Physiology Lab (Phsl 3063, Phsl 3701)

Fall 2012

UNIVERSITY OF MINNESOTA

 

 

 

 

 

 

 

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Syllabus

Schedule

Lesson Summaries

Review Material

Guidelines


Atlas of Human Cardiac Anatomy

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BME Department

Physiology Department

Institute of Technology

 

 

 

 

 

 

 

Lesson 16 - Blood Pressure Tutorial

In Lesson 16 you learn how to take someone's blood pressure using a sphygmomanometer or blood pressure cuff and a stethoscope. A blood pressure reading consists of two numbers. The first is the systolic pressure, which tells you how hard your heart is pumping. Since systole is a measure of contraction and the cardiac cycle spends the least amount of time in contraction, systole represents only a small portion of your overall blood pressure or Mean Arterial Pressure (MAP). The second number is the diastolic pressure, which is a measure of arterial relaxation. This is the part of the cardiac cycle where your ventricles are relaxing and filling with blood. So how do you figure these numbers out with a cuff and stethoscope?

 cuff
Image taken from www.merck.com/media/mmhe2/figures/fg022_2.gif

Here you can see that when the cuff is inflated, it "squeezes" the brachial artery. When the cuff is inflated enough to create a pressure greater than systolic, then you will hear no pulse with a stethoscope. As you slowly release the pressure, the flow of blood keeps pushing to get through the artery until the pressure falls to just below systolic. Now your blood pressure is greater than the pressure created by the cuff and flow is restored; however there is resistance to flow due to the smaller diameter of the artery. This smaller diameter causes turbulence in the normally laminar (smooth) flow and this turbulence is what you hear in the stethoscope as "blowing or swishing" sounds. When the diameter of the artery is large enough to allow laminar flow, the resistance is gone and therefore this marks the second Korotkoff sound and the approximate diastolic pressure. If you want to visualize this mathematically, look at the following equation:

FLOW = PRESSURE/RESISTANCE

Resistance = (Viscosity*Length)/Diameter4

So for resistance, you can see that diameter plays the biggest role because it is to the power of 4. Also, resistance is inversely related to diameter; meaning that the smaller the diameter, the greater the resistance.

 

 

 

 

 

 

This page was last updated on August 11, 2008