Building artificial hearts to learn about circulatory system physiology

Transcription

1 Bilding artificial hearts to learn abot circlatory system physiology David Brock Vertebrate anatomy and physiology as gides to the strctre and fnction of the hman body seem to captre stdent interest like no other topic in the life sciences. In my experience, stdents are fascinated by the circlatory system especially the heart. It is no wonder why: Until modern medicine discovered how to make it start again, a beating heart literally defined the line between life and death. Despite stdent interest, the heart is often a poorly nderstood topic in biology. Stdents are sally able to memorize its anatomical information and report the seqence of events in a contraction. However, if asked to explain the heart s role in the body in their own words (a reasonable analysis qestion) or why injring it weakens a person (a good synthesis problem), stdents can seldom do more than parrot back words sch as sinoatrial node, which reveals a lack of tre nderstanding. In short, stdents often do not nderstand the physiology or fnction of the heart. To help stdents nderstand this vital organ s physiology, I created an investigation activity involving the mammalian heart and its role in the circlatory system. Stdents design, bild, and demonstrate working artificial hearts to exhibit what they know abot the circlatory system. These physiological models provide a great opportnity to test for hidden misconceptions and illstrate other essential concepts stdents need to learn abot circlation (e.g., blood pressre and the path blood takes). This article describes how to incorporate the model heart activity into the classroom, inclding material that can be taght sing the working prodct. Relation to the Standards Stdying anatomy and physiology provides a general introdction to scientific investigation. The working hearts described in this article provide a way to teach and evalate basic problem-solving skills that spport the development of stdent nderstanding [of the character of scientific inqiry] and nrtre a commnity of science learners (NRC 1996, p. 31). Becase the activity reqires stdents to work in small grops, it generates the kind of discorse that enables them to experience science as a hman endeavor (NRC 1996, p. 200). Perhaps the most significant National Science Edcation Standard addressed is developing in stdents the abilities, nderstanding, and perspective necessary to engage in athentic scientific thinking (NRC 1996, p. 173). Figre 1 Diagram of the heart. Blood from the body Right atrim Right ventricle Aorta Blood to the body To the lngs From the lngs Left atrim Left ventricle 36

2 These models also address perhaps the most critical theme in biology: the relationship between strctre and fnction, and the direct dependency of the latter on the former at all levels of organization from the moleclar to the ecological (Content Standard C; NRC 1996, p. 181). The artificial heart activity provides a concrete opportnity to teach this central life science idea. For example, stdents can nderstand why the larger size of the ventricles generates the larger force of the systolic blood pressre becase they can literally feel how mch firmer the plastic tbing gets when the extra blood is forced into it by the larger pmping action of the model s ventricles. Finally, becase stdents have to design their hearts before they can bild and test them, they mst engage in thoghtfl planning for a piece of technology and test [the] soltion against the needs and criteria it was designed to meet (NRC 1996, p. 192). This project provides the ideal opportnity to discss how society identifies needs and ses science to create the technology needed to meet them (Content Standard E; NRC 1996, p. 190). With Robert Jarvik a scientist who developed an artificial heart appearing daily in television ads and a plethora of information abot artificial hearts available on the internet, there are many opportnities to explore the convergence of science, technology, and society (Content Standard F; NRC 1996, p. 193). A good extension to this project is having stdents research the models of artificial hearts available and then debate or write abot the merits of their own models. Materials and methods Before developing models, stdents first need to become familiar with the anatomy of the heart (see Athor s note, p. 38, and Figre 1). Once stdents have done this, have them bild a working model of the hman heart. Becase this is a physiology activity and not an anatomy lesson, stdents hearts do not have to look like actal mammalian hearts. However becase fnction is dependent on strctre each heart mst meet the following criteria: Each vital strctre mst be present in some fashion (i.e., two atria, two ventricles). December

3 All strctres mst be properly linked to one another according to their physiological relationship (e.g., the ventricles shold not be able to pass flid directly between each other). Each heart mst enclose a self-contained, sealed, and contigos water soltion (i.e., the blood ), so that no transfsions are needed and no bleeding occrs dring the heart s operation. The heart mst actally be able to move flid bt does not have to fnction atonomosly (i.e., stdents may prodce the necessary forces). Everything (i.e., valves, contractions) mst work in its proper order and manner when the model is operated. Once stdents nderstand the criteria, provide the necessary materials to grops of three or for stdents and give them space to be creative. Becase stdents design their own models, the materials needed vary. I typically provide stdents with the materials listed below bt allow them to sggest additional materials (e.g., food coloring): for vinyl examination gloves (or plastic water bottles, or balloons, or eqivalent) (Safety note: Becase of the potential for latex allergies, se only vinyl F I G U R E 2 examination gloves.) 1 2 m of 5/16 in. vinyl or plastic tbing one tbe of silicon aqarim sealant, cement, or bath calk pinch cock (or other form of simple clamp) water (to simlate blood) Athor s note. I sometimes begin the artificial heart project with the dissection of a real heart to increase my stdents familiarity with the organ s actal strctre. Cow hearts have proved to be the most sccessfl becase their larger size allows stdents to explore all of the openings and connections more easily. (Safety note: Given the risk of microbial/viral infection sorces, secre only commercially prepared cow hearts.) This kinesthetic learning seems to improve stdents ability to bild and demonstrate the workings of their models more effectively. Standard dissection safety cations apply: Stdents shold be cationed to wear indirectly vented chemical-splash goggles, aprons, and gloves, and take care with sharp instrments. However, I want to emphasize that having stdents perform dissections is not a necessary part of the activity. Alternative methods (inclding simple diagrams sch as that in Figre 1, p. 36) qite adeqately introdce stdents to the heart s basic anatomy, and I have sed these alternatives eqally sccessflly in my own classes. Photo of a correctly finished heart model. In this model heart, each glove represents a specific chamber of the heart. Tbing lengths are relative to the distance blood wold actally have to travel between the varios chambers, and the clamps represent the varios valve strctres that gide the direction of the blood s flow. Stdents have two 70-minte classes to complete the activity. Dring that time, they brainstorm how to solve the problem, develop their soltions into a workable design, and bild and demonstrate their models. Taking two days to complete the activity allows yo to acqire and provide any nanticipated materials that stdents may sggest. For example, I have had stdents ask for marbles in an attempt to mimic how artificial replacement valves work, and for different widths of plastic tbing to simlate the different arterial diameters in the aorta and plmonary artery. Right atrim Tricspid valve Right ventricle Aortic valve Plminary valve Left ventricle Left atrim Bicspid valve 38

4 Working Model Hearts Some stdents have troble getting past the literal anatomy of the heart and may gle glove chambers directly to each other ntil yo ask: How are yo going to separate the flow so that the blood does not mix? Others forget to leave an (easily closed) opening to fill their hearts with blood (e.g., a ct glove finger that can be knotted). Some will assemble the varios parts correctly, only to tape, rather than gle, seams sht their system will bleed ntil yo remind them that even an opening as narrow as a paper ct bleeds ntil clotting completely cements the gap. With teacher gidance, most stdents find their way to a soltion that looks similar to the heart in Figre 2. Ideally, the heart shold have short, roghly eqal pieces of tbing between the right atrim and right ventricle and between the left atrim and left ventricle. There shold be significantly FIGURE 3 Assessment rbric. Strctral components (50%): for chambers (4%) atria chambers same size (4%) ventricle chambers larger than atria chambers and different in size (medim and large) (6%) for valves (4%) for lengths of tbing (12%) two short one medim one long correct arrangement of chambers, tbing, and valves (16%) small-size right atrim attached to medimsize right ventricle by short tbe with valve between them small-size left atrim attached to large-size right ventricle by short tbe with valve between them right ventricle attached to left atrim by medim tbe with valve between them left ventricle attached to right atrim by long tbe with valve between them water (4%) Fnctional components (50%): stdents sqeeze both atria simltaneosly (8%) stdents open both valves between the atria and the ventricles before sqeezing the atria (8%) stdents sqeeze both ventricles simltaneosly (8%) stdents open both valves between the ventricles and the atria before sqeezing the ventricles (8%) valves work to case water to flow in correct direction (9%) system does not leak (9%) longer pieces of tbing between the right ventricle and left atrim and between the left ventricle and right atrim to reflect the longer pathways Keywords: Heart the blood mst take between these at chambers; the tbe between the left Enter code: TST and right atria shold be the longest. There mst be valves in place between each chamber to gide the direction of blood flow when the heart is operated. Ultimately, stdents make their own choices and the final design will be somewhat niqe. The strctre of the model does not have to reflect exact anatomy, bt it mst spport proper fnctioning of the artificial heart. The basic activity takes two days; if yo inclde the possible extension activities described in Discssions and extensions (p. 40) the project will take abot a week. Assessment This is primarily a physiology assignment, not an anatomy lesson. When evalating stdent models, it is critical to focs more on how the heart model works, and less on how it looks. For example, when yo are assessing the models, do not base the evalation on atria chambers being directly above ventricles or on a physically replicated septm. Stdents do not need to create a biological representation of the body or lngs. From a physiological standpoint, the heart is a closed system that pmps blood to itself: The body and lngs are simply very large networks of tbes connecting the pmps to one another. Therefore, as long as the flid in a stdent model moves in the same defined pattern that is fond in the real hman circlatory system, it is not essential for the heart to display all of the anatomical relationships. If stdents avoid the common error of first sqeezing the Safety considerations. The materials stdents se to constrct the hearts can vary; therefore, so can the safety concerns. Stdents shold se vinyl gloves, standard plastic tbing, and silicon cement as sggested in Materials and methods, (p. 37). Either ct the lengths of plastic tbing ahead of time or have stdents se scissors (rather than a knife) to redce risk of injry. In addition to indirectly vented chemical-splash goggles, stdents shold wear aprons and protective gloves dring model constrction to prevent the silicon cement from directly toching their skin (prolonged exposre can prodce a mild chemical brn). Secre and review the Material Safety Data Sheet (MSDS) for sealant or calk, and make sre ventilation can accommodate fmes. Have stdents work on a tarp or shrink wrap so that any sealant can be wrapped p and thrown away. After abot 24 hors, the models are typically dried and cred and are safe to toch with bare hands. December

5 Working Model Hearts right atrim followed by the right ventricle, and then the left atrim followed by the left ventricle bt instead correctly contract both atria followed by both ventricles they shold receive fll credit for that portion of the assessment. Finished hearts, however, mst have some strctral reqirements, specifically: The for chambers shold be connected properly with appropriate tbing lengths to represent the physiological demands of moving the blood different distances. The chambers shold be different sizes to represent the difference in force they can generate to move the blood different distances. The chambers shold be separated by the correct valves. The link between the right atrim and left ventricle and the link between the left atrim and right ventricle mst cross over as they do in a living system (Figre 2, p. 38). In addition, the completed system shold be flly closed and watertight with no (or minimal) leaks stdents shold not have to transfse water at any stage of their demonstration. As for physiology reqirements, when stdents demonstrate how their hearts work, it is critical that they remember to contract both atria simltaneosly (Figre 2; the two gloves labeled RA and LA ) while opening only the valves between the corresponding atria and ventricles (Figre 2; the clamps between gloves RA and RV and between gloves LA and LV). Then, stdents shold sqeeze both ventricles simltaneosly (Figre 2; the two gloves labeled RV and LV ) while closing the first set of valves and opening the second two (Figre 2; the clamps between gloves RV and LA and between gloves LV and RA). The key is to replicate the contraction so the chambers react as they wold in a real heart both atria and then both ventricles and to maniplate the valves so that the liqid flows in only one direction (from atria to ventricles to atria) withot any backflow. I have watched many grops with otherwise pictre-perfect hearts either move their flid only one chamber at a time or sqeeze blood from one atrim into two ventricles. Therefore, when observing model presentations, make sre the chambers are operated together and that the correct valves are opened in pairs. For frther information on assessing this activity, see the sample rbric in Figre 3 (p. 39). Discssion and extensions The primary goal of this activity is to give stdents a tangible way to nderstand the actions of the hman circlatory system. Stdents can see how the heart is essentially a coordinated set of pmps moving liqid one way in a closed loop. For example, stdents better comprehend the sophisticated link between sinoatrial and atrioventriclar nodes the two parts of the heart that coordinate the alternating contractions of the atria and the ventricles when they have to coordinate which chambers are pmping at a given time. They are also able to nderstand why the different chambers of the heart are different sizes becase they can literally see and feel how mch force it takes to move blood between chambers. From a developmental standpoint, stdents can nderstand why the heart forms before any other organ in the mammalian body. The simplified models help stdents visalize the heart basically pmping to itself, as it does in the first weeks of embryonic growth. The developing body and lngs simply become the increasingly more complex plmbing that connects two sets of the heart s for pmps. Even the heart s dysfnctions make more sense when it is possible to observe conseqences. Stdents can simlate damage to one of the model s pmps (e.g., by gling it to a rigid srface to decrease its flexibility) and watch what happens to the blood s flow, or remove a valve to see how the blood travels backwards. These visals allow stdents to witness the physiology of disorders and better nderstand why heart attacks and heart mrmrs prodce the associated symptoms. Stdents can observe how damage to certain parts of the heart (e.g., the left ventricle) is more medically significant by simlating different possibilities and seeing the reslts: Stdents can investigate the amont of stress plaqe deposits in the arteries pt on the heart by injecting some gle into the tbing near a chamber. Once stdents nderstand the physiology of the heart, yo can give them a problem to solve abot the organ and its role in circlation, and encorage them to hypothesize and test possible answers. From healthy physiology to bleeding, clotting, anerisms, strokes, bypass srgery, and more, providing stdents with the opportnity to create fnctional, artificial hearts allows them to explore endless circlatory-system concepts. David Brock (brockda@rpcs.org) is a science teacher at Roland Park Contry School in Baltimore, Maryland. Reference National Research Concil (NRC) National science edcation standards. Washington, DC: National Academy Press. Frther resorces. Campbell, N., and J. Reece Biology. 8th ed. San Francisco: Benjamin Cmmings. Hall-Craggs, E.C.B Anatomy as a basis for clinical medicine. 3rd ed. London: Williams and Wilkins Waverly. Pblic Broadcasting System (PBS) NOVA online: The electric heart. Shier, D., J. Btler, and R. Lewis Hole s essentials of hman anatomy and physiology. 8th ed. St. Lois: WCB/McGraw-Hill. 40