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From: 07077090@sdcc6.UUCP (07077090)
Newsgroups: net.med
Subject: Re: blood pressure
Message-ID: <1779@sdcc6.UUCP>
Date: Wed, 10-Oct-84 02:28:31 EDT
Article-I.D.: sdcc6.1779
Posted: Wed Oct 10 02:28:31 1984
Date-Received: Fri, 12-Oct-84 05:44:18 EDT
References: <3778@tektronix.UUCP>, <127@logico.UUCP>
Organization: U.C. San Diego, Computer Center
Lines: 27


As you imply, blood pressure is a complex
phenomenon. In the most basic terms, it
is determined by cardiac output and vascular
impedance which has both resistive and  
capacitance components. The pulse pressure
(systolic-diastolic) increases with increased
stroke volume and decreased vessel compliance
(capacitance) while mean arterial pressure
depends on cardiac output and vascular resistance.
BP of 120/100 (vs 120/90) implies a higher resistance
(or higher output) and more work for the heart.
As far as we know, there is no lower limit for the
beneficial effects of low blood pressure, until
low pressure itself becomes symptomatic (e.g. with
lightheadedness or fainting). There is a complex
feedback system as you suggest, involving hormones
etc, which is one reason BP is hard to control. 
For example, one cause of high BP could be thought of
as the body (kidneys?) "thinking" there is low
intravascular blood volume, and thus "conserving"
as much water in the circulation as possible.

         Mikc Blyth
         ..sdcc3!sdcc6!07077090
         UCSD Medical School