Re: Why does diastolic pressure decrease when heavy exercise is performed?

Shannon, sorry this reply took so long...

When we talk about blood pressure from a physical point of view we use the 
equation F=P/R, where F is Flow rate, P is pressure, and R is resistance. 
Flow rate can be changed by modulating cardiac output (how much blood is 
pumped by the heart per minute; resistance can be changed by two things: 
changing the viscosity of the blood and changing the diameter of the blood 
vessels the blood flows through. The narrower the blood vessel, the more 
resistance.

As you exercise, your cardiac output increases by virtue of an increased 
heart rate and the Frank-Starling mechanism (where, when there is more flow 
into the heart, the heart pumps more strongly in order to get all of the 
blood out). This greatly increases flow. Since flow is increasing, you 
would expect pressure to increase as well (F=P/R).

This flow increase causes changes in the blood vessels. In order to relieve 
excessive blood pressure without sacrificing flow, the smooth muscle in the 
blood vessels relaxes, causing the expansion of the blood vessels. This is 
called vasodilation. The vasodilation is mediated by the autonomic nervous 
system. It is important to note that not all vessels become dilated. When 
you exercise, you typically dilate blood vessels leading to those muscles 
that are being worked as well as those that supply the brain and heart. The 
vessels leading to the digestive system, by comparison, tend to constrict 
to help shunt blood towards the places where it's most needed.

In any case, the reason that diastolic blood pressure falls during exercise 
is that the vasodilation seems proportionate to the systolic pressure 
(which is much higher than the diastolic) because it represents the 
greatest danger of stroke, aneurysm, or hemorrhage. The diastolic pressure 
is the lowest pressure, and if the vessel that you are measuring pressure 
on is significantly dilated, your diastolic blood pressure will drop 
despite the increase in cardiac output.

Hope this helps. Feel free to ask if you have any more questions.
:)
Noah Raizman