. Biophysics: concepts and mechanisms. Biophysics. 32 SOME PHYSICAL FORCES EXEMPLIFIED IN MAN. f .. pt +p2 + p3+p4 (pressure = force per unit area) Figure 2-2. Pressure and Force. It has been found that 15 psi can support a column of mercury about 30 in. (76 cm or 760 mm) high. That is, if a glass tube of any diameter (the larger the cross-sectional area the larger the force, since the pressure is 15 psi) is mounted vertically in a pool of mercury, and if the air in the tube above the mercury is exhausted substantially to zero pressure, the air pres- sure on the outside of the pool will force
. Biophysics: concepts and mechanisms. Biophysics. 32 SOME PHYSICAL FORCES EXEMPLIFIED IN MAN. f .. pt +p2 + p3+p4 (pressure = force per unit area) Figure 2-2. Pressure and Force. It has been found that 15 psi can support a column of mercury about 30 in. (76 cm or 760 mm) high. That is, if a glass tube of any diameter (the larger the cross-sectional area the larger the force, since the pressure is 15 psi) is mounted vertically in a pool of mercury, and if the air in the tube above the mercury is exhausted substantially to zero pressure, the air pres- sure on the outside of the pool will force the mercury up the tube to a height of about 30 in. above the level in the pool. If the supporting pressure (dif- ference between air pressure on the mercury in the pool, open to air, and on the mercury in the column) is less than 15 psi, the height of the column is correspondingly less. Atmospheric pressure varies with the weather, from about 29 to 31 in. of mercury between very stormy, low-pressure weather and fine, high-pressure weather. Living systems operate under this continuous pressure of 15 psi, but do not collapse for two reasons. Firstly tissue is about 80 per cent water by weight, and water is nearly incompressible. Secondly, air can pass fairly freely into those interior parts which are not solid or liquid, and the internal gas pressure is about the same as the external. A large reduction in pressure (, 12 psi) over a small area of the skin surface can be tolerated for some minutes without ill effects. On the other hand, pressure-increases up to 327 psi at a new record depth in water of 726 ft were recently tolerated. The cur- rent skin-diving record is 378 ft, where the total pressure, P, is of the order of 12atm! The total pressure (psi) is given by: P = Patm + D where Palm = psi, D is the depth in feet, and is the weight, in pounds, of a column of water 1 in area and 1 ft high. At the record skin- diving depth, the total force on the body
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