Lab Activity 3: Blood Pressure (Manual Method)
What is blood pressure?
Blood pressure is the pressure exerted by the blood on the walls of the blood vessels. Unless
indicated otherwise, blood pressure refers to systemic arterial blood pressure, i.e., the pressure in
the large arteries delivering blood to body parts other than the lungs, such as the brachial artery
(in the arm). The pressure of the blood in other vessels is lower than the arterial pressure. Blood
pressure values are universally stated in millimeters of mercury (mm Hg). The systolic pressure is
defined as the peak pressure in the arteries during the cardiac cycle; the diastolic pressure is the
lowest pressure (at the resting phase of the cardiac cycle).
Typical values for a resting, healthy adult human are approximately 120 mm Hg systolic and
80 mm Hg diastolic (written as 120/80 mm Hg), with large individual variations. These measures
of blood pressure are not static, but undergo natural variations from one heartbeat to another or
throughout the day (in a circadian rhythm); they also change in response to stress, nutritional
factors, drugs, or disease.
Figure 1. Sphygmomanometer - used for measuring blood pressure.
The non-invasive auscultatory (from the Latin for listening) measurements are simple and quick.
These methods actually measure the pressure of an inflated cuff at the points where it just
occludes blood flow, and where it just permits unrestricted flow. These are the methods more
commonly used for routine examinations and monitoring.
Basic digital blood pressure monitors are relatively inexpensive, making it easy for patients to
monitor their own blood pressure. Their accuracy can vary greatly, as most have not been
validated for accuracy. Upper arm, rather than wrist, monitors usually give readings closer to
auscultatory. Some meters are automatic, with pumps to inflate the cuff without squeezing a bulb.
The auscultatory method uses a stethoscope and a blood pressure cuff. This comprises an
inflatable cuff placed around the upper arm at roughly the same vertical height as the heart,
attached to an aneroid manometer. The cuff of an appropriate size is inflated manually by
repeatedly squeezing a rubber bulb until the artery is completely occluded. It is very important
that the correct size cuff is selected for the patient, based on the circumference of the patient's
Too small a cuff yields too high a pressure. Too large a cuff yields too low a pressure. Listening
with the stethoscope to the brachial artery at the elbow, the examiner slowly releases the
pressure in the cuff. When blood just starts to flow in the artery, a "whooshing" or pounding sound
is heard. The pressure at which this sound is first heard is the systolic blood pressure. The cuff
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pressure is further released until no sound can be heard, at the diastolic blood pressure.
Sometimes, the pressure is palpated (felt by hand) to get an estimate before auscultation.
Normal values of blood pressure:
Normal ranges for blood pressure in adult humans are:
Systolic between 90 and 135 mm Hg
Diastolic between 50 and 90 mm Hg
In veterinary medicine, blood pressure values for dogs and cats are:
Systolic between 150 and 150 mm Hg
Diastolic between 50 and 110 mm Hg
Effects of high blood pressure:
Blood pressure exceeding normal values is called hypertension. It itself is only rarely an acute
problem- called a hypertensive crisis. But because of its long-term indirect effects (and also as an
indicator of other problems) it is a serious worry to physicians diagnosing it.
All level of blood pressure puts mechanical stress on the arterial walls. Higher pressures increase
heart workload and progression of unhealthy tissue growth that develops within the walls of
arteries. The higher the pressure, the more stress that is present, the more the heart muscle
tends to thicken, enlarge and become weaker over time.
Persistent hypertension is one of the risk factors for strokes, heart attacks, heart failure,
aneurysms, and is the second leading cause of kidney failure after diabetes.
Effects of low blood pressure:
Blood pressure that is too low is known as hypotension. Low blood pressure may be a sign of
severe disease and requires urgent medical attention.
When blood pressure and blood flow decrease beyond a certain point, the perfusion of the brain
becomes critically decreased (i.e., the blood supply is not sufficient), causing lightheadedness,
dizziness, weakness and fainting.
However, people who function well while maintaining low blood pressures have lower rates of
cardiovascular disease events than people with normal blood pressures.
Factors influencing blood pressure:
The physics of the circulatory system, as of any fluid system, are very complex. That said, there
are many physical factors that influence blood pressure. Each of these may in turn be influenced
by physiological factors, such as diet, exercise, disease, drugs, and alcohol etc.
Some physical factors are:
• Rate of pumping. In the circulatory system, this rate is called heart rate, the rate at which
blood (the fluid) is pumped by the heart. The higher the heart rate, the higher (potentially,
assuming no change in stroke volume) the blood pressure.
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• Volume of fluid. In the case of the circulatory system, this is blood volume, the amount of
blood present in the body. The more blood present in the body, the higher the rate of
blood return to the heart and the resulting cardiac output. There is some relationship
between dietary salt intake and increased blood volume, potentially resulting in higher
blood pressure, though this varies with the individual and is highly dependent on
autonomic nervous system response.
• In cardiac physiology, the rate and volume of flow are accounted for in a combined
fashion by cardiac output, which is the heart rate, the rate of contraction, multiplied by the
stroke volume, the amount of blood pumped out from the heart with each contraction. It
represents the efficiency with which the heart circulates blood throughout the body.
• Resistance. In the circulatory system, this is the resistance of the blood vessels. The
higher the resistance, the higher the blood pressure. Resistance is related to size (The
larger the blood vessel, the lower the resistance), as well as the smoothness of the blood
vessel walls. Smoothness is reduced by the buildup of fatty deposits on the arterial walls.
• Viscosity or thickness of the fluid. If the blood gets thicker, the result is an increase in
blood pressure. Certain medical conditions can change the viscosity of the blood. For
instance, low red blood cell concentration, anemia, reduces viscosity, whereas increased
red blood cell concentration increases viscosity. Viscosity also increases with blood sugar
concentration—visualize pumping syrup. (It was thought that aspirin and other drugs
decreased the viscosity of blood, but this has been found not to be so "blood thinners"
reduce the tendency of the blood to clot, not viscosity.)
In practice, each individual's autonomic nervous system responds to and regulates all these
interacting factors so that, although the above issues are important, the actual blood pressure
response of a given individual varies widely because of both split-second and slow-moving
responses of the nervous system and end organs. These responses are very effective in
changing the variables and resulting blood pressure from moment to moment.
"Blood pressure." Wikipedia, The Free Encyclopedia. 28 Sep 2006, 19:06 UTC. Wikimedia
Foundation, Inc. 3 Oct 2006
Pre-Lab Activities and Questions:
-Go to http://www.blaufuss.org/ and use the heart sounds tutorial. Just focus on the normal heart
-“Lub” represents what occurance? “Dub” represents what occurance?
In this experiment you will determine blood pressure using a blood pressure cuff.
Blood Pressure Cuff
1. Deflate the bladder of the cuff and place it around the upper arm so it fits snugly, but not too
tightly. If you're right handed, you should hold the bulb/pump in your left hand to inflate the cuff.
Hold it in the palm so your fingers can easily reach the valve at the top to open/close the outlet to
the air bladder wrapped around the person's arm.
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2. Put the head of the stethoscope just under the edge of the cuff, a little above the crease of the
person's elbow. Hold it there firmly with the thumb, or with a few fingers of your right hand. Put
the ear pieces of the stethoscope in your ears.
3. Inflate the cuff with brisk squeezes of the bulb. Watch the pressure gage as you do it. For most
kids, you shouldn't need to go over 150 (the markings indicate "pressure" in mm Hg or mercury).
4. At 150, slightly open the valve on the air pump (held in your left hand, as above). This part
takes practice. It's important that you don't let the air out too suddenly. Likewise, your friend will
be quite irritated with you if you let it out too slowly.
5. Now, pay attention *very carefully* to what you hear through the stethoscope as the needle on
the pressure gage falls. You will be listening for a slight "blrrpp" or a something that sounds like
"prrpshh." The first time you hear this sound, note what the reading was on the pressure gage.
This value represents the systolic blood pressure (described above).
6. The sounds should continue and become louder in intensity. Note the pressure reading when
you hear the sound for the last time. This value represents the diastolic blood pressure.
7. Afterwards, open the air valve completely to release any remaining pressure.
You and your partner should each take their blood pressure twice.
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