The absolute necessity of chest-wall collapse during diving in breath-hold diving mammals

It has been suggested that the chest-wall (rib-cage, diaphragm, abdominal wall) of a diving animal is nearly incollapsible, and at deep depths can sustain a considerable trans-thoracic pressure PTT (=Pbody surface Pintra-thorax). In strong contrast, we argue on circulatory, blood pooling, biological material strength, and ventilatory grounds that this is incompatible with life. At a PTT of more than a third of an atm, or that experienced at a depth of only 3.5 m if the interior of the thorax remains at Psea-level, the heart is unable to perfuse tissues outside the thorax. To the extent that PTT is substantially larger than at sea level, blood will pool in the thoracic vessels with potentially lethal consequences of vascular rupture. From estimates of supporting tissue thickness and geometry, a PTT >1 atm would exceed the mechanical strength of known biological materials. Finally, a chest-wall sufficiently stiff to support a PTT >1 atm is not compatible with breathing when the animal is at the surface.