32 



SOME PHYSICAL FORCES EXEMPLIFIED IN MAN 



f .. p t + p 2 + p 3+ p 4 

 (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 

 (e.g., 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 = P atm + 0.43 D 



where P alm = 14.8 psi, D is the depth in feet, and 0.43 is the weight, in 

 pounds, of a column of water 1 in. 2 in area and 1 ft high. At the record skin- 

 diving depth, the total force on the body (20 sq ft) is about 270 tons! 



The troubles start when pressure changes occur rapidly, such as during 

 collisions or impact. Consider the skin diver equilibrated 200 ft below the 



