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Ph
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Developer Notes
I  Equations
Blood pressure is normally measured in mmHg. Introduce this unit of pressure?
More examples/ activities other than physiology?
This needs A LOT of work!
Goals
Understand pressure increases with depth within a fluid
Discuss blood pressure
Concepts & Skills Introduced
Area
Physics
Physiology
Concept
Pressure within a fluid
Ph
Blood pressure
Time Required
Warm-up Questions
Why do healthcare professionals take your blood pressure at your upper arm (rather than
your feet)?
Aside from drowning, why is scuba diving considered such a hazardous activity?
Presentation
POE/Demo- Multi-hole cup. Punch holes at different levels toward the bottom of a cup
(plastic, paper, Styrofoam). Fill with water.
Why do the streams of water have different paths?
Disc- What happens to your blood pressure in your legs and feet compared to your heart?
What happens as you go deeper under water?
Pressure increases with depth. We know this from swimming and diving.
Pressure in a liquid of uniform density varies with depth. We can derive an equation to
explain this!!!
P = F/A = mg/A
D = m/V, so m = DV, substitute for m in above eq.
P = DVg/A
V = Ah, substitute for V in above eq.
P = DAhg/A
P = Dhg [units: (kg/m3) (m) (m/s2) = (kg) (m/s2)/ (m2) = N/m2 = Pa]
If D is constant and g is constant, pressure increases with increasing depth (height below
surface)!!! (P = Dgh)
Difference in P depends on difference in h.
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Ph
Deeper water has higher pressure, which means more force, which means acceleration,
which means fluids are self-leveling - "Water seeks its own level."
We looked at how the heart exerts a pressure on the blood. Now let’s look at the pressure
within the blood. Since the pressure in a fluid varies with depth, blood in the lower body
has a higher pressure than in the upper body. Blood pressure is usually measured at the
same level as the heart, in order to measure the pressure of the blood leaving the heart.
Many other factors influence blood pressure including friction encountered in blood
vessels and dilation or constriction of arteries. Postural effects on blood pressure result
from the Ph relationship (i.e. feeling light-headed when you stand up too quickly).
Act/Demo- (Not sure if this works- need to try it)
Try to weight a balloon for neutral buoyancy in a tub of water. If it starts sinking, it will
shrink and sink faster. If it starts rising, it will expand and rise faster (I think). Do bubbles
expand as they rise? I think so. Can you see the change?
Another unit for pressure is mmHg (millimeters of mercury). It refers to how far the
pressure would raise a column of mercury. This is the unit of pressure commonly used to
measure blood pressure. 1 mmHg = 133 Pa
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Background/History
We’ve seen what happens when pressure is applied to a liquid (Pascal’s principle). But
what about inside the liquid? What happens when you’re swimming and you swim
deeper and deeper? You have more water above you at a deeper level, what does this do?
(This intro. sucks!!!!!!!!!)
We’ve looked at how the heart applies pressure to the blood in order to move it around.
But what about the pressure within the blood itself? The term blood pressure refers to the
pressure that blood exerts on the walls of the blood vessels. Blood pressure can be an
indicator of health. Do you think that your blood pressure is the same throughout your
body? Why or why not? If you think blood pressure is different in different places in
your body, where would it be highest?
Problem
Is pressure the same within a liquid? (this question sucks!!!!!)
Activity
Act/Demo- (Not sure if this works- need to try it)
Try to weight a balloon for neutral buoyancy in a tub of water. If it starts sinking, it will
shrink and sink faster. If it starts rising, it will expand and rise faster. Or, try to observe
bubbles???
Act- Blood pressure.
Work in pairs
Measure your partner’s blood pressure:
 at the upper arm
 at the lower leg
 with the arm raised over the head
 ?????
Summary
Exercises
1. Calculate the difference in blood pressure between an artery in a foot and in the
main artery leaving the heart (called the aorta). The foot is 1.35 m below the
heart and the density of blood is 1.05 x 103 kg/m3.
Convert your answer to mmHg. The average pressure in the aorta is 100 mmHg.
What is total blood pressure in the foot artery?
2. What is the difference in blood pressure between the heart and the head when a
person is lying flat? What is the difference in blood pressure when the person is
sitting up straight, if the head is .45 m above the heart? If the average pressure at
the heart is 100 mmHg, what is the pressure in the head (in mmHg)?
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Ph
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3.
4.
5.
6.
7.
8.
Normally your body makes adjustments to compensate for this difference in
pressure. Explain what happens if you sit up too fast.
In the absence of a gravitational force, what happens to the pressure within a
liquid? Explain.
People who have professions that require them to stand for long periods of time
often develop problems with swollen veins (called varicose veins). Why?
Two water pipes, of the same diameter, go from the top of a building to the
bottom. One is zig-zag while the other is straight. When filled, which one will
have higher water pressure at the bottom?
Mercury has a density that is 13.6 times greater than water. How does the
pressure 0.5 m below the surface of a sample of mercury compare to the pressure
0.5 m below the surface of water?
What is the force on the bottom of a fish tank that is .30 m wide and .50 m long,
and has a water level of .20 m. (Density of water = 1.0 x 103 kg/m3). (Ignore
pressure of the atmosphere).
How high above the arm does a transfusion bag of blood need to be in order to
enter the vein with a pressure of 1.33 x 104 Pa?
Challenge
1. You have a tube that is shaped like a U (called a U-tube in chemistry) that has
some water in it. You pour some oil into one side of the tube that makes a
column 13 cm high (the oil and water don’t mix). If the density of the oil is 650
kg/m3, what pressure does the oil put on the water?
How high does the water rise on the other side?
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