragm descends like a
parachute. The lung follows passively; as the lung stretches
pleural pressure falls, and this would cause the ribs to move
inwards (an expiratory maneuver). However, downward displacement
of the diaphragm compresses the abdominal content. Thanks
to the apposition of diaphragmatic muscle and ribs, the increased
intra-abdominal pressure is transferred to the lower ribs,
which are pushed outwards. The lower thoracic aperture widens,
causing the thorax to be lifted. The net effect is an inspiratory
maneuver.
In the case of severe hyperinflation the diaphragm is much flatter and the costophrenic angle much larger (right panel). Much of the force delivered by the diaphragmatic muscle is now translated into pulling the lower ribs inward, and little in downward displacement of the diaphragm. Hence there is little change in abdominal pressure which can be translated into inspiratory movement of the thoracic cage. In extreme cases the diaphragm acts a muscle of expiration, ventilation being dependent solely on the activity of respiratory muscles which lift the thoracic cage. Inspiratory movement of the lower ribs (‘Hoover’s sign’) is therefore an ominous sign in adults.