Download The Heart as a Pump

Instructor Version
The Heart as a Pump
I. Introduction
Learning about the cardiovascular system is central to understanding how a number of physiological
systems work because it is intimately involved in their functioning. Furthermore, cardiovascular disease is
a leading health issue in the United States. A solid understanding of cardiac mechanics underlies the
ability to learn cardiac regulation, normal heart function and cardiac pathologies. Traditionally, cardiac
mechanics and regulation are taught in lectures, but the advent of multimedia has resulted in the addition
of computerized animations that deal more dynamically with some aspects of cardiac function. The
ADAM Interactive Physiology® series is an example of this latter approach. The traditional approach
frequently results in students memorizing formulas like, CO = HR X SV. After traditional pedagogy, a
probing assessment of students’ understanding about the factors determining SV often reveals little more
than superficial learning, and no ability to accurately predict cardiac responses to normal and abnormal
conditions. In some ways the technology-based approach appears to offer better learning, but it is most
often implemented as an animation of the cardiac cycle, which makes the cycle even more difficult to
understand than the already complicated one-page diagram included in almost every A&P and physiology
text. Both the traditional and multimedia approaches assume that students have some direct experience
with fluid flows, pressure gradients and force development.
The model used in this activity (seen in Fig. 1.1) is made entirely of off-the-shelf parts. It consists of a
glass 50cc medical-grade syringe, ‘T’ luerlock fitting, two polypropylene check valves, a glass beaker, a
modified graduate cylinder, a ½” PVC valve and assorted clear tubings. This set up has the important
cardiac characteristics of filling by positive pressure, one-way flow produced by two check valves, preload and after-load. We have found this to be an effective tool for helping students master concepts that
were intractable using other methods. Prior to the explorations with the models it would be a rare student
who could predict the difficulty of raising cardiac output by raising heart rate alone. With the model,
students directly experience the importance of filling rate in determining output. Students followup the
guided, hands-on exploration, with examinations of a preserved sheep heart and of plastic models of
human hearts, relating the function they have just learned to the features of a real hearts. This teaching
order places the functional properties first, providing a conceptual framework and making structural
details meaningful.
The learning activities in this exercise have evolved, in response to the experiences of our students
and instructors, to include a set of standard components: 1) a pre-laboratory investigation to prepare
students for the lab session; 2) a brief in-lab exploration that creates a "need to know" about the topic of
the lab; 3) a series of guided discovery exercises during which the students work collaboratively in groups
of three or four to identify and work with the major concepts; 4) an open-ended exploration of a problem
uses the knowledge gained in the guided portion of the laboratory and 5) a problem set that challenges
students to apply the concepts during or after the lab period.
II. Learning Benchmarks
These benchmarks are written so that students and instructors are clear about what is to be learned
and how learning is to be demonstrated. The statements are operational, specifying some action
students will be able to take when they have succeeded with the material. They avoid words like
understand which is ambiguous. When assessment items are written, they should be derived from
these benchmarks.
1. Define the following terminology of cardiac pumping in reference to both the heart and the physical
heart-pump model: end diastolic volume (EDV), end systolic volume (ESV), stroke volume (SV),
ejection fraction (EF), heart rate (HR), ejection, filling, systole, diastole, cardiac output (CO), cycle
2. Intervals and Rates: a) define the terms interval and rate; b) show how rate is inversely related to the
Copyright 2000 by the American Physiological Society. Permission is granted to reproduce with proper citation for
classroom or workshop use only. For all other purposes, contact the American Physiological Society Education
Office. ( [email protected])
1
interval; c) be able to calculate either the rate or the interval from the other
3. Predict the CO based on variation of HR, SV, EDV, ESV, EF
4. Describe the two mechanisms by which cardiac output may be increased or decreased
Explain how diastolic filling rate may be increased.
5.
Predict and calculate how stroke volume (SV) would vary following changes in EDV and/or ESV
6.
7. Identify the macroscopic features and locations of the key regions of the heart including the chambers,
valves, tributary vessels and vascular supply (major coronary arteries and veins), stating the way in
which their structures serve their primary functions.
8. Predict the open or closed state of the cardiac valves when the pressures in the heart chambers and
outflow vessels are known.
9. Trace the movement of blood through the various chambers of the heart, identifying the sequence of
important structures and explaining how this particular sequence is necessary for the efficient pumping
of the heart
III. Underlying Concepts
This exercise requires very little prior knowledge. Most students have observed some kind of pump,
but may not have identified it as such. They are also familiar with cycles, but may not be able to say
what makes a cycle, when one starts and ends, and how we determine its frequency. Since so many
physiological phenomenon are cyclical this is an important prior concept to build on in this exercise.
The pre-lab exercise is assigned to students to be completed before they come to the laboratory. This
addresses a common problem with any class - unprepared students realize a low yield for their time
invested in class. We always begin labs with a short quiz that covers material from the previous lab
and material they are to have covered that is related to the lab of the day, including the pre-lab
exercise. This tends to result in promptness and preparedness.
Pre-Lab Exercise
During the lab you will use the apparatus shown in Figure
1-1. Important functions of the heart are represented by the
syringe which fills with fluid coming from one source (the
beaker) and then dispenses the fluid to another location (the
metering reservoir). This happens because there is a check valve
(labeled “Inflow valve”) between the syringe and the beaker
which allows flow toward the syringe but prevents flow back
into the beaker. A second outflow check valve in the tubing
between the syringe and the metering reservoir allows the water
to flow up the tube but not back into the syringe. If the plunger
of the syringe is not held in place the water pressure in the
beaker will cause water to flow into the syringe body. Pushing
on the plunger causes water to flow out of the syringe, up
through the outflow valve, and into the metering reservoir.
When the syringe fills and empties repeatedly, the water in
the beaker is gradually pumped into the metering reservoir.
Figure 1-1 Syringe output setup
Each emptying and refilling of the syringe constitutes one
pumping cycle as illustrated in Figure 1-2. In the figure the
plunger is positioned at the 40 ml mark at the beginning of the cycle. It is then pushed to the 20 ml mark
during the emptying phase. Next, when the plunger is no longer being pushed, water begins flowing into
the syringe from the beaker. This starts the filling phase which ends when the syringe fills to the 40 ml
mark again. When the plunger is pushed forward again, a new pumping cycle begins.
Copyright 2000 by the American Physiological Society. Permission is granted to reproduce with proper citation for
classroom or workshop use only. For all other purposes, contact the American Physiological Society Education
Office. ( [email protected])
2
*
From the above description, define a pumping cycle: A pumping cycle includes all of the events from
the time the syringe is completely filled, immediately before pumping, until it has refilled after emptying
and is ready for another ejection. In other words it spans from the state of being completely filled at the
start of one cycle to the state of being filled at the start of the next cycle.
The volume of fluid in the syringe immediately before the plunger is pushed is called the Filled Volume
(Filled_Vol). In this example Filled_Vol is 40 ml. The volume left in the syringe at the end of the
inward pumping stroke is called the Emptied Volume (Emptied_Vol). In this example Emptied_Vol is
20 ml.
The terms were created so that they indicate as clearly as possible what they represent. The terms for
the syringe pump can be used to develop physiological definitions for the heart. For instance, End
Diastolic Volume is the volume of blood in the heart when it is filled at the end of diastole.
*
How would you find the amount pumped in one cycle, the Cycle Volume, Cycle_Vol. ?
Subtract the Emptied_Vol from the Filled_Vol
* What is Cycle_Vol. in the example of Fig. 1.2?
20 ml
* The fraction of the Filled_Vol that is pumped out
in one cycle is called the Ejection Fraction, EF, and
is calculated as follows:
EF = Cycle_Vol / Filled_Vol
*
What is the EF shown in Fig. 1.2? 20/40 or ½
The time from the beginning of one pumping
cycle to the beginning of the next is called the Intercycle interval. It is the time for one complete cycle,
and its units are seconds/cycle.
*
If the syringe fills and empties every 5 seconds,
what is the inter-cycle interval? 5 seconds
The Pump Rate, PR, is the number of pumping
cycles per unit time, thus it is the reciprocal of the
inter-cycle interval. PR is normally expressed as
Cycles Per Minute, CPM. It can be converted from
cycles/sec to cycles/min as shown below:
Figure 1-2 Syringe pumping volumes
1 Cycle
60 sec
12 Cycles / Min (CPM)
×
5 sec
1 min
*
What is PR if the Inter-cycle interval is 5 seconds? There is one cycle per 5 seconds so the pump
rate is 12 CPM
The Pump Output, PO, is the total amount of fluid pumped in one minute.
Copyright 2000 by the American Physiological Society. Permission is granted to reproduce with proper citation for
classroom or workshop use only. For all other purposes, contact the American Physiological Society Education
Office. ( [email protected])
3
* If the average inter-cycle interval for the pumping illustrated in Fig. 1.2 is 10 seconds, what is the
pump output, PO? 6 cycle/min * 20 ml / cycle = 120 ml/min
IV. Basic Principles: The fundamentals upon which the activity is based
1. Pressure-Flow: Students may have had little if any first-hand experience with pressure-flow
phenomenon in real life. Sometimes it is important to review or point out the way that flow rate depends
upon the height difference between the reservoir and the syringe. They may not know how gravity affects
flow, or that it is not the absolute height of the reservoir, but rather the height relative to the syringe
height. We have a second lab which examines pressure-resistance-flow relationships in tube flow and
which may precede this lab.
2. Cycles: Cycles are so fundamental to life, yet students often find it difficult to define them. The notion
that the end of one cycle is for all practical purposes the same point in time as the beginning of the next
one doesn’t always seem to be obvious. Often it seems that the end of a cycle is some time before the
beginning of the next. In terms of the annual cycle, December 1 might be thought to be the end of one
yearly cycle and January 1 the beginning of the next. Of course, 11:59 PM of December 31 is the end of
one annual cycle and 12:00 AM January 1 is the beginning of the next. Theoretically the actually end to
beginning time is infinitesimally small, being a tiny fraction of a second between 11:59 PM and 12:00
AM. The same is true for the cardiac cycle in which diastole begins the instant that systole ends.
V. Misconceptions: Students often bring a number of misconceptions to the activity or they may
develop others while carrying out the exercise.
A. Misconceptions frequently present before carrying out the exercise:
1. The only way to increase cardiac output is via an increased heart rate: Rate is the cardiac variable that
we are most aware of. Not surprisingly, most students usually identify only this variable when asked
about how the heart increases its output from the resting level. The exercise addresses the role of both rate
and stroke volume in determining output. Rate is independent of stroke volume, but stroke volume can be
dependent upon rate so both of these final determinants must be considered.
2. Increasing pump rate automatically increases output or “the faster you pump, the more you pump”: In
this exercise, students discover that increasing pump rate does not significantly increase pump output
because there is a reduced filling time. Only when compensatory adjustments to filling rate occur, can
increasing heart rate significantly increase cardiac output.
3. Valves actively open to allow flow to occur at specific times. Students usually describe valves as
having an active role, as in,”the aortic semi-lunar valve opens so the blood can be ejected into the aorta.”
In the same way they view the chordae tendineae as actively opening the AV valves. The model valves
are indisputably passive and students can observe the functional result of their passive behavior in
response to changes in the pressure gradients across them.
4. Atria fill with blood and then pump it into the ventricles. This is the usual understanding of how the
flow through the heart occurs. When there is no atrium it is apparent that the heart can function pretty
well as a one-chambered pump as it mostly does at rest. Students should come to the understanding that
the blood flows through the atria and into the ventricles during atrial diastole.
5. Ventricles completely empty during systole. It is reasonable to assume that efficient pumping would
produce an ESV of zero. The exercise is set up in a way to convey that ESV is not zero. Usually during
the exercise students ask whether it is also the case with the real ventricle that there is a significant ESV.
B. Misconceptions that may develop during the exercise:
1. Atria are unimportant in filling. The model used is one-chambered and students may reach the
conclusion that the atria are unimportant. They should observe that one of the ways to increase CO via
heart rate increase is to add a filling role for the atria, but this needs to be pointed out.
2. There is a single input/output port in the ventricle. Since the inflow and outflow of the model are via
the syringe outlet with the ‘Y’ splitting the input from the output, literal-minded students might
misconstrue the actual anatomy. Hopefully, the anatomical correlations at the end of the lab dislodge this
error and any other strictly anatomical misconceptions resulting from the use of an abstract model.
Copyright 2000 by the American Physiological Society. Permission is granted to reproduce with proper citation for
classroom or workshop use only. For all other purposes, contact the American Physiological Society Education
Office. ( [email protected])
4
VI. Heart-as-a-Pump Learning Activity
A. Exploration
We always begin the lab exercise by asking students to briefly confer in their groups about a question
that draws upon their own experiences which pertain to the lab. This exercise causes them to draw
upon the understanding they already have, and it usually also reveals misconceptions they have. Often
the answer to the Exploration question is not apparent until the end of the lab, thus, it sets the stage
for the lab exercise. By doing moderate exercise students will notice that their heart rate is increased
but if they observe well they will also notice that their pulse is stronger, though it may take some
probing to draw out this aspect of the cardiovascular change.
Go to a stairway in the building and run a total of 4 flights of stairs. When you get back to the lab
pause and note the state of your cardiovascular system. List the changes in the heart’s function you are
aware of?
B. Task Assignments
We use collaborative laboratory groups of 3-4 students to carry out the exercise. Students (like
professors) rarely know how to organize themselves effectively into groups to perform a task. Clarity
about roles helps, so, the students are asked to make specific assignments for the days exercise. Over
the course of the term the groups are asked to have different individuals play different roles on
successive weeks in the lab. Perhaps the most important role is the Reader/Coordinator who must
keep the group on track and make sure that every part of the exercise is carried out in sequence.
Despite all attempts to explain the importance of this function, students are so used to ignoring
written instructions (which they have frequently been told to ignore in other classes) that they need
strong reinforcement in carrying out this task. We tell the person in this role to read everything out
load, and to check off the boxes on the left as each part is completed. Italicized text means actions to
be performed. The Information, materials gatherer/timekeeper is the group member designated to
confer with the instructor and to seek needed materials. Having one person per group assigned to this
role eliminates a fair amount of traffic congestion and instructor fatigue. The Recorder is assigned the
role of writing down everything. The others in the group can obtain the information from him/her
later. The groups need to understand that missing information leads to the need to repeat portions of
the experiment. There may be more than one Instrument Operator.
Task Assignment
Group Member
Reader / Coordinator
Information, materials
gatherer/Timekeeper
Recorder
Instrument operator(s)
C. Procedures
The Instrument Operators will operate the syringe-pump as directed by the Timekeeper who can use
a wrist watch or the wall clock to synchronize the pumping cycles specified for each part of the exercise.
Copyright 2000 by the American Physiological Society. Permission is granted to reproduce with proper citation for
classroom or workshop use only. For all other purposes, contact the American Physiological Society Education
Office. ( [email protected])
5
* One of the instrument operators should let the syringe fill and empty several times until the large air
bubbles are purged from the syringe and the outflow tubing. The outflow and inflow tubes should be
filled with water after this is done. Note: The syringe will fill on its own because of the water pressure
from the beaker, and the plunger could come out, so one of the operators needs to hold the plunger
until it is time to begin one of the exercises.
* Set the syringe on the table top when you are ready to begin. Think about where you want to have
the syringe placed during each part of the experiment. For consistent results this should always be
the same level relative to the reservoir.
* Prior to performing each of the exercises below, open the reset valve to empty fluid from the
metering reservoir into the beaker, and then close it again. Pay attention to the level of fluid in the
metering reservoir since it can overflow.
1. Case 1, 10 second Inter-cycle interval:
Now you will begin to explore the relationship between pump rate, Filled_Vol and Emptied_Vol. The
goal of this exercise is to understand how these factors are inter-related and then how to use your
understanding of their relationships to produce the highest possible pump output. Your lab group is
competing with the other lab groups to see who can obtain the greatest PO.
In each of the three Cases, the Filled_Vol and Emptied_Vol will be determined by the timing of the
Inter-cycle interval. In Case 1, you will use an Inter-cycle interval of 10 seconds. Allow 50% of the Intercycle interval for filling and 50% of the time for emptying:
* how many seconds will be spent filling? 5 seconds
* how many seconds for emptying? 5 seconds
* how many cycles/min. will be completed? 6
* a. Push the syringe plunger to the 10 ml mark, hold it there and empty the metering reservoir,
closing the valve when it is empty. Orient the syringe in a way that allows you to read the
volumes, because you will need to record the Filled_Vol and Emptied_Vol for each cycle.
* b. When the Timekeeper tells you to, start a minute of pumping, allow the syringe to begin filling
until the filling period has elapsed at which time your Timekeeper will tell you to begin emptying
for the predetermined emptying time. Continue pumping for one minute. Note: Don’t pull on the
plunger; it will fill on its own. Record in Table Ia the Filled_Vol and Emptied_Vol for each
p
umping cycle during the minute. These volumes will not be the
s
ame for each cycle.
Table Ia
Cycle #
Filled_Vol
Emptied_Vol
Cycle_Vol
1
2
3
4
5
6
Avg
Copyright 2000 by the American Physiological Society. Permission is granted to reproduce with proper citation for
classroom or workshop use only. For all other purposes, contact the American Physiological Society Education
Office. ( [email protected])
6
*
c. Calculate the Cycle_Vol for each pumping cycle and then compute an average for each of the
volumes in Table Ia. Cycle_Vol = Filled_Vol - Emptied_Vol
* d. What was the PO, for Case 1? PO may be found by summing the Cycle_Vol column or noting
the accumulated volume in the graduated cylinder; they should be very close to the same
* e. Calculate the Cycle_Vol from PO: PO divided by the number of cycles gives the calculated
Cycle_Vol
* f. How do the calculated Cycle_Vol and the averaged Cycle_Vol from Table Ia compare?.
____________________________________________________________________________________
* g. State in your own words, the relationship between filled volume (Filled_Vol), emptied volume
(Emptied_Vol) and the cycle volume (Cycle_Vol).
Cycle_Vol is the difference between the Filled_Vol and the Emptied_Vol.
* h. Calculate and fill in the values in Table Ib. Students should use the averaged Cycle_Vol and
Filled_Vol to calculate the ejection fraction, EF
Table Ib, Inter-cycle Interval = 10 sec.
PR (CPM)
Cycle_Vol (ml)
EF
2. Case 2: 5 second Inter-cycle interval.
Repeat the experiment as in Case 1 above, but this time use a 5 second Inter-cycle interval still
allocating 50% of the time to filling and 50% to emptying
* a. Predict what the PO will be at this higher pumping rate?
______________
Make sure that
* b. Fill in the parameters to be used:
students make and
#
Emptying time: _______
Filling time: ________
record their prediction
of cycles
of how doubling the
in a
pumping rate will
minute:
affect the pump output,
____
PO. They need to refer
Record
the
data
in
Table
IIa
and
then
record
the
summary
data in
c.
*
back to their prediction
Table IIb.
later. Generally,
predicting feels risky
to students and they
are hesitant to record
there predictions.
Insist.
There are more rows
than need to be used in
this table and in Table
Ia. If students aren’t
clear about what
constitutes a cycle they
may end up with an
extra one. There
should be no data
below row 12 of this
Table IIa
Cycle #
Filled_Vol
Emptied_Vol
Cycle_Vol
1
2
3
4
5
6
7
8
Copyright 2000 by the American Physiological Society. Permission is granted to reproduce with proper citation for
classroom or workshop use only. For all other purposes, contact the American Physiological Society Education
Office. ( [email protected])
7
9
10
11
12
Avg
*
*
d. What was the PO, for Case 2? ________
e. Calculate the Cycle_Vol from PO:
* f.
Calculate and fill in the values in Table IIb.
Table IIb, Inter-cycle Interval = 5 sec.
PR (CPM)
*
g.
Cycle_Vol (ml)
EF
Fill in Table III
Table III Comparing Cases 1 & 2
Case
PR (CPM)
Filled_Vol
(ml)
Emptied_Vol (ml)
Cycle_Vol (ml)
PO (ml)
1
2
* h. State in your own words how the shorter Inter-cycle interval (higher CPM) affected:
Filled_Vol: was reduced by the shorter filling time of the smaller inter-cycle interval
Emptied_Vol: was not changed or may have been increased by the smaller inter-cycle interval
Cycle_Vol: was reduced by the smaller inter-cycle interval
PO: was almost the same as with the lower pumping rate of 6 cycles/min
*
i. What is the relationship between Inter-cycle interval and Cycle_Vol?
Cycle-Vol appears to be almost directly related to the Inter-cycle interval
* j. Based on the data in Table III, does increasing pump rate increase pump output? Explain your
conclusion explaining any differences from your prediction in a., made before you did the
experiment.
Increasing pumping rate does not appear to increase pump output. The reason is that increasing the
rate shortens the inter-cycle interval, part of which is used for filling. If the rate double then filling time is
cut in half and Filled_Vol is about half of what it was. This is probably different from what was predicted
Copyright 2000 by the American Physiological Society. Permission is granted to reproduce with proper citation for
classroom or workshop use only. For all other purposes, contact the American Physiological Society Education
Office. ( [email protected])
8
since it is logical to assume at the outset that doubling rate will double the pump output.
3. Case 3: Maximizing pump output
This could be called collaborative competition. The groups are given the open-ended task of figuring
out how to get the most out of the pump and to see if they can best the other groups. There are two
kinds of approaches they can use alone or in combination. This part of the exercise should draw upon
what the students have learned in the guided part, Cases 1 and 2.
*
Reflecting on your investigations so far, what kind of changes might maximize pump output,
PO, in addition to increasing the rate? So far pumping rate is the only factor you have changed so
think about other ways to alter the pumping cycle, the apparatus or the way you do the
experiment.
Raising the reservoir until there is the maximal vertical distance to the syringe can increase the filling
pressure more than two-fold. Shortening the emptying part of the cycle by pumping harder leaves a
greater proportion of the inter-cycle interval for filling, which is still passive.
* b. Based on your answer to the previous question, estimate the maximum output of the pump:
A combination of these approaches can more than double the PO. Be careful, a very effective group can
overflow the 250 ml cylinder. The maximum PO we have seen is around 270 ml.
* c. Use the setup to test your prediction. Note: Be careful not to push too hard on the syringe, it
could break.
Record your PO: _______________
* d. If there was a difference between the predicted and observed maximum outputs, explain them.
____________________________________________________________________________________
________________________________________________________________________
* e. List in order of importance and explain what seem to be the most important factors determining
the maximum output of the pump?
Filling rate appears to be the most important limiting factor in raising cardiac output. Flow into the
heart during diastole is gradient driven and there are limits to how much this can be increased. Second in
importance is heart rate. Without large increases in heart rate, CO could only increase by the percentage
that SV can increase, which is probably around 50%.
a.
4. Sheep heart examination Now that you understand the heart as a special kind of pump, you will
examine a sheep heart to learn how its design allows it to function as a very efficient dual pump. Work
with a lab partner for this exercise and following the instructions below as you examine a pre-dissected
preserved sheep heart:
This anatomical study is designed to bring the student to the mammalian heart’s structure with the
purpose of viewing its structures in light of the functions that must be performed by it. This approach
puts function first, since function determines structure, rather than the other way around. It creates a
meaningful context in which to carry out structural study. We use pre-dissected hearts because
students are usually inefficient at exposing the structures adequately, and since learning to dissect
preserved materials is not one of our benchmarks, the extra time that would be required is not
justified. Plasticized hearts are now available and they work very well, lasting for many terms. It is
assumed that students have available to them a text and perhaps a lab manual with labeled figures to
aid this exploration.
a.
Examination of pre-dissected heart:
Copyright 2000 by the American Physiological Society. Permission is granted to reproduce with proper citation for
classroom or workshop use only. For all other purposes, contact the American Physiological Society Education
Office. ( [email protected])
9
*
(1) Use cold tap water to rinse excess preservative from the heart . Look for remnants of the
pericardium that surrounded the heart. This fibroserous membrane has already been removed, but
portions of it may still be attached at the bases of the large vessels at the top of the heart. Notice
the deposits of fat on the surface of the heart surrounding the coronary vessels.
* (2) Study the external topography of the heart. All vessels of the heart enter and exit from the
superior aspect. The opposite, pointed end, or the apex, is directed inferiorly. Extending
diagonally from right to left on the anterior surface is one of the coronary vessels. It is usually
covered by fat deposits. This vessel is over the region that separates the two ventricles.
Determine the position of each ventricle and the two atria.
* (3) Using the blunt probe locate the superior and inferior venae cavae . These vessels are not always
intact, but at least the openings to a heart chamber will be present. While one of you gently
pushes the probe into the superior vena cava the other should open the heart at one of the
incisions already present. The probe will enter one of the heart chambers. Which chamber is it?
Right atrium (RA)
* (4) What direction would the blood be flowing by following the path of the blunt probe? Into the
RA
* (5) Where would blood be coming from that enters this heart chamber? Systemic circulation
* (6) Gently push the probe on to the next heart chamber. What chamber does it enter? Right ventricle
* (7) What structure did the probe pass through as it entered this chamber? Tricuspid or AV valve
* (8) What is the function of the structure it passed through?
The tricuspid valve closes when right ventricular pressure exceeds right atrial pressure preventing
regurgitation (backflow) into the atrium.
* (9) What part of the syringe pump does the structure in (7) above correspond to? The check valve
between the reservoir and the syringe
* (10) Examine the interior of the heart chamber the probe is entering. Identify the papillary muscles
and the chordae tendineae attached to the three flaps (cusps) of the structure identified in (7)
above. What do you think they do? They prevent the valve flaps from everting during
ventricular systole when the pressure in the ventricle is high.
* (11) Where does the blood go when it leaves this chamber and what vessel(s) does it travel in? To
the lung via the pulmonary trunk and pulmonary arteries
*
*
*
*
(12) Pull the probe out and then gently push it through the place where blood exits this heart
chamber. What structure does the blood pass through as it exits this chamber? Pulmonary
semilunar valve
(13) Try to figure out how this structure actually works. Look at it from inside the heart and from
outside. What part of the syringe pump does this correspond to? The check valve between the
syringe and the tube leading to the graduated cylinder (metering vessel)
(14) Which side of the heart have you been exploring? Right
(15) How do you know this is the side of the heart you think it is? What characteristics are you
relying on to make this judgement? Inflow from the systemic circulation enters the right heart.
* (16) Gauging from the thickness of the walls of the four heart chambers, list the chambers in order of
increasing strength: right atrium , left atrium , right ventricle , left ventricle
* (17) Why might the chambers be capable of different levels of force development?
The atria do not need to develop a great deal of force since they pump blood into the ventricles when
ventricular pressure is near zero. Therefore, they have relatively thin walls. The right ventricle pumps the
full cardiac output into the pulmonary circuit whose total resistance is much lower than the systemic
circuit. This means much less pressure is required to push the blood through the pulmonary circuit and
therefore the right ventricle is less muscular than the left ventricle. The left ventricle supplies the
Copyright 2000 by the American Physiological Society. Permission is granted to reproduce with proper citation for
classroom or workshop use only. For all other purposes, contact the American Physiological Society Education
Office. ( [email protected])
10
systemic circulation which has a high resistance and pressure. As a result it must be the most muscular
chamber.
* (18) Explore the other side of the heart, identifying its inflow vessels, chambers and valves. You
should be able to identify the structures on the sheep and/or human heart that correspond to the
parts of the syringe pump used in the first part of this exercise.
Copyright 2000 by the American Physiological Society. Permission is granted to reproduce with proper citation for
classroom or workshop use only. For all other purposes, contact the American Physiological Society Education
Office. ( [email protected])
11
5. Correlation: How does the sheep and/or human heart relate to that of the syringe pump model?
* (1) In order to relate the function of the syringe-pump to those of the sheep and/or human heart,
define and explain each of the following terms. In the lab define these terms for the syringepump (middle column), then, after the lab session, define them for the heart (right column)
stating the correct physiological terms and adapting the syringe pump definition. (Note: refer to
the text for help)
Term
Syringe Pump
Heart
Filled_Vol
the volume of fluid in the
syringe immediately before
the emptying phase begins
End Diastolic Volume (EDV): the amount
of blood in the ventricle immediately
before ventricular systole begins
Emptied_Vol
the volume of fluid in the
syringe at the end of the
emptying phase
End Systolic Volume (ESV): the volume
of blood left in the ventricle at the end of
systolic ejection
Cycle_Vol
the volume of fluid pumped
out during an individual
pumping cycle
Systolic Volume (SV): the volume ejected
by the ventricle during each systole
Ejection Fraction (EF)
the fraction of the
Filled_Vol that is pumped
out each pumping cycle
Ejection Fraction (EF): the fraction of
EDV ejected during each systole, or
SV/EDV
Inter-Cycle Interval
the time from the beginning
of one pumping cycle to the
beginning of the next
Interbeat Interval: the time from the
beginning of one cardiac cycle to the
beginning of the next; usually measured
from ‘R’ wave to ‘R’ wave on an ECG
the number of pumping
cycles/ min
Heart Rate (HR): the number of heart
beats or cardiac cycles/min
the volume of fluid pumped
by the syringe in one
minute or PR X Cycle_Vol
Cardiac Output (CO): the volume of blood
pumped into the systemic circulation in
one minute or HR X SV
Pump Rate (PR)
Pump Output (PO)
*
*
(2) In the syringe-pump setup: explain what the beaker which supplies fluid to the syringe
represents? It represents the venous return to the heart - either the pulmonary or the systemic
(3) Explain what the tube leading to the metering reservoir represents? It represents the arterial
circulation, either the pulmonary or systemic
*
(4) In what ways does the syringe-pump differ from the hearts you examined?
1) It lacks an atrium, 2) its valves are not arranged in the same way because there is only one opening
to the ventricle (syringe) and the ‘Y’ to the reservoir or the metering vessel is external to the pump, 3) in
Copyright 2000 by the American Physiological Society. Permission is granted to reproduce with proper citation for
classroom or workshop use only. For all other purposes, contact the American Physiological Society Education
Office. ( [email protected])
12
the syringe both valves are constructed in the same way while in the heart they are built differently
Copyright 2000 by the American Physiological Society. Permission is granted to reproduce with proper citation for
classroom or workshop use only. For all other purposes, contact the American Physiological Society Education
Office. ( [email protected])
13
D. Post-lab Self-Assessment
1. A well trained athlete has a very low resting heart rate of 50 beats per minute. His heart is fit,
however, having a resting stroke volume of 120 ml.
What is his resting cardiac output (ml/min)?
a.
CO = HR X SV; CO = 50 beats/min X 120 ml/beat = 6000 ml/min
b.
If he exercises heavily his heart rate may increase to 200 beats per minute. What would his
cardiac output be then?
Assuming no change in SV: CO = 200 beats/min X 120 ml/beat = 24,000 ml/min
c.
How else could his heart change its pumping to further increase cardiac output?
The stroke volume could also increase somewhat although given the much shorter interbeat interval it
might be difficult to increase this very much because there so little time for filling to occur. (Sympathetic
activity can increase atrial contractility and venous tone which both increase filling; it also increases
ventricular contractility shortening the ejection period, and it speeds repolarization, leaving a greater
percentage of the cycle for filling. These can lead to increased stroke volume at high heart rates.)
2. Someone who is in heart failure may have a greatly enlarged heart which holds a large volume of blood
but is unable to pump very much into the aorta. If the end diastolic volume of such a heart is 200 ml, the
ejection fraction is 0.25, and the heart rate is 100 BPM, what is the cardiac output?
CO = HR X SV; HR is known but not SV; to find SV use the EF and EDV;
EF = SV/EDV; SV = EF X EDV = .25 X 200 ml = 50 ml
CO = 100 beats/min X 50 ml/beat = 5000 ml/min
3. Convert the following intervals between heart beats into heart rates in beats/min:
1 beats/5 sec X 60 sec/min = 12 bpm
a.
5 sec.
b.
7 sec. 1 beats/7 sec X 60 sec/min = 8.6 bpm
c.
12 sec.
1 beats/12 sec X 60 sec/min = 5 bpm
d.
15 sec.
1 beats/15 sec X 60 sec/min = 4 bpm
4. A student’s body requires a cardiac output of 5600 ml (5.6 L) per minute to support resting activity. It
was determined that this student has a resting heart rate of 70 BPM.
What is the student’s stroke volume? Show your calculations.
a.
CO = HR X SV; SV = CO/HR = 5600 ml/min ÷ 70 beats/min = 80 ml/beat
b.
Measurements under these conditions show that end diastolic volume is 150 ml. What is the
ejection fraction? Show your work.
EF = SV/EDV = 80 ml ÷ 150 ml = 0.53
Copyright 2000 by the American Physiological Society. Permission is granted to reproduce with proper citation for
classroom or workshop use only. For all other purposes, contact the American Physiological Society Education
Office. ( [email protected])
14
VII. Student Assessment: This assessment would ascertain whether students accomplished the
benchmarks. They are keyed to the benchmarks at the beginning of the exercise.
** TRUE/FALSE
1. The aortic semilunar valve closes when the pressure in the left ventricle is higher than the pressure in
the aorta. (Benchmark 8) False
2. During diastole, the A-V valves open because the papillary muscles contract and pull their flaps apart.
(Benchmark 7) False
3. The semilunar valves open when the pressure in the ventricles is higher than the pressure in the aorta
and pulmonary artery. (Benchmark 8) True
4. A pumping cycle includes all of the events from the beginning of pump filling to the end of pump
emptying. (Benchmark 1) True
* *MULTIPLE-CHOICE
5. A young woman has a cardiac output of 6300ml, a resting heart rate of 70 BPM, an end diastolic
volume of 130ml and a heart rate of 170 BPM at her maximum exercise level. If she is able to maintain a
constant stroke volume until she reaches her maximum exercise level, what would her cardiac output be?
(Benchmark 3)
A. 13,500ml
B. 26,000ml
C. 42,300ml
D. 36,700ml
* E. 15,300ml
6. When the heart rate increases during exercise, the interbeat interval shortens. Which of the following
mechanisms helps to keep stroke volume from decreasing? (Benchmark 5)
A. Lowering of blood pressure in the vena cava.
B. Lengthening the duration of ventricular systole.
* C. Increasing the force of the atrial contraction.
D. Lengthening the duration of ventricular diastole.
*ESSAY/SHORT ANSWER/PROBLEM
7. If the heart rate of an individual is 60 BPM and diastole and systole each take the same amount of time
for each heart beat, how long is the filling time? (Benchmark 2)
8. Discuss the factors that determine whether cardiac output will increase in a manner that is
proportionate to increases in heart rate. (Benchmark 4 and 5)
*LAB PRACTICAL
Benchmarks 7 and 9 are addressed in practical exams via both identification and identification/function
questions.
Copyright 2000 by the American Physiological Society. Permission is granted to reproduce with proper citation for
classroom or workshop use only. For all other purposes, contact the American Physiological Society Education
Office. ( [email protected])
15
VIII. Resources and Materials
Room facilities:
- lab benches or tables; one per group of 3-4 students
- the Exploration asks the students to run up a few flights of stairs; a jog around the building will also
serve
Supplies (other than the syringe pump):
- plastic models of hearts and/or preserved sheep hearts; plasticized sheep hearts are now available
which last for a number of years
Syringe pump components (one set for each 3-4 students):
1
6"
1
1
1
2'
2
1
1
300ml
1
1
500 ml aspirator bottle - Baxter #B7581-500
Masterflex tubing - Cole Parmer #6424-17
Ratchet tubing clamp - Cole Parmer #533-50 (TJP340L code #916)
250 ml plastic graduated cylinder - Baxter #C9074-250
1/4" Universal stopcock - Cole Parmer #H06225-60
Tygon tubing - Cole Parmer #H06409-25
Polypropylene check valves - Cole Parmer #Ho6304-30
50 ml multifit luer-lock glass syringe - Baxter #59430-3A
“T” luer lock - Cole Parmer #H06359-47
rheoscopic fluid (1:3 dilution with water) - Novostar Design, Inc., 111 West Pine St.,
Graham, NC 27253; 800-659-3197
standard lab stand with 30" pole
burette clamps
The only fabrication involves taping a 1/4" npt hole in the bottom of the graduated cylinder so the
universal stopcock can be threaded into it. Teflon tape should be used on the threads before it is screwed
into place.
Correspondence:
Dr. Daniel E. Lemons
Dept. of Biology, J526
City College of New York
138th St. and Convent Ave.
New York, NY 10031
(212) 650-8543
[email protected]
http://harold.sci.ccny.cuny.edu
Copyright 2000 by the American Physiological Society. Permission is granted to reproduce with proper citation for
classroom or workshop use only. For all other purposes, contact the American Physiological Society Education
Office. ( [email protected])
16