There is greater pressure the deeper you go in water. So if you go too deep, the pressure in the human lungs will be so great and too much that the persons lungs will collapse.
Inspiration happens when the pressure inside the lungs is lower than the atmospheric pressure (outside) and air rushes into the lungs. Expiration is when the air inside the lungs is higher than the atmospheric pressure and the air rushes out of the lungs. If the intrapleural pressure (pressure within the pleura of the lungs) isn't maintained then the pressure in the lungs can't differentiate between inspiration and expiration and so the lung collapses.
When pressure inside the lungs is lower than outside, air flows into the lungs to equalize the pressure. This is known as inhalation, where the diaphragm contracts and the rib cage expands to create more space for air to enter the lungs.
If the pressure in the lung is greater than atmospheric pressure, air will flow out of the lungs in order to equalize the pressure. This is called exhalation or breathing out.
Pressure within the thorax decreases and air is drawn into the lungs
If intraalveolar pressure exceeds atmospheric pressure, air will flow out of the lungs due to the pressure gradient. This process is known as exhalation or expiration. It allows the lungs to get rid of carbon dioxide and regulate the levels of gases in the body.
When intrapulmonary volume increases, the intrapulmonary pressure decreases. This creates a pressure gradient that allows air to flow from higher pressure outside the lungs to the lower pressure inside the lungs during inspiration.
Normally, the pressure in the lungs is greater than the pressure in the pleural space surrounding the lungs
the lungs recoil/contract to push air out using its smooth muscle
As the pressure increases in the space between the chest wall and the lungs from an influx of blood, the collapse of the lung due to a relatively low pressure (within the lung) will occur.
Pressure in the lungs increases during exhalation-when you breath out
when air moves out of the lungs, the air pressure decreases
As your diaphragm expands, the pressure in your thoracic cavity decreases. Air rushes into the partial vacuum, and you inhale.