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Practical
Physiology mini-practicals
Prof. Tony Gardner-Medwin
[ further info: www.ucl.ac.uk/lapt/med ]
Aims
To give direct experience of simple physiological concepts and measurement
techniques.
To encourage discussion of the basis of everyday physiological experiences.
To help identify questions that puzzle you, for discussion in your portfolio notes.
Lab practicalities
Work in pairs. Discuss things fully. Don't let someone else answer questions for you.
Ask questions!
There is only a short time (20 min) for each topic. Spend 5 – 10 min of that writing
notes.
You don't have to volunteer as a subject, or even give reasons for not volunteering.
But being a subject is the best way to benefit from the practicals. If you have worries
about this, please discuss them with a demonstrator or your personal tutor.
If you think you have a relevant medical condition, discuss this (confidentially) with
a teacher before being a volunteer. Don’t take risks with your own or anybody else’s
safety. That said, all the practicals are extremely unlikely to be hazardous in any
foreseeable circumstances.
After the sessions
There are LABVIEW computer simulations that go with the practicals, available during
the class and afterwards (accessible within LAPT). You can download these, using the
usual LAPT facilities (see www.ucl.ac.uk/lapt/med ) for use at home. There are also
model answers to the questions in the notes, available on the web site. If you have
technical or academic questions, email to cusplap@ucl.ac.uk .
Assessment
Many of these topics will come up in assessments. If you have trouble with them later
in the year, it's really important to sort this. Ask for help. If necessary, we can arrange
special tutorials or help sessions.
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Measurement of the arterial pulse with a wrist monitor
Objectives
Use the wrist monitor to get 3 measurements (Systolic and Diastolic pressure and Heart Rate),
and assess how important it is to have the monitor at heart level. Wrist monitors are convenient,
though in some people not as accurate as measurements made on the upper arm.
Risk assessment
This is a standard, common and safe procedure. Prolonged stasis of blood (>10 min) can cause
thrombosis if there are other risk factors. Don’t measure > 5 times on one subject, or at intervals
< 3 min.
Make the measurements on yourself, holding still, with the monitor at heart level. Note the
diagram showing correct mounting of the monitor on the wrist. Secure the velcro firmly……….
Write definitions for each measurement, and label them on a sketch graph of arterial pressure
vs. time ….
Give a definition and an illustrative example to explain the meaning of the pressure unit
'mmHg'….
Suppose that you had let the wrist hang down (0.6m lower), instead of being at heart level.
Calculate what you would expect the systolic pressure to be at this lower level. (NB density of
Hg is 13.7 timeas that of water) …
Make measurements both at heart level and with the wrist dependent (without removing the
monitor in between) and compare the results with what you expect…….
Convert your systolic pressure to kilo-Pascals (kPa) and the heart rate to an inter-pulse interval.
(NB 1 mmHg = 133Pa).
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How much error in monitor height would lead to a 10 mmHg error in a clinical measurement?
What components do you think the wrist monitor contains (apart from the electronic circuitry)?
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2.
Measurement of blood flow in the arm
Objectives
You use plethysmography to measure blood flow by changes of “fullness” of an organ such as
the arm. Insert the forearm into the apparatus, so that any changes of its volume are registered
by displacements of water from the chamber. Make sure the arm is supported at both ends and
that the subject is seated in a comfortable and stable way so that limb movements do not occur.
Risk assessment
Prolonged inflation of a cuff can lead to discomfort due to swelling of veins and (if the subject
also has a disorder of blood clotting) to thrombosis. In this experiment, you need not inflate for
more than 1 min, which is quite safe. Local discomfort (squeezing) in the upper arm is no cause
for concern at all.
Explain how sudden inflation of a cuff on the upper arm may alter the volume of the forearm.
Explain how pressures of
a.
50 mmHg,
b.
100 mmHg
c.
200 mmHg
would have different effects. Which would give the most discomfort, and why? [NB the right
answer is not the obvious one!]
Measure the rate of increase of forearm volume under condition (a), increasing the cuff pressure
from zero as fast as possible. Measure the time taken for a 1ml volume change. Deflate the cuff
after 30s.
Sketch a graph of the time-course of the changes of volume in this experiment, indicating what
aspect of the graph is related to forearm blood flow and what the measurement is that you have
made.
Write down one or (preferably) two ideas about why the volume of the forearm does not
continue rising at the same rate indefinitely in this experiment.
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Explain how and why you might expect the results to be different
3.
a.
at a different temperature,
b.
after exercising the arm
c.
after the cuff had been inflated for a few minutes to 200mmHg. [NB this involves
physiology you haven’t yet learned, but you can discuss and probably figure out the
answers.]
Response to stimulation of peripheral nerve
Objectives
The ulnar nerve is stimulated at the wrist with a brief (0.1ms) electric pulse (on the side distant
from the thumb). This causes action potentials in both sensory and motor nerves. You study the
effects on the adductor policis muscle that pulls the thumb toward the little finger.
Risk assessment
Nerve stimulation is a common clinical procedure. It is completely safe with apparatus
designed to clinical standards, but it feels weird. You should experience it for yourself.
Although small shocks are uncomfortable rather than painful, large shocks can be painful.
Remember that the pain results not from damage to your body, but from action potentials set
off in the nerve fibres that normally signal damage. Since the experiment involves low
resistance electrodes attached to your arm, note that a shock from the mains across your heart
could be lethal. Under no circumstances handle mains equipment or lark around with electrodes
attached.
Describe the sensations of stimulating the ulnar nerve at the wrist with single electrical pulses,
gradually increasing the voltage from zero until twitches of muscles can be observed.
List some of the different kinds of afferent and efferent nerve fibres that such stimuli would
afffect.
Measure the duration of the electrical activity evoked in the adductor policis muscle by the
stimuli.
Measure the duration of mechanical force developed by the adductor policis muscle with these
stimuli.
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Sketch a graph of the two effects on a single time scale and label it with what is happening at
different stages.
Musicians can produce brief forces shorter than a muscle twitch. How do you think they can do
this? They can also produce forces much bigger than the ones you have measured. How can
they do that?
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Partial pressures in a gas mixture
The (non-aqueous) gases in the atmosphere are normally in proportions:
O2 21%
CO2 0.04%
N2(+inert gases) 79%
These proportions are the same whether you measure them in terms of the numbers of
molecules, number of mols, or the volumes of extracted pure gas measured at equal pressures.
They vary little with place or height above sea level (though the very small %CO 2 can be
significantly higher in a stuffy room).
Given the total atmospheric pressure is 760 mmHg (101 kPa) at sea level and about half this
(380mmHg or 50 kPa) at the highest habitable altitude (5500m), calculate the partial pressure of
O2 (PO2) in dry air in both places. Partial pressure is the part of the total pressure that is due to
one gas, and is given by
Partial pressure = total pressure  molar fraction of the one gas in the mixture.
PO2 at sea level (dry fresh air) =
PO2 at 5500m (dry fresh air) =
Suppose you take fresh air into the mouth and bronchi (normal atmospheric pressure, but at
37oC and with moist walls). It becomes saturated with water vapour, with vapour pressure = 47
mmHg (6.3 kPa) whatever the ambient pressure.
Calculate
PO2 in the fresh moist air at 37oC at sea level =
PO2 in the fresh moist air at 37oC at 5500m =
NB PCO2 will be about 500 times smaller. It would in fact make no difference physiologically if
PCO2 were zero in fresh air (except for plants, relying on photosynthesis!).
Risk assessment.
The only risk in this practical is infection via unwashed mouthpieces. Clean tubes are provided.
Take responsibility to use a clean tube or wash it between users.
With demonstrator help, collect gas mixture from the alveoli, deep in the lungs. Explain how to
do this:-
This gas mixture is of course different from fresh air. Measure the % CO2 in it, using the meter
provided, and noting that the apparatus dries the gas before the measurement is made. Note
down:% CO2 in the dried alveolar air =
PCO2 in the dried alveolar air at 760 mmHg =
Calculate:PCO2 in the alveolar air when it was moist at 37oC (in the lungs) at 760 mmHg =
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Hint:- Draw a box (representing total pressure) divided into compartments for partial pressures.
Write down the total pressure and the pressure in the H2O compartment. What is the pressure
due to everything other than H2O? What fraction of these other gases is CO2? What is therefore
the pressure due to CO2?
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With a healthy circulation, oxygenated blood coming from the lungs is in equilibrium with the
gas mixture in the alveoli, with the same partial pressures.
What is the PCO2 in the arterial blood in your subject =
What assumptions (if any) do you make in coming to this conclusion?
If a clinical test report says “PCO2 = 40 mmHg” about a blood sample, mark these statements
true or false:a.
The amount of CO2 per ml in the sample equals that in a gas mixture with PCO2 =
40 mm Hg.
b.
A gas mixture with PCO2 = 40 mmhg would be in equilibrium with the sample.
c.
The blood would neither gain nor lose CO2 if you shook it with a gas mixture with
PCO2= 40 mm Hg
d.
The amount of CO2 per ml in the blood is the same as in pure water with PCO2 = 40
mm Hg
Make sure you can tick these 3 boxes, and consult a demonstrator if you can’t!
O
I understand what the partial pressure of a gas means, in a mixture of gases.
O
I understand how it changes if the mixture is dried or equilibrated with water.
O
I understand what it means to talk about the partial pressure of a gas in a solution.
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