It is true that mediastinal shift occurs following pneumonectomy (toward the side of resected lung). As to the forces at play, this is rather complex. Using a somewhat simplified model, first recall that the lung stays "inflated" because of slight negative pressure in the pleural space around the lung, generated by the "outward" action of the chest wall and diaphragm. When you inhale, the diaphragm moves downward, the chest wall expands, increasing negative pressure in the pleural space, in turn drawing the lungs "outward", i.e. causing them to expand. When something causes air to leak into the pleural space, whether from a ruptured alveolus from barotrauma or penetration of the chest wall (eg icepick), this disrupts this negative pressure and the lung can collapse. In the immediate post operative state, there is air in the space previously occupied by the removed lung. However, this air is not (normally) in contiguity with the outside world, and in fact this space will develop slight negative pressure (relative to external ambient air pressure). This is in part due to gas absorption into the blood through the pleural surface, and in part due to the previously mentioned action of chest wall and diaphragm. Pleural fluid, which the pleura normally secretes to lubricate the lung surface, moves into the space, and begins to fill the affected hemithorax. Get a chest x-ray initially, with a person upright, and you can see an air-fluid level of residual air over a line of layering fluid. As previously mentioned, the gas will be absorbed over time, and fluid continues to accumulate, until the hemithorax is filled, which on chest x-ray makes that half of the chest look "white" or opacified. The pleural space for the right and left lung is anatomically distinct, which in part accounts for why the fluid accumulates mainly on one side of the chest. The fluid accumulation does not go unchecked however as increased fluid pressure in the pleural space decreases the pressure gradient for secretion of fluid, and increases the pressure gradient for absorption of fluid (fluid moves across membranes driven by pressure gradients, or drawn by osmotic gradients, the latter not being that important here). Anyhow, here's a thumbnail of a chest x-ray from a person who had a left pneumonectomy (conventionally, a chest x-ray is displayed as if the patient imaged is facing you, and the right side of the picture is the patient's left side). Note the "white" opacified chest, as opposed to the normally aerated "dark" lung. For the connoisseur, the image has some mediastinal shift (to the person's left) and hyperexpansion of the remaining right lung. All that aside, and back to what's important, I think it's great that divebuddies is enthused about sharing our love of scuba diving with his/her father.
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