Sperm whales are obviously mammals and breathe air from the surface. They can stay underwater for around 45 minutes and routine dives reach 1,600 – 3,200 feet. They have been recorded going as deep as 1.2 miles! It is very interesting how these whales evolved to allow them to swim so deep and for so long.
Boyles law states that gas pressure is inversely proportional to volume. So, as pressure increases, volume decreases. While this is something we notice when we go to high altitude and struggle for breath, it is orders of magnitude greater underwater. At sea level, the 50,000 feet of air in the atmosphere above us presses on us at 14.5 pounds per square inch. We don’t notice that pressure because all the gasses and fluids in our bodies are pushing out with the same force.
As we go underwater the pressure changes dramatically. At a mere 32 feet of depth, there is another 14.5 psi of pressure meaning that the air in a whale’s lungs is half the volume and twice the pressure in order to counterbalance the outside pressure. This continues proportionally as depth increases. At half a mile below the surface (2,640 feet) the whale’s lungs are reduced to less than 1% of their surface volume and the pressure of that compressed air inside their lungs is massive (about 100x greater). In order to prevent the massive air pressure from pushing extra nitrogen and oxygen into the the bloodstream, which would give the whale “the bends”, the whale has evolved so that their alveoli – the tiny sacs in their lungs where oxygen and CO2 is exchanged in the bloodstream – are shut down. So, from the time the whale dives below the surface, any air in its lungs is not moved into the bloodstream. The lungs collapse down as described above and they don’t use that air to breathe until they hit the surface again.
How do they stay down so long? Whales access reserve oxygen in its blood and muscles – not from the air in its lungs. Whales have more than 2x the hemoglobin in their blood as humans have and 10x the myoglobin. At the surface the whale’s breathing recharges the oxygen stores in the blood and muscles to “breathe” from while diving. This is different than what happens when we humans go underwater. We take a deep breath and our alveoli continue to operate, moving oxygen from our lungs into our bloodstream.
Another thing that allows whales to dive so deeply: their bodies are more flexible. According to the National Ocean Service: “their ribs are bound by loose, bendable cartilage, which allows the rib cage to collapse at pressures that would easily snap our bones.”
Primary source for this IFOD: “Storm in a Teacup: The Physics of Everyday Life” by Helen Czerski.