Thorax and diaphragm: form and function
The way in which the diaphragm is
extended within the thorax differs between neonate and adolescent,
and the difference matters. Let us first look at neonate and
adult in the erect posture. In adults the muscular part of
the diaphragm is apposed to the lower rib cage and rises very
steeply, whereas the membranous part is extended horizontally.
This is a very advantageous construction. When the diaphragmatic
muscle contracts it pulls the membranous part down like the
dome of a parachute. This extends the lung downwards and results
in an inspiratory movement. At the same time the pleural pressure
falls. This might cause an inwards, and hence an expiratory,
movement of the thorax. However, the downward displacement
of the diaphragd compresses the abdominal content, elevating
the abdominal pressure. On that account the lower ribcage
is pushed outward. The relatively stiff thorax hinges outwards
and upwards, and this reinforces the inspiratory maneuver.
In the neonate the geometry is less favorable. The diaphragm is more level, and the muscular part is not closely apposed to the rib cage. This diminishes the potential for displacing the diaphragm downwards; also when the muscle contracts the lower rib cage is pulled inwards, a paradox movement with a view to inspiration. Whilst the downward displacement of the lung leads to inspiring air, this is associated with less increase in abdominal pressure than in the adult. In addition the abdominal pressure is less efficiently translated into an outword acting force on the lower rib cage, because the area of apposition is much more limited. Finally, because the rib cage is so pliable, some of the drop in pleural pressure is associated with an inward movement of the rib cage, counteracting to some extent the efficiency of the inspiratory maneuver.
In the supine position the intra-abdominal mass displaces the diaphragm into the thoracic cavity. Lung volume in the supine posture is therefore smaller than in the erect posture. The displacement of the diaphragm by the abominal contents also influences its geometry, the upper part being at a greater angle to the rib cage; this is a less favorable position for the muscle to move the diaphragm towards the abdomen, but on the other hand the larger stretch helps the muscle to operate at a more efficient part of its force-length curve.
- The most favorable shape for an efficient function of the diaphragm is that in an healthy adult, in whom muscle shortening is most efficiently translated into downward displacement of the diaphragm and the creation of and increased intra-abdonal pressure that pushes the rib cage outward.
- It is of great advantage that during growth the thorax becomes stiffer and more elongated, so that the apposition between muscle and thoracic cage increases. Such changes in the shape occur between birth and adulthood.