artigo 20 - construction of a model demonstrating cardiovascular principles

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277:67-83, 1999.Advan Physiol EducDavid W. Rodenbaugh, Heidi L. Collins, Chao-Yin Chen and Stephen E. DiCarlo

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http://www.the-aps.org/.Physiological Society. ISSN: 1043-4046, ESSN: 1522-1229. Visit our website atthe American Physiological Society, 9650 Rockville Pike, Bethesda MD 20814-3991. Copyright 2005 by the Americanand in the broader context of general biology education. It is published four times a year in March, June, September and December by

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CONSTRUCTION OF A MODEL DEMONSTRATING

CARDIOVASCULAR PRINCIPLES

David W. Rodenbaugh, Heidi L. Collins, Chao-Yin Chen, and Stephen E. DiCarlo

Departm ent of Physiology, Wayne State Un iv ersity School of Medicin e, Detr oit, M ichigan 48201

We developed a laboratory exercise that involves the construction and

subsequent manipulation of a model of the cardiovascular system. The

laboratory was designed to engage students in interactive, inquiry-based

learningandto stimulateinterestforfuturesciencestudy. Themodel presentsaconcrete

meansby whichcardiovascular mechanicscan be understoodas well as a focal pointfor

student interaction and discussion of cardiovascularprinciples. The laboratory contains

directions for the construction of an inexpensive, easy-to-build model as well as an

experimental protocol. From this experience students may gain an appreciation for

science that cannot be obtained by reading a book or interacting with a computer.

Students not only learn the significant physiological concepts but also appreciate the

importance of laboratory experimentation for understanding complex concepts. Model

construction provides a hands-on experience that may substantially improve perfor-

mance in science processes. We believe that model construction is an appropriate

method for teachingadvancedconcepts.

AM. J. PHYSIOL. 27 7 (AD V. PHYSIOL. EDUC. 22) : S67S83, 1 999.

Key words:mechanical heart; experiment; education; hands-on

Employment opportunities in the future will requirehigher skills and understanding of math and science.Furthermore, employees will be expected to workcooperatively to solve problems and develop solu-tions. These expectations will require educators todesign curricula that include a thorough backgroundin math andscienceandarepresented in amatter thatcontributes to the development of thinking practitio-ners (4, 10, 16). The thinking practitioner is compe-tent in basic science facts as well as the application ofa scientific knowledge base to analyze and solve

problems. Therefore, students must be taught in amanner that fosters analytic thought processes andteamwork (9, 20). For students to develop indepen-dent critical thinking skills, educational material mustrequire the students active involvement and encour-age them to take responsibility for their learning (8,14). Without proper trainingof the work forceforthe

future, the effect on the economy, society, and ourstandard of living will be detrimental. Therefore, it isin the best interest of our nation to raise the level ofeducation of all its citizens in an effort to meet thedemands of a changing society and remain competi-tive in the world arena.

Model construction is one way to improve thinkingskillsas well as enhancecollaborativeefforts (17, 21).Models have been shown to change the focus andorganization of scientific thinking (15). Models also

encouragelogic, reasoning, andcreativity,all of whichareassetsto thescientific thought process.

Model construction also requires students to recog-nize the reality that perfect solutions and answersto problemsencountered in thereal world aredifficultto obtain. Students quickly understand that perfect

I N N O V A T I O N S A N D I D E A S

1043 - 4046 / 99 $5.00 COPYRIGHT 1999THE AMERICAN PHYSIOLOGICAL SOCIETY

VOLUME22: NUMBER 1 ADVANCESIN PHYSIOLOGY EDUCATION DECEMBER 1999

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solutions are substituted with optimal solutions, thatis, solutions that minimize problems while maximiz-ing the utility of the finished model. To perform thisintellectual balancing act, students explore conceptsin greater depth and across disciplines to obtain theoptimal solutions. Students quickly realize that theymustconsiderandincorporatethisself-acquired novelinformation into the construction of the model to findthose optimal solutions.

For thesereasons, we developed alaboratoryexercisethat involvestheconstructionandsubsequentmanipu-lation of a model of the cardiovascular system. Con-struction of the model only requires inexpensivematerials, which can be found around the classroomor purchased through scientific catalogs. Students are

challenged to build the model and subsequentlymanipulate and measure changes in cardiovascularparameters.Onthebasisof dataobtainedfrommanipu-latingthe model, students answer questions designedto provoke thinking on integrated cardiovascularphysiology. Finally, studentscomplete a table summa-rizing the effects of the manipulations that are per-formed.

Our intention was to provide a discussion on theeducational advantages of model construction as wellas providingahands-onlaboratoryexercisethat couldbe adapted to a variety of educational situations. This

paper includes the model construction procedures(APPENDIX A), laboratory exercise to manipulate themodel (APPENDIX B), andexercisesolutions(APPENDIX C).

Construction and subsequent manipulation of thiscardiovascular model encourage active and coopera-tive learning strategies. These strategies are appropri-ateforscienceeducationandaretheheartof problem-based learning (3). Cooperative learning strategiesshould be employed whenever the learning goals ofconceptually complex subjects encompass divergentthinking, critical thinking, long-termretention, and/orthesocial development of students (1113). Evidencesuggests that with the use of activity-based scienceprograms, teachers can expect substantially improvedperformances in scienceprocesses(3).

Wehave used thecardiovascularmodel and accompa-nying laboratory in several arenas with great success.The model has been constructed and used as a

demonstration during lecture for medical school stu-dents. One of the greatest benefits occurred afterlecturewhen students manipulated themodel to gain

a better comprehension of a concept that was notfully understood. We also used the model to illustratepoints and answerquestions that cameup duringandafter lecture. Students often used the model beforeexams to review concepts. During this time thestudentsmanipulatedthemodel to reinforceconcepts

addressed in lecture through problem-based learning.All the feedback we received from medical studentsafter lecture and exams were positive, and studentsreported that the model was useful as part of theirreview.

Although the model is useful as a demonstration, the

greatest benefits occur when students construct andsubsequently manipulate the model. This model andexercise is successful with students for enhancingself-esteem from a job well done and encouraging

group learning, as well as exposing students to basiccardiovascular concepts. For this experience, stu-dents work in groups of three or four. The studentsare challenged to work together, follow instructions,and build the model. Judging by the enthusiasm and

commentsmadeduringthisexperience, it isclear thatactive, problem-solving experiences foster confi-dence, understanding, and self-esteem. Students who

complete themodel have agreat sense of satisfaction,agood understanding of cardiovascular concepts, andenhanced social skills. These skills are required for

employment opportunities in the future. Greater em-phasis will be placed on cooperative experiences tosolveproblemsandfind solutions(19).

Even though the benefits of model construction arenumerous, model construction is not a cure-all solu-tion foreducators. The roleof the instructoriscrucialforthe successof the classandthe construction of themodel. It is important to note that construction alonedoes notguarantee learning. Theimportant partisnotthat students manipulate things physically but thatthey do so for a purpose and engage in discussionabout it (22). Therefore, it is important for theinstructor to specify the objectives for the lesson,assignthegroups,explainthetaskandgoal, andassessthe effectiveness of the learning groups and theindividual studentsachievement (10, 16, 17, 20).


VOLUME 22 : NUMBER 1 ADVANCES IN PHYSIOLOGY EDUCATION DECEMBER 1999

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The instructors primary roleis to explain the goalsofthe laboratory for the students to complete the tasksand achieve the established objectives. To clarify thegoals of education, a hierarchy was created thatincludes, in order from the simple to most complex,knowledge, comprehension, application, analysis, syn-thesis, and evaluation (2). This hierarchy is known asBlooms Taxonomy. The benefits of using the cardio-

vascular model permiteducators to incorporate all sixlevelsof learninginto their goals.

Thegoalsof thislaboratory areas follows.

1) The students should learn to identify the compo-nents of the model in relation to their physiologicalanalogs and begin to understand how they work

together to simulate the cardiovascular system. Dur-ing this process students also begin to appreciate thesubtle qualities inherent in the cardiovascular systemthat may bedifficult to conveyvialecture(knowledgeand comprehension).

2) Students are challenged to operate their modelsunder different conditions such as constriction of

vesselsorchanges inheart rate (application).

3) The students must interpret the output of themodel and determine the results of the differentperturbations by completing tables (analysis and syn-

thesis).

4) The students are continually evaluating their mod-els performance, which in turn translates into aself-evaluation of their own knowledge. In addition,the educator can informally assess the knowledgebase of the group or individual by asking and/oranswering questions during the laboratory (assess-ment).

Addressing the class, the group, and even the indi-vidual results in several crucial benefits at the highestlevel of learning, evaluation. By interacting with thegroups, the instructor can quickly develop an ideaofthe level of comprehension and whether the goalshave been met. As the comprehension level is deter-mined, the instructor can quickly adapt to the needsof the individual, group, or entire lab. This type ofinformal assessment is an ongoing communicationprocesswith immediate payoffs as opposed to lecture

and test taking(formal assessment). Lecture generallybecomes unilateral communication, with the instruc-tor relaying information that may or may not bepertinent and/or retained. Student retention is thenonlyformally assessed by a testafter all thematerial iscovered. This is very counterintuitive considering thefact that instructors generally move on to the nexttopic after the grade is earned from the test andseldomaddresstheneedsdemonstratedby theresultsof the test.

In conclusion, we have provided a supplement totraditional teaching, which includesalaboratoryexer-cise that involves the construction and subsequentmanipulation of a model that demonstrates manyimportant principles of cardiovascular physiology. By

constructing and manipulating this model, studentsgain an understanding of cardiovascular function andinquiry-based learning. In addition, we have providedsources for supplemental reading, the laboratory it-self, and solutions for the instructor. With thesesources, instructors will have a project with clearlydefined goals, relevant content, and questions andanswers, all of which ensure a positive cooperativelearningexperiencefor bothstudentsand instructors.

APPENDIX A: CONSTRUCTION OF MODEL

Faculty and students are encouraged to discuss basic principles of

cardiovascular physiology before constructing the model. Several

relevant educational reviews addressing cardiovascular principles

arerecommended (see Refs. 1, 57, 18).

To make the directions for the construction of the cardiovascular

model as clear as possible, each component has been assigned a

letter and illustrated. A list of all the materials required for model

construction can be found in Table 1. Students are challenged to

construct and manipulate the model and to understand what each

component of themodel represents.

Part1: ConstructingtheLeftVentricleof the Heart

Connect the one-way valves (Fig. 1, part D) by inserting them into

the twoopeningson the topof thetwo-hole rubberstopper (Fig. 1,part A2; top denotes widest part of stopper). Be sure to place the

valves so that flow can occur into and out of the stopper. One

one-way valve will allow fluid to flow in whereas the second

one-way valve will allow fluid to flow out. Insert the heart-shaped

balloon (Fig. 1, part C) into the 60-ml syringe (plunger removed)

(Fig. 1, partK). Stretch the mouth of the balloon around the end of

the syringe, sealing the syringe where the plunger would have

been. Insertthevalve/stopper combination into theballoon/syringe



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combination, firmlysealingthe end where theplunger would have

been. Refer to Fig. 2 for an image of how the valve/stopper

combination fits inside the balloon/syringe combination. The last

step in assembling the left ventricle is the pump. Cut and attach a

24-cm piece of Tygon tubing(Fig. 1, part E) to the tip of the 60-ml

syringe (Fig. 1, part K) that holds the balloon. This complex will

function as the ventricle. Attach the other 60-ml syringe with the

plungerin place (Fig. 1, part J) to the freeend of the Tygon tubing

just attached to the left ventricle. Again, refer to Fig. 2 for an

example of the completed left ventricle and pump. Pulling on theplunger (Fig. 1, part J) will create a negativepressure inside theleft

ventricle. When thishappens, the balloon inthe syringe (Fig. 1, part

K) will inflate and fill with air. Pumping the plungerwill enablethe

student to act as the muscle for the heart to pump the blood

throughout thesystem.

Part2: AssemblingtheCardiacOutputMeasurement Device

Measureand cutone24-cmsection of Tygon tubing(Fig. 3, part E).

Insert the narrow end of the tubing connector (Fig. 3, part B) into

theone-holerubber stopper (Fig. 3,partA1). Attach thetubing(Fig.

3, part E) to the free end of the tubing connector in the rubber

stopper. Usethestopperto seal theopeningof a60-ml syringe(Fig.

3, part K). Note that the plunger for this syringe is removed. The

student should have a cardiac output measurement device resem-

bling the device in Fig. 4. Attach the left ventricle to the cardiac

output measurementdevice. Determine whichvalve allowsfluid to

flow out of the left ventriclebyblowing air through the valves. The

valve that does not allow air to flow in is the one that allows flow

out of the left ventricle. Connect the tubing going into the rubber

stopper sealing the cardiac output measurement device (Fig. 4)

onto the end of the one-way valve that allows flowsout of the left

FIG. 1.

Parts for construction of left ventricle: E, Tygon tubing; D, 1-way valves (2); C, medium

balloon (red); A2, 2-hole rubber stopper; J, 60-ml syringe with the plunger; K, 60-ml syringe

withoutplunger.

TABLE 1

Materials required for model construction

2 1-Hole rubber stoppers(A1)

1 2-Hole rubber stopper (A2)4 Tubingconnectors(B)

1 12-in. Round or heart-shaped

balloon (C)

4 1-Way valves(D)

200 cm 14-in. innerdiameter,

3/8-in. outerdiametertygon

tubing(E)

1 Large (5-ft inflated) longblue

balloon (F)

2 Rubberbands(G)

1 Rubber bulb (H)

1 T connector (I)1 Syringe (60ml) with plunger

(J)

3 Syringes(60ml) without

plunger (K)

1 Ruler or meter stick(L)

2 Clamps(M)

2 Ringstands

Scotch tape

Letters in parentheses correspond to letters referencing parts

necessary for assembly in Figs. 118. Many of the supplies can be

found at home, in a grocery store, or in the school laboratory.

Science supply companies are another good source for materials

used in this laboratory: Cole Palmer (Niles, IL), Sargent-WelchScientific (Skokie, IL), and VWRScientific (Boston, MA).



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FIG. 2.

Completed left ventricle. Model demonstrates how

stopper and balloon areput together.

FIG. 3.

Parts for construction of cardiac output (CO) measurement device: E, Tygon tubing; K,

60-mlsyringe; A1, 1-hole rubber stopper; B, tubingconnector.

FIG. 4.

Completed COmeasurementdevice.



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ventricle (Fig. 2). Once these components are connected, the

model should resemble the device shown in Fig. 5, wheredevice1

is theleftventricle and device 2 is thecardiac output Measurement

device.

Part3:Construction of theBlood PressureMeasurement Device

Cut a 60-cm piece of Tygon tubing (Fig. 6, part E). Usescotch tape

to attach the tubing (Fig. 6, part E) lengthwise to one side of the

meter stick (Fig. 6, part L). Be sure to leave approximately a 5-cm

length of tubing below the end of the meter stick as illustrated in

Fig. 7. This 5-cm section of tubing will be attached to the T

connector(Fig. 6,partI). Cut and connectthe30-cmpiece of Tygon

tubing (Fig. 6, part E) to the end of the T connector opposite the

endleadingto theblood pressure measurement device.Referto Fig.

7.Oncethisstepiscomplete, connectthetubingleadingfromtheT

connector of thebloodpressuremeasurementdeviceto thesyringe

tip of the cardiac output measurement device (Fig. 4) as illustrated

in Fig. 8.

Part4:Construction of theVenousReservoir

To visualizetheeffectof blood poolingin theveins,stretchthe

balloon with airandallow it sit for15minutes. Thiswillallowthe

water blood to pool and not simply run through the balloon.

After the balloon hasbeen stretched, cut anarrowhole inthe endof

thelong balloon(Fig. 9, partF). Firmlyattach themouthpieceof the

long balloon to the wide end of the tubing connector (Fig. 9, part

B). Wrap a rubber band (Fig. 9, part G) around the balloon and the

tubingconnector to ensure afirm, leak-free connection. Repeat this

procedure using the second tubing connector (Fig. 9, part B) and

the opening cut in the end of the balloon. Referto Fig. 10. Cuttwo

24-cm sectionsof Tygon tubing (Fig. 9, part E). Connect onepiece

of tubing to each of the tubing connectorsexiting the ends of the

balloon. Figure 10 illustrates the completed venous reservoir. To

connectthevenousreservoir to thepreviouslyconstructed portion

of the model, locate the third remaining end on the T connector

(Fig. 7). Connect the completed venousreservoir by attaching one

tubing of the completed venous reservoir to the third end on the T

connector.Figure 11illustrateshow themodel shouldappearatthis

point.

Part5: Construction of theMusclePump

Insert aone-way valve(Fig. 12, partD) intotherubber bulb(Fig.12,

part H). Determine which way this valve allows fluid to flow by

blowing air into it. Connect the second one-way valve(Fig. 12, part

D) to theoppositeend of the rubberbulb. Be certain that this valveis pointingin thesamedirection asthefirst valve. Onceassembled,

the muscle pump should resemble the pump shown in Fig. 13.

Notice that this assemblyallows fluid to flow in only onedirection.

To attach the muscle pump to the rest of the model, determine

which valveallows flow into therubber bulb. Connect thisvalveto

the Tygon tubing exitingthe venousreservoir(Fig. 10). Themodel

shouldresemble themodel illustratedin Fig. 14.

FIG. 5.

Connected left ventricle (device 1) and CO measurement device (device 2). Model demon-

strateshow leftventricle is attached toCOmeasurementdevice.



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FIG. 6.

Partsfor construction of bloodpressure measurementdevice: L, meter stick;E,Tygon tubing;

I, Tconnector.

FIG. 7.

Completed blood pressuremeasurement device.



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FIG. 8.

Left ventricle (1), CO measurement device (2), and blood pressure measuring device (3).

Model demonstrates how blood pressure measurement device is attached to CO measure-

ment device.

FIG. 9.

Parts for assemblingvenousreservoir: E, Tygontubing; F, large longballoon(blue); B, tubing

connectors(2); G,rubber bands(2).



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FIG. 10.

Completed venous reservoir. Note how rubber bands are wrapped around balloon/tubing

connector, producingaleak-free seal.

FIG. 11.

Leftventricle(1), COmeasurement device(2), andbloodpressure measuringdevice(3), and

venous reservoir (4). Model demonstrates how the venous reservoir is attached to rest of

circulatory model.



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FIG. 12.

Partsfor construction of muscle pump: H, rubber bulb; D, 1-way valves (2).

FIG. 13.

Completedmusclepump.Notedirectionof flow through 1-way valvesandhow fluid flowsin

only 1 direction when bulb is pumped.



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Part 6: Construction of the Right Atriumand ItsConnection to the Left Ventricle

Insert the tubing connector (Fig. 15, part B) into the one-hole

rubber stopper (Fig. 15, part A1). Seal the end of the60-ml syringe(Fig. 15, part K) by placing the stopper into the end where the

plunger was removed. Next, cut and connect a 40-cm section of

Tygon tubing (Fig. 15, part E) to the tubing connector that is

inserted into the rubber stopper. Cut and connect a 10-cm section

of Tygontubing(Fig. 15, partE) tothe tipof the 60-mlsyringe.Usea

drill or Phillipshead screwdriver to punch a hole in the syringe just

below therubber stopper insertedinto thesyringe. Refer to Fig. 16

foranexampleof thecompletedright atriumandthelocation of the

hole. Attach thefree end of thetubingleadingto therubber stopper

to the one-way valve flowing out of the muscle pump (Fig. 13).

Connect the 10-cm section of tubing attached to the tip of the

syringe to the one-way valve entering into the completed left

ventricle(Fig. 2). Thiscompletes the circulatory system andshould

resemble the completed systemillustrated in Fig. 17.

Part7:SettingUp theCardiovascular Model

Figure 18illustrateshow themodel shouldappearaftercompletion.

Suspendthesyringerepresentingthe right atrium(Fig. 18, device6)

andthe left ventricle(Fig. 18, device 1) on aringstand. Thebottom

of the syringe representing the right atrium should be higher than

the top of the two one-way valves enteringthe left ventricle. Next,

suspend thecardiac output measurement device (Fig. 18, device 2)

on the second ring stand with the tip of the syringe facing down.

Finally, suspend the blood pressure measurement device (Fig. 18,

device 3) from the same ring stand, which supports the cardiac

output measurementdevice. The rest of the circulatory systemcan

remain on the table. Now that everything is connected and sealed,fill the system with water. Carefully remove the stopper from the

top of the right atrium and slowly pour100 ml of water into the

right atrium while simultaneously pumping the left ventricle.

Continue until the entire systemis filled, all the air is removed, and

only fluid can be pumped back to the right atrium via the muscle

pump (Fig. 18, device 5). Place a clamp (Fig. 18, part M) along the

section of tubing connectingthe left ventricle(Fig. 18, device1) to

the cardiac output measurement device (Fig. 18, device 2) to

prevent spontaneous flow into the cardiac output measurement

device. Place a second clamp (Fig. 18, part M) along the section of

tubingconnectingtheblood pressure measurement device(Fig. 18,

device 3) to the venousreservoir(Fig. 18, device4). Thisclamp will

allow the students to alter peripheral resistance (PR). Before

proceeding with the experiments, become acquainted with the

modelscomponentsandhow thewholemodel works.

Explanation of theComponentsof theCardiovascular Model

Thetwo one-way valves,which control flow into andout of the left

ventricle, simulate the atrial-ventricular (flow in) and aortic (flow

out) valves, respectively. The valvesare very important in keeping

the blood moving in one direction. In the cardiovascular system,

FIG. 14.

Left ventricle (1), CO measurement device (2), and blood pressure measuring device (3),

venous reservoir (4), and muscle pump (5). Model how muscle pump is connected to

circulatory system.



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FIG. 15.

Partsfor construction of right atrium: K, 60-ml syringe; A1, 1-hole rubber stopper; E, Tygon

tubing; B, tubingconnector.

FIG. 16.

Completedright atrium. Notelocation of hole in syringe, which allows pressure to equalize

throughoutmodel.



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valves also exist in the heart andin the veins for the same purpose.

Without these valves, blood would travel in one direction during

systole and then in thereversedirection duringdiastole. Thisis due

to the fact that fluid flows from a region of high pressure toward a

region of low pressure. Theleft ventricle is the lastchamber of theheart through which blood travels before being pumped through-

out the vasculature. Therefore, in the body, the left ventricle is the

strongest chamber becauseit must generatea pressure greaterthan

that foundin the arteries to forceblood out of the heart.

The second portion of the model constructed will be used to

measure cardiac output(CO).Thevolumeof bloodejectedfromthe

left ventricle after each systole can be measured on the side of the

syringe. This is known as stroke volume (SV). CO can be deter-

mined by multiplying SV by heart rate (HR). Pressure can be

measured in the tubing artery during systole and diastole. The

pumping beating of the balloon heart will cause the water

blood in the tubing to rise or fall during systole and diastole.

Changes in arterial pressure will occur with changes in resistance

and canbe measured usingthe blood pressure measurement deviceconstructed in Part 3. By increasing the tension on the clamp and

squeezingdown on the vessel, blood flow is decreased due to

the increase in resistance. This will cause an increase in pressure

within thevessel.

The long balloon represents the venous reservoir in which blood

pools during normal, nonexercise states. The rubber bulb repre-

sents the muscle pump, which actively pumps blood out of the

venousreservoir and back into the circulation. One-way valves are

important herebecause they ensurethat blood is flowing in one

direction, toward the heart, and that no blood returns to the

venous reservoir. The syringe acts as the right atrium and allows

students to observe venousreturn and itseffects on preload.

APPENDIX B: LABORATORY PROCEDURE

In completing this laboratory, students are challenged to manipu-

late the model of the cardiovascular system and observe the

changes that occur.

Part1: EstablishingControls

Control values for HR, CO, arterial pressure, and venous return

must be established. Setting and obtaining the control values

provides a baseline that can be compared with values obtained

when students experimentally change one or more hemodynamic

variable.

A. It isimportant that all theairwithinthe model beremoved prior

to theinitiation of the exercise.

B.Students should work in teams of three to four members. One

student should operate the left ventricular pump. The second

student should operate the muscle pump and monitor the volume

of fluid in the right atrium. The third student should monitor and

record thedata.

FIG. 17.

Left ventricle (1), CO measurement device (2), and blood pressure measuring device (3),

venous reservoir (4), muscle pump (5), and right atrium (6). Model illustrates how right

atrium is connected tomuscle pump andleft ventricle.



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C. By pumping the plunger attached to the left ventricle at a

constant rate and volume of air, one is able to cause the balloonheart to pump water blood. Start pumpingso that the HR is 6

beats per minute and the volume of air changes 30 ml. Simulta-

neously, begin squeezingthe muscle pump to force blood out of

the venous reservoir and back to the right atrium. It is important

that the muscle pump is pumped in conjunction with the left

ventricle to maintain a constant level in the right atrium. This is

known as preload. For the control measurements, maintain a

preload of30ml.

D. Turn the small clamp (Fig. 18, part M) located between the

venous reservoir (Fig. 18, device 4) and blood pressure measuring

device (Fig. 18, device3) so that systolic blood pressure (SBP) is

20 cm and diastolic blood pressure (DBP) is 10 cm. To

measure these pressures, students use the device constructed in

Part 3. SBP is measuredwhentheheart is forcedto contract, i.e.,when the syringe is forcing air into the other syringe. DBP is

measuredwhen theheart isrelaxing, i.e.,whenone pullsback on

the syringe.

E. Measure CO by using the syringe constructed in Part 2. The

syringe measures SV. To calculate CO, students must record the

volume ejected in a one-minute period of time. Students can

determine this value usingeither of two methods. One method to

determine CO is by estimating SV, the volume ejected per heart

beat, using the cardiac output measurement device. Calculate CO

usingtheequation CO (SV)(HR). A secondmethodto determine

CO is by collecting the volume ejected in a one-minute period of

time. To do this, remove the stopper going into the top of thecardiac output measurement device and clamp the tubing exiting

the cardiac output measurement device completely shut. Measure

thevolumeejected intothecardiac outputmeasurementdevicein a

one-minute period. Students must note that other values cannotbe

obtained while using this method to determine CO. Students are

also encouraged to develop their own ideas for determining CO

using the cardiac output measurement device and the equation as

theirtools.

F.Values recorded for HR, SV, CO, SBP, DBP, and venous return

should be used to reestablish control settings after each part of the

experiment. Record thesecontrol values in Table2.

Part 2: Effects of ChangingPR

A.After establishing control values, increase PR by tightening the

clamplocatedbetweenthevenousreservoirand theblood pressure

measurement device. Be sure to maintain the established control

parameters for left ventricle preload via the muscle pump, SV and

HR. Measurearterial pressures.

1. What effect does increasing PR have on mean arterial pressure

(MAP), SBP, and DBP?Record valuesin Table2.

2. In the human, what is the analogous situation to an increase in

PR?

3. Predict theeffectof adecreasein PRon thepressure parameters.

Reduce PR, measure the effects on the pressure parameters, and

recordthesevalues in Table2.

FIG. 18.

Fully assembled model on ring stands. Note position

of left ventricle(1), CO measurementdevice(2), blood

pressure measurement device (3), venous reservoir

(4), muscle pump (5), and right atria (6). Clamps (M)

can bepositioned anywhere alonglengthsof tubingas

indicated.

TABLE 2

Summary sheetfor experimentsincluded in laboratory

MAP Systolic

Pressure

Diastolic

Pressure

Cardiac

Output

Control

Increase PR

Decrease PR

Increase SV

Increase HR

Muslce Pump

Supine

Students should complete table using actual values on denoting

whether values increase () or decrease () from control. MAP,

mean arterial pressure; PR, peripheral resistance; SV, stroke vol-

ume; HR, heart rate.



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Part3:Effectsof ChangingSV

A. Returnthemodel to control settingsdetailedin Part1.

B. While maintainingthesameHR, increaseSVby pumping60mlofair within thesyringeconnectedto theleftventricle.

4. What are the effects of an increase in SV on the pressure

parametersand CO?Recordthe valuesin Table2.

Part 4: Effects of ChangingHR


B. While maintaining thecontrol level of SV, doubletheHR.

5. What are the effects of an increase in HR on the pressure

parametersand CO?Recordthe valuesin Table2.

Part5:Effectsof ChangingtheMusclePump


B.As one studentmaintains these control settings, another student

should increase themusclepump activity so that preload increases

to 50ml in the right atrium.

6. What are theeffects on thepressure parameters and CO?Record

thesevalues in Table2.

7. Fromwhereis themuscle pump pumpingthe blood?

C. Stopthemusclepump activity so that preload fallsto 0 ml in the

rightatrium.

8. What are theeffects on thepressure parameters and CO?Record

thesevalues Table2.

Part 6: Effects of Posture on Cardiac Function

A. Returnthemodel to thecontrol settingsdetailedin Part1.

B. Place theright atriumon thetable to simulateanindividual in the

supineposition(lyingdown).Begin pumpingatthecontrol rateand

record CO.

C. Place the right atrium back on the ring stand to simulate an

individual standing upright. Begin pumpingat the control rate and

record CO.

9. What effect does lying down and standing up have on CO and

arterial pressure?Why do these changesoccur?Record these values

in Table 2. Changing from the supine to standing position too

quickly causesone to feel dizzy. Why doesthisoccur?

APPENDIX C: INSTRUCTORS SOLUTIONSTO EXERCISE QUESTIONS

Before discussing the overall regulation of the cardiovascular

system, it may be useful to review the basic relationship betweenMAP, CO,and PR, that is,

MAP CO PR

/\

HR SV

It is obvious from the above relationship that any condition that

increases CO and/or PR will result in an increase in MAP, with the

assumptionthat theother factordoes notchange. Answersto Table

2 arepresented in Table3.

Question 1

PR reflects the state of vasoconstriction and/or vasodilation and

measurestheresistanceto bloodflow. Resistance is directly relatedto blood viscosity and vessel length and is inversely related to

(vessel radius)4. Accordingly, the radius of the vessel is the single

mostimportant factorregulatingPR. Thus, in thebody, avery small

change in arteriole radius has an exponential effect on PR. In the

cardiovascular model, an increase in PR results in an increase in

MAP.

SBPisthepressure exertedby thebloodagainst thewallsof arteries

when the heart is contracting (systole). SBP is a function of the

volume of blood ejected into the arteries during systole and

compliance of the vasculature. By increasing PR, the ability of the

blood to flow out of the arterial segment was decreased. Thus a

greater volume of blood remained in the vessel and exerted more

pressure on thewallsof thearterial segment, andSBPincreased.

DBP is the pressure exerted by the volume of blood remaining in

the arterieswhen the heartisat rest(diastole). Itisafunctionof the

length of time the heart is at rest (or HR), compliance of the

vasculature, andtheoutflow of bloodinto the periphery. Increases

in PR impede the outflow of blood from the arterial segment. This

TABLE 3

Solutionsto Table 2

MAP Systolic

Pressure

Diastolic

Pressure

Cardiac

Output

Control

Increase PR

Decrease PR Increase SV

Increase HR

Muscle Pump

Supine

Although students haveactual recorded values, thistable illustrates

whether an increase () or decrease () occurred compared with

control.



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results in an increase in the volume of blood in the arterial

segment duringdiastole, and thusDBPincreases.

Question 2

Inhumans,anincreasein PRis analogousto thecondition known as

hypertension. When a person is said to have hypertension, it is

generally meant that hisor her MAPis higher than theupperrange

of the accepted norm. An individual with a MAP of 110 mmHg

underrestingconditionsis considered to behypertensive. ThisMAP

occurswhenDBP is approximately 90mmHg and SBP is approxi-

mately 140mmHg.

Hypertension hastwo primaryeffectson thecardiovascular system:

1) increased work of the heart and 2) damage to arteries and

peripheral organs. For blood to be ejected from the left ventricle

intotheaorta,theaortic valvemustopen. Thisis achievedwhen left

ventricular pressure is greater than aortic pressure. Think of the

aortic valve as a swinging door. For example, left ventricular

pressure opens the door in one direction, whereas aortic

pressure closes the door in the opposite direction. In hyperten-sion, thepressure against which theleftventricle must beat against

issignificantlyincreased. Thusthe pressure requiredbytheheartto

open the aortic valve is higher than that pressure generated in

normotensive individuals. Consequently, theworkload of the heart

increases. In responseto theincreasedworkload, theheartenlarges

(hypertrophies).

Arteries also suffer fromtheprolonged exposure to the increase in

blood pressure. High pressure in thearteriescausesarteriosclerosis.

The formation of blood clots and weakening of the vascular wall

characterize the arteriosclerotic process. These vessels, therefore,

frequently thrombose and/or rupture. In either situation, marked

organ damage can occur throughout the body. This can be life

threateningif damage to thebrain and/or kidneyoccurs.

Question 3

A decrease in PR resultsin a decrease in MAP, SBP, andDBP due to

the increased abilityforbloodto flow out of the arterial section.

Question 4

Review the basic relationship discussed at the beginning of this

appendix.

Cardiac output is the product of HR and SV. It is the volume of

blood pumped by the heart in one minute. An increase in the

amount of blood pumped into the aortawith each heart beat (SV),

increases the amount of blood pumpedbytheheartin oneminute.

Thus an increasein SV willresult in an increasein CO.

As previously discussed, SBP is a function of the volume of blood

ejected into the aorta duringsystole (i.e., SV). Because a greaterSV

increases the volume of blood within the arteries during systole,

SBPincreases.

DBP is a function of the volume of blood in the arteries during

diastole. Because there is an increasein the volume of blood in the

artery due to thegreater SV, therewill also beanincreaseDBP.The

increasein DBPwill not beassignificant asthe increasein SBP.

Question 5

An elevated HR increases the time the heart contracts, and thus

more blood is pumped into the aorta in one minute. Thus CO

increases.

Mean arterial pressure, determined by SBPand DBP, is the product

of COandPR. FactorsinfluencingCOandPRwill alterSBPandDBP.

Ultimately, MAP will change. The elevated HR will increase the

amountof bloodlocatedin thearteries, thusincreasingSBP.In turn,

theelevatedSBP will contributeto anincreasein MAP.

The elevated HR also effects DBP. An increase in HR reduces the

length of time spent between each cardiac contraction. Conse-

quently, theinflow of bloodintothevasculature isincreased during

diastole.ThusDBPincreases,whichalsocontributesto theincrease

in MAP.

Question 6

In combination with venous valves, contracting skeletal muscles

serve as an effective and powerful pump, driving blood back to the

heart (venous return). Immediately after contraction, the veins are

empty and competent venous valves prevent any backflow. At this

instant, pressure in the emptied veinsfalls to zero. This providesa

maximumpressure gradient. This is important because blood flow

can be characterized as movement down a pressure gradient. Thus

blood flowsinto the veins.

Venousreturn isincreased by meansofthemuscle pump compress-

ing and expanding peripheral veins. The increase in venousreturn

lengthens the cardiac muscle fiber. Thus, in accordance with the

Frank-Starling mechanism or length-tension relationship, the force

of contraction increases. The increase in contractility ejects a

greater quantity of blood out of the heart, and thus SV increases.

Because SV is a component that determines SBP, SBP is in turn

increased. In addition, thegreaterSVincreases thevolume of blood

in the artery during diastole. Thus there will also be an increase in

DBP.Taken together, theincreasein SBPandDBPwill contributeto

anincreasein MAP.

Question 7

The muscle pump removes blood from venous storage. Although

the same amount of blood is in the body, much of it was stored in

thevenousside of thecirculation(balloon).

Question 8

Without blood in the atrium, the ventricle will have no blood to

eject; thus COand MAPwill equal zero.

Question 9

Changing from a supine to an upright position causes an abrupt

translocation of blood from the thorax into the veins of the lower



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extremities. Because veins (balloons) are distensible, they distend

and blood pools in the lower body. The sudden loss of venous

return and consequent inability to maintain CO leads to abrupt

syncope. If veinshadrigid walls, no distension would occurandno

change in COwouldbeobserved. Thus syncopewouldbeavoided.

We thank Hanna R. Hilditch for support and expert technical

assistance with the photography.

Address for reprint requests and other correspondence: S. E.

DiCarlo, Dept.of Physiology,WayneStateUniv. School of Medicine,

Scott Hall, Detroit, MI 48201(E-mail:[email protected]).

Received11May 1998; acceptedin final form22 July1999.

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artigo 20 - construction of a model demonstrating cardiovascular principles

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