Why does hydrostatic pressure decrease when a blood vessel is constricted?

[scilearner]

If a blood vessel consticts, the resistance increases and kinetic energy decreases, if kinetic energy decreases hyrdrostatic pressure must go up but why does other occur?

 

"The average capillary hydrostatic pressure is determined by arterial and venous pressures (PA and PV), and by the ratio of post-to-precapillary resistances (RV/RA). An increase in either arterial or venous pressure will increase capillary pressure; however, a given change in PA is only about one-fifth as effective in changing PC as the same absolute change in PV. Because venous resistance is relatively low, changes in PV are readily transmitted back to the capillary, and conversely, because arterial resistance is relatively high, changes in PA are poorly transmitted downstream to the capillary. Therefore, PC is much more influenced by changes in PV than by changes in PA. Furthermore, PC is increased by precapillary vasodilation (particularly by arteriolar dilation); precapillary vasoconstriction decreases PC. Venous constriction increases PC, whereas venous dilation decreases PC."

 

If anyone can explain this I'll be very greatful.

Edited January 4, 2010 by scilearner