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I understand that the pressure can be 50 atmospheres: https://en.wikipedia.org/wiki/Deep_diving.

Okay, I understand also that the pressure is equal all around but if someone was put between two metal plates the pressure of 50 atmospheres would I guess certainly crush him mechanically, like the flesh on his bones would crushed against the bones or his bones themselves would break.

So how is oceanic/liquid pressure different? It just seems so obvious that 750 pounds/square inch would crush a person and yet this does not happen obviously.

I know this has been discussed before but I am asking very specifically: why is flesh being pressed against bone at 50 atmospheres not damaged while if you put your arm into a vise exerting such pressure the flesh would, I would bet money on, be ruptured at the cellular level.

EDIT: And if an individual cell is not damaged at 50 atmospheres, how much pressure does it take? I thought cells are quite fragile but maybe I am wrong. So, if experimentally we put human tissue in a test tube filled with water and increased the pressure, at what point would we see cell damage?

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Think of a baggie full of water. It would not be crushed at depth. The water is almost totally incompressible. If outside water presses on it, it presses back hard enough to prevent crushing and shrinking in volume.

By contrast, it has no rigidity. If crushed in a vice, pressure is applied non-uniformly. The pressure changes the shape of the water, and the baggie bursts easily.

The human body is mostly water. None of the liquid or solid parts are compressible, so they are fine at depth.

However, there are air spaces inside that are compressible. These would easily be crushed, reducing their volume. The work around is to carefully increase the air pressure in these spaces to match the surrounding water. If this is done, pressure in the air spaces balances, and no crushing takes place. It does mean moving up and down slowly, giving ears a chance to clear and lungs a chance to breathe in or out as needed.

Another reason for concern is that under pressure, $N_2$ dissolves in blood. That causes problems in itself. But another comes on the return to the surface. As the pressure drops, $N_2$ comes out of solution, forming bubbles. This increases the volume of a body that is not designed for it. This is called the Bends. It is painful and/or fatal. Pausing during rises allows the $N_2$ to be breathed out without coming out of solution.

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    $\begingroup$ but in our own daily experience, 750 lbs/square inch would be painful as flesh is pressed against bone -- why does this not happen in deep water? and the bag represents a single cell -- what if that bag was placed around a hard core, representing bone, so that the pressure would not be uniform? $\endgroup$ – releseabe Jun 20 '20 at 21:22
  • $\begingroup$ It depends how it is applied. It isn't the pressure itself. It is the distortion from applying a non-uniform force that stretch the body in ways that damage it. Also, if something hits the body and comes to a sudden stop, the momentary forces may be much larger that $750$ lb/sq inch. $\endgroup$ – mmesser314 Jun 20 '20 at 21:27
  • $\begingroup$ Another reason is that a vice is hard. Water at pressure conforms to the body's shape. The body must conform to a vice. This also leads to damage. $\endgroup$ – mmesser314 Jun 20 '20 at 21:28
  • $\begingroup$ so say instead of a vise we had like a blood pressure sleeve which one can experience discomfort from at much lower than 50 atmospheres. and with this soft "vise" we still have the bone which is not soft but hard. $\endgroup$ – releseabe Jun 20 '20 at 21:40
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    $\begingroup$ That still squeezes one place more than another. It forces blood out of vessels under it to other areas that are not being squeezed. This closes off blood vessels. It also pushes down under it, but not outside of it. This causes uncomfortable stretching. A bruise occurs when the stretching is enough to break small blood vessels. $\endgroup$ – mmesser314 Jun 20 '20 at 21:47
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You're identified the difference between a dilatational load (alternatively, an equitriaxial load, in this case called the hydrostatic load or pressure) and a deviatoric load (which is analogous to shear, although it can involve nonshear components in the stress tensor). A dilatational load consists only of normal stresses that are equal in every direction. The deviatoric load consists of the load minus the dilatational load; in practice, this means that we subtract one-third the trace of the stress tensor from the diagonal to effectively separate and remove the pressure.

Purely hydrostatic loads, meaning compressive dilatational loads, don't damage uniform materials; they simply squeeze the atoms together in a reversible way (barring exotic events such as black hole formation). The Titanic wreckage site, for example, yielded extremely fragile but intact glass objects; they were undamaged because they were pressurized equally on all sides.

Deviatoric loads do damage solids through shearing; for instance, submarine hulls at excessive depths fail from deviatoric loads in the wall material, which isn't uniformly pressurized because the submarine is hollow. The smallest deviatoric load (e.g., 10 Pa, or 0.0001 atm) can easily tear a biological cell apart.

Pressurizing your body to a pressure P is in itself harmless as long as your insides are also at pressure P and you're breathing a gas at pressure P.

I've written more about the different types of stress here, including the distinction between normal and shear stress states in 2D and dilational and deviatoric stress states in 3D. You can verify from the tables there that all condensed matter has large and fairly similar bulk moduli (within an order of magnitude for biological tissue). This is the material property that mediates shrinkage upon hydrostatic loading. In contrast, the shear modulus, which mediates shear from deviatoric loads, varies from ~1 GPa for bone to zero for blood. Avoid excessive deviatoric loads at all costs!

Does this satisfactorily address your question?

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The crushing you describe does not happen, because the diver is breathing air that is at the same pressure as his or her surroundings. This means the pressure inside their body is in equilibrium with the pressure outside their body, and so nothing gets crushed.

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    $\begingroup$ I tried to be clear: flesh is being pressed against bone. It seems to me that if my arm was being mechanically compressed against bone in a vise, the flesh would be damaged. I am wrong about a vise at 50 atmospheres of pressure damaging flesh against bone? $\endgroup$ – releseabe Jun 20 '20 at 20:38
  • $\begingroup$ I am not worried about flesh and bones, but what happens to blood pressure. I mean if you meadure blood pressure, your flesh is gettibng compressed moderately and blood flow stops. So what is happening in diving? $\endgroup$ – lalala Jun 20 '20 at 21:16
  • $\begingroup$ @lalala, your heart provides a difference in pressure to push blood through your body ... it does not have an actual "pressure setpoint". This means that as the ambient pressure goes up both inside and outside your body, your heart does the same amount of work as it did at normal ambient pressures. $\endgroup$ – David White Jun 20 '20 at 23:54

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