Charlie99
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Water is nearly incompressible, so the volume of water changes very little as the pressure changes. This is also true of most of the body. What does compress is any gas filled areas of the body .... lungs, ears, etc.
Regarding blood pressure, I think you are confusing the absolute pressure of the blood with the differential pressure between different parts of the circulatory system.
Blood pressure readings are normally expressed in millimeters of mercury. 760mm of mercury is 1 atm, so a normal blood pressure of 120mmHg is about 0.16atm. On the surface, your arterial blood pressure is about 0.16atm (120mm Hg) above the ambient 1ata pressure. When you are down at 100' or 4ata, your arterial blood pressure will still be about 0.16ata relative to the ambient. The fact that your absolute blood pressure when at 100' is about 4.16ata doesn't matter.
Regarding blood pressure, I think you are confusing the absolute pressure of the blood with the differential pressure between different parts of the circulatory system.
Blood pressure readings are normally expressed in millimeters of mercury. 760mm of mercury is 1 atm, so a normal blood pressure of 120mmHg is about 0.16atm. On the surface, your arterial blood pressure is about 0.16atm (120mm Hg) above the ambient 1ata pressure. When you are down at 100' or 4ata, your arterial blood pressure will still be about 0.16ata relative to the ambient. The fact that your absolute blood pressure when at 100' is about 4.16ata doesn't matter.