ABSTRACT
By its own weight made steadfast and immovable (William Congreve, 16701729)
I. Introduction
The possibility that gravity affected the distribution of pulmonary blood flow was recognized many years ago as a factor in the apical distribution of pulmonary tuberculosis and one reason for its treatment by supine bed rest (1). Gravity, the force that gives a body its normal weight, will cause an unsupported body to fall at an acceleration of 9.81 m seC', the gravitational constant, with which all land dwellers are so familiar that it is taken for granted. Increased levels of acceleration are less familiar, but are inevitably associated with change in speed (linear acceleration/deceleration), or change in direction when traveling at a uniform speed (centrifugal acceleration). Linear accelerations tend to be of modest magnitude and of short duration due to the great speeds generated, the exception being the launch forces in space flight, but considerable centrifugal forces are encountered in aerobatics, or even in car racing and fairgrounds.
