https://link.springer.com/chapter/10.1007/978-1-4757-0393-1_3
TESommerville
In teaching the physiology of the cardiovascular system, an analogy can be set up between the anatomical components of the system and corresponding physical components of a water reticulation system. The heart itself matches the pump, the blood is the liquid being pumped, and the blood vessels are the pipes distributing the liquid. A blend is constructed and then elaborated to accentuate the specific characteristics of the physiological system when compared to and contrasted with the mechanical system
The pump analogy, and the implied blend, is so obvious that, as in so many other cases of a well-fitting analogy, the simpler and more readily understood function of the mechanical pump is likely to be the dominant frame. The important ways in which the cardiovascular system differs from a simple pump-and-pipe system arise out of the blend, but require the teacher to provoke the learner by means of probing questions to elaborate the ideas in the blend. A number of aspects, if skilfully handled, will allow understanding to emerge from the blend; in some areas the understanding sought is obscured if the mechanical pump input is dominant, and a few aspects require the cardiovascular input to dominate where there is no immediate analogy with a mechanical pump.
At the level of tertiary education, some knowledge of how a simple reciprocating pump works can be taken as a starting point. This begs the question: Is a mechanical pump in fact a concept sufficiently familiar to the learners to make a blend that is helpful in explaining the properties of the cardiovascular system? The idea of the heart as a pump is in fact a fairly standard analogy that is made, even in picture-books for young children. The mechanics of pumps are dealt with in high-school science, and members of rural communities are familiar with the pumps of various kinds that have been installed to enable them to draw water from reservoirs. This background perhaps explains the surprise of my first-year students that I should even ask whether they knew about pumps.
Whether or not we are making an unwarranted assumption about knowledge of pumps in general, in our Physiology laboratory we have available a number of mechanical pumps, each with attached tubes in a circular system, filled with water and capable of being modified in various ways: the pump can be worked faster or more slowly, the water can be drained out or added to, the calibre of the tubing can be increased or decreased, and the effects of these manoeuvres on the flow and the pressure in the system can be gauged. Since the systems are mechanical, they allow each component to be varied separately or simultaneously but, because they are mechanical, they do not illustrate all the features of the physiological system, and so a blended space must be constructed and its components elaborated in order for understanding of the cardiovascular system to grow.
The converse question may also be asked: Why draw out this analogy at all – it is surely self-evident that the heart is a pump? In fact, one can describe the heart in other terms: as an endocrine organ; as an electrical organ; as a metabolic organ. While each of these aspects is related to the functioning of the cardiovascular system, the endocrine, electrical and metabolic aspects do not relate as directly as does the physical/physiological pump function to the mechanics of blood circulation.
In Fauconnier and Turner’s terms, the generic space (meaning a mental, conceptual, rather than a physical, space) is sufficiently abstract to accommodate non-identical features. The generic space for the cardiovascular/pump inputs contains three concepts common to both systems (see figure 7.2), reflecting the necessity of moving a liquid, a physical system for doing so, and the terminology that describes the elements of the system. Each of these aspects is represented by components in the two input spaces, which include respectively the cardiovascular system and a pumping system. The blended space contains the common aspects of both inputs in the same mental conception and, as noted already, includes several items that are obviously analogous.
Relationshipsbetweendifferent spacesintheheart-pump blend
These three types of mental space constitute a mirror network, in which the generic and blended spaces contain ‘reflections’ of near-identical elements contained in the two input spaces on either side, and in which all the spaces share the same organising frame (i.e. the physics of fluid movement). Cross-space mapping demonstrates the obvious similarities in all the spaces.
Once the blend has been created, composition contributes to the identification of corresponding features of the cardiovascular and the pump system; completion occurs as the learner initially looks for features of the pump system that may be analogously present in the cardiovascular system. The emergent structure that arises in the blend is chiefly as a result of elaboration (‘running the blend’) – allowing the new conception created by composition and completion to develop in directions that the original inputs would not have suggested separately.
The ready identification in the blended space of corresponding elements from the input spaces may easily make for dominance of the mechanical pump input, leading to a simple – and incorrect – perception of the cardiovascular system’s functioning – human scale achieved at the cost of accuracy. In order for this unhelpful dominance to be removed, the teacher has to engage learners in elaboration; for example:-
Cursory examination of the heart, or illustrations of it, reveals four pumping chambers, and four valves. Tracing the direction of blood flow through the system shows that these four chambers are arranged in series, in two pairs. (A closer look at the size, function and name of the two atria suggests that they serve merely as antechambers to the two ventricles.) One can fairly easily deduce the existence of a pulmonary and a systemic circulation, each with its own pump, separated from each other by the pipes/blood vessels carrying blood through the lungs and those carrying blood through the rest of the body.
New conceptual relationships arising in the blended space are called, by Fauconnier and Turner, vital relations [p92] – presumably to indicate the ‘living’ interaction between different aspects. Two of these relations are analogy [p98] and disanalogy [p99], which they regard as two sides of the same coin: just as one perceives similarities in creating an analogy between two concepts, one must perceive at least one similar feature that they have in common in order to draw out a disanalogy. While the chambers of the heart match the chamber of the pump (analogy), the rigid walls of the latter are notably different from the distensible and actively moving walls of the former (disanalogy). The pump’s operation implies a reservoir of liquid from which it draws its prime; the venous system acts as reservoir in the cardiovascular system (analogy). However, the heart’s output into the arterial tree feeds (via the capillaries) into the veins, forming a circular, rather than a purely unidirectional in-one-side-and-out-the-other system (disanalogy). This aspect of flow can be further elaborated – strictly speaking, each part of the heart is a unidirectional pump; it just happens that the end-point of the left heart is the intake point of the right heart, and vice versa. (The blood that the heart pumps is regarded in the blended space as a liquid, and everyday experience demonstrates that blood behaves as such. The understanding that one of its major functions, to carry oxygen, is served by small, living cellular elements which are not liquid, but are suspended in liquid, is not particularly problematic and does not conceptually challenge the accuracy of the blend.)
Even without detailed knowledge of water reticulation systems, it can be grasped that for the pump to distribute the liquid, a system of tubing is necessary. Consideration of the fact that the blood circulation is closed, feeding back into itself, should stimulate elaboration of the disanalogy compared to the rigid pipes in a mechanical system. There must be distensibility of the pipes/arteries into which the pump/heart empties its load on each stroke, in order for the intermittent pressure build-up not to rupture the system, and there must be some capacity (vena cava and right atrium on the right side of the heart; pulmonary veins and left atrium on the left) adjacent to the inlet into each pump, from which it can fill. Further thought yields the insight that the heart, unlike mechanical pumps, cannot produce negative pressure to suck liquid into its chambers – nor could the thin-walled veins and atria avoid collapsing if suction were applied to them. The cardiovascular system must thus operate at a positive pressure throughout, not only on the output side.
The terminology that enables us to compress concepts into single words or phrases is an aspect of disciplinary discourse that, like all technical jargon, is itself a series of blends. Fortunately, in this case, each physiological term has a direct analogy with a mechanical term, thus making the initial discomfort of the blend easier to assimilate. (Technical terms are almost always uncomfortable at first – either the term is an eponym, a neologism or, worst of all, an ordinary word given a special technical meaning in the context of the discipline that has appropriated it. A well-known example in my field is the term shock, which in medical parlance has nothing to do with electricity or emotion but denotes one of a number of conditions in which the cardiovascular system is unable to perfuse the tissues of the body adequately.)
So cardiac output, and its unit of measurement – litres/minute – may be intuitively understood in the blend by analogy with pump output.
Heart rate is even more obvious. (Although one might elaborate – and thereby provide a lead-in to discussion of the electrical activity of the heart – by mentioning that the rate at which the heart beats, in abnormal circumstances, may not correlate with the rate obtained by counting the pulse.)
Stroke volume, again in the blend, may be understood by disanalogy, since, while the concept of a certain volume of liquid being expelled with each pumping cycle is common to both, the stroke of a piston pump in a rigid bore bears little direct resemblance to the complex contraction of every part of the walls of the heart.
The relationship of the above three aspects: Cardiac Output = Stroke Volume x Heart Rate can also be readily understood when operating in the blend.
Preload, by analogy with the prime that must be offered the pump in order for it to have something to act on, is a term that makes sense when explained. However, the fact that the preload of the heart (either left or right ventricle) can vary contrasts immediately with a mechanical pump with a fixed input and output. Here again, elaboration of disanalogy serves to make the distinction. Exploration of the Frank-Starling relationship (figure 7.3) takes cardiovascular function even further away from the blend. As with most muscles, the more cardiac muscle is stretched before it contracts (i.e. the greater its preload), the more forceful its contraction.
This leads us to discuss inotropy – the power of the muscle to contract. Inotropy depends upon the preload, the physical condition of the heart muscle and the ionic and hormonal concentrations in the blood that perfuses it. These provide more contrasts: no mechanical pump is able to vary its power expenditure depending on circumstances. Here, one may introduce, in terms of the vital relations cause and effect in tension with intentionality, the initially startling concept that cardiac output from minute to minute is determined largely by factors external to the heart. The heart as a pump does not determine its own output; the requirements of the body tissues, communicated by the autonomic nervous system and hormones – neurohumeral regulation – dictate what the heart’s output must be. It is at this point that the balance in the blended space must be weighted heavily towards the cardiovascular rather than the mechanical pump frame; persistence of the mechanical pump analogy will obstruct elaboration of this sort of information. (Again, in contrast to the mechanical pump, the fact that the heart is responsible for its own power supply is far away from the blended space – it in fact provides for its own energy supply by means of the flow that it generates through the coronary arteries that supply the muscular walls of its own ventricles.)
Afterload is not an intuitively obvious concept, but makes sense when one considers that any pump has to eject against some sort of opposition to outflow – even if it is only the mass of the liquid being ejected. The size of the outlet, the viscosity of the liquid, the amount of liquid in the pipes into which the pump is ejecting, the stiffness of those pipes; all of these contribute to afterload. Here again there is a contrast with the mechanical pumping system; the ability of the piping to vary its calibre is the major determinant of cardiac afterload, and thus of blood pressure. (By analogy with Ohm’s law, Cardiac Output x Peripheral Vascular Resistance = Blood Pressure).
I hope to have shown in the foregoing how analogy in its broadest sense may contribute to effective teaching and learning, and its usefulness in negotiating the terminology-rich field of the health sciences. I have described various aspects of the pedagogic experience in terms of Fauconnier and Turner’s own terminology. I have elaborated considerably on some aspects of cardiovascular physiology, at the risk of providing unnecessary and tedious detail, in order to provide examples of how analogy, disanalogy and other ‘vital relations’ help make the transition by elaboration from the more obvious elements of the blended space to the less apparent, and from the area of dominance of the pump model to dominance of the cardiovascular system concept.
One can stretch – or even stray far from – the blended space to the extent that time and one’s learners allow. In my opinion, the risk greater than that of losing one’s audience is that of analogies in general: that the more easily grasped input dominates the blend, resulting in inappropriate understanding of the input to which the analogy was designed to lead. Elaboration is crucial to extend the concepts established in the blended space, and thereby to expand the capacity of the minds that one has led into and through the blend.
References:
Fauconnier, G. and Turner, M. (2002).The way we think. Conceptual blending and the mind’s hidden complexities. Basic Books, New York.
Ganong, W.F. (2005). Ch.29 The heart as a pump.In W.F. Ganong.Review of medical physiology (22 ed., pp. 565-576). Lange, New York.
Guyton A.C. and Hall, J.E. (2006). Ch.9 Heart muscle: the heart as a pump and function of the heart valves. In A.C. Guyton and J.E Hall.Textbook of medical physiology (11 ed., pp. 103-114). Elsevier, Philadelphia.
Abstract
The human heart is an unusual and efficiently designed pump. Its main function is to pump blood through the entire circulation to meet the hematologic requirements of all cells of the body. This pump is most unusual in that the source of energy for the pump is located in the walls of the pump itself. It also has an unusual circulatory system, a system which must function in an organ which shrinks and dilates many times a day as it pumps blood. The main vessels are located on the surface of the heart, and the smaller vessels which deliver the blood to the cells of the pump penetrate the wall, the source of power. These smaller vessels are so constructed that they can deliver blood to each cell even though the vessels are being firmly squeezed upon during systole and must function in the organ while it is continuously changing in size and shape during the cardiac cycle. The major volume of blood flow occurs during diastole when tissue pressure is lowest. With research and clinical experience the astute and master clinician and cardiologist can determine at the bedside the functional state of the cardiac pump with sufficient accuracy to provide diagnosis, treatment and prognosis.
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The heart as a pump |
:Schematicrepresentationoftheheart asapump comparedtoamechanicalpiston- drivenpump
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TESommerville
In teaching the physiology of the cardiovascular system, an analogy can be set up between the anatomical components of the system and corresponding physical components of a water reticulation system. The heart itself matches the pump, the blood is the liquid being pumped, and the blood vessels are the pipes distributing the liquid. A blend is constructed and then elaborated to accentuate the specific characteristics of the physiological system when compared to and contrasted with the mechanical system
The pump analogy, and the implied blend, is so obvious that, as in so many other cases of a well-fitting analogy, the simpler and more readily understood function of the mechanical pump is likely to be the dominant frame. The important ways in which the cardiovascular system differs from a simple pump-and-pipe system arise out of the blend, but require the teacher to provoke the learner by means of probing questions to elaborate the ideas in the blend. A number of aspects, if skilfully handled, will allow understanding to emerge from the blend; in some areas the understanding sought is obscured if the mechanical pump input is dominant, and a few aspects require the cardiovascular input to dominate where there is no immediate analogy with a mechanical pump.
At the level of tertiary education, some knowledge of how a simple reciprocating pump works can be taken as a starting point. This begs the question: Is a mechanical pump in fact a concept sufficiently familiar to the learners to make a blend that is helpful in explaining the properties of the cardiovascular system? The idea of the heart as a pump is in fact a fairly standard analogy that is made, even in picture-books for young children. The mechanics of pumps are dealt with in high-school science, and members of rural communities are familiar with the pumps of various kinds that have been installed to enable them to draw water from reservoirs. This background perhaps explains the surprise of my first-year students that I should even ask whether they knew about pumps.
Whether or not we are making an unwarranted assumption about knowledge of pumps in general, in our Physiology laboratory we have available a number of mechanical pumps, each with attached tubes in a circular system, filled with water and capable of being modified in various ways: the pump can be worked faster or more slowly, the water can be drained out or added to, the calibre of the tubing can be increased or decreased, and the effects of these manoeuvres on the flow and the pressure in the system can be gauged. Since the systems are mechanical, they allow each component to be varied separately or simultaneously but, because they are mechanical, they do not illustrate all the features of the physiological system, and so a blended space must be constructed and its components elaborated in order for understanding of the cardiovascular system to grow.
The converse question may also be asked: Why draw out this analogy at all – it is surely self-evident that the heart is a pump? In fact, one can describe the heart in other terms: as an endocrine organ; as an electrical organ; as a metabolic organ. While each of these aspects is related to the functioning of the cardiovascular system, the endocrine, electrical and metabolic aspects do not relate as directly as does the physical/physiological pump function to the mechanics of blood circulation.
In Fauconnier and Turner’s terms, the generic space (meaning a mental, conceptual, rather than a physical, space) is sufficiently abstract to accommodate non-identical features. The generic space for the cardiovascular/pump inputs contains three concepts common to both systems (see figure 7.2), reflecting the necessity of moving a liquid, a physical system for doing so, and the terminology that describes the elements of the system. Each of these aspects is represented by components in the two input spaces, which include respectively the cardiovascular system and a pumping system. The blended space contains the common aspects of both inputs in the same mental conception and, as noted already, includes several items that are obviously analogous.
Relationshipsbetweendifferent spacesintheheart-pump blend
These three types of mental space constitute a mirror network, in which the generic and blended spaces contain ‘reflections’ of near-identical elements contained in the two input spaces on either side, and in which all the spaces share the same organising frame (i.e. the physics of fluid movement). Cross-space mapping demonstrates the obvious similarities in all the spaces.
Once the blend has been created, composition contributes to the identification of corresponding features of the cardiovascular and the pump system; completion occurs as the learner initially looks for features of the pump system that may be analogously present in the cardiovascular system. The emergent structure that arises in the blend is chiefly as a result of elaboration (‘running the blend’) – allowing the new conception created by composition and completion to develop in directions that the original inputs would not have suggested separately.
The ready identification in the blended space of corresponding elements from the input spaces may easily make for dominance of the mechanical pump input, leading to a simple – and incorrect – perception of the cardiovascular system’s functioning – human scale achieved at the cost of accuracy. In order for this unhelpful dominance to be removed, the teacher has to engage learners in elaboration; for example:-
Cursory examination of the heart, or illustrations of it, reveals four pumping chambers, and four valves. Tracing the direction of blood flow through the system shows that these four chambers are arranged in series, in two pairs. (A closer look at the size, function and name of the two atria suggests that they serve merely as antechambers to the two ventricles.) One can fairly easily deduce the existence of a pulmonary and a systemic circulation, each with its own pump, separated from each other by the pipes/blood vessels carrying blood through the lungs and those carrying blood through the rest of the body.
New conceptual relationships arising in the blended space are called, by Fauconnier and Turner, vital relations [p92] – presumably to indicate the ‘living’ interaction between different aspects. Two of these relations are analogy [p98] and disanalogy [p99], which they regard as two sides of the same coin: just as one perceives similarities in creating an analogy between two concepts, one must perceive at least one similar feature that they have in common in order to draw out a disanalogy. While the chambers of the heart match the chamber of the pump (analogy), the rigid walls of the latter are notably different from the distensible and actively moving walls of the former (disanalogy). The pump’s operation implies a reservoir of liquid from which it draws its prime; the venous system acts as reservoir in the cardiovascular system (analogy). However, the heart’s output into the arterial tree feeds (via the capillaries) into the veins, forming a circular, rather than a purely unidirectional in-one-side-and-out-the-other system (disanalogy). This aspect of flow can be further elaborated – strictly speaking, each part of the heart is a unidirectional pump; it just happens that the end-point of the left heart is the intake point of the right heart, and vice versa. (The blood that the heart pumps is regarded in the blended space as a liquid, and everyday experience demonstrates that blood behaves as such. The understanding that one of its major functions, to carry oxygen, is served by small, living cellular elements which are not liquid, but are suspended in liquid, is not particularly problematic and does not conceptually challenge the accuracy of the blend.)
Even without detailed knowledge of water reticulation systems, it can be grasped that for the pump to distribute the liquid, a system of tubing is necessary. Consideration of the fact that the blood circulation is closed, feeding back into itself, should stimulate elaboration of the disanalogy compared to the rigid pipes in a mechanical system. There must be distensibility of the pipes/arteries into which the pump/heart empties its load on each stroke, in order for the intermittent pressure build-up not to rupture the system, and there must be some capacity (vena cava and right atrium on the right side of the heart; pulmonary veins and left atrium on the left) adjacent to the inlet into each pump, from which it can fill. Further thought yields the insight that the heart, unlike mechanical pumps, cannot produce negative pressure to suck liquid into its chambers – nor could the thin-walled veins and atria avoid collapsing if suction were applied to them. The cardiovascular system must thus operate at a positive pressure throughout, not only on the output side.
The terminology that enables us to compress concepts into single words or phrases is an aspect of disciplinary discourse that, like all technical jargon, is itself a series of blends. Fortunately, in this case, each physiological term has a direct analogy with a mechanical term, thus making the initial discomfort of the blend easier to assimilate. (Technical terms are almost always uncomfortable at first – either the term is an eponym, a neologism or, worst of all, an ordinary word given a special technical meaning in the context of the discipline that has appropriated it. A well-known example in my field is the term shock, which in medical parlance has nothing to do with electricity or emotion but denotes one of a number of conditions in which the cardiovascular system is unable to perfuse the tissues of the body adequately.)
So cardiac output, and its unit of measurement – litres/minute – may be intuitively understood in the blend by analogy with pump output.
Heart rate is even more obvious. (Although one might elaborate – and thereby provide a lead-in to discussion of the electrical activity of the heart – by mentioning that the rate at which the heart beats, in abnormal circumstances, may not correlate with the rate obtained by counting the pulse.)
Stroke volume, again in the blend, may be understood by disanalogy, since, while the concept of a certain volume of liquid being expelled with each pumping cycle is common to both, the stroke of a piston pump in a rigid bore bears little direct resemblance to the complex contraction of every part of the walls of the heart.
The relationship of the above three aspects: Cardiac Output = Stroke Volume x Heart Rate can also be readily understood when operating in the blend.
Preload, by analogy with the prime that must be offered the pump in order for it to have something to act on, is a term that makes sense when explained. However, the fact that the preload of the heart (either left or right ventricle) can vary contrasts immediately with a mechanical pump with a fixed input and output. Here again, elaboration of disanalogy serves to make the distinction. Exploration of the Frank-Starling relationship (figure 7.3) takes cardiovascular function even further away from the blend. As with most muscles, the more cardiac muscle is stretched before it contracts (i.e. the greater its preload), the more forceful its contraction.
This leads us to discuss inotropy – the power of the muscle to contract. Inotropy depends upon the preload, the physical condition of the heart muscle and the ionic and hormonal concentrations in the blood that perfuses it. These provide more contrasts: no mechanical pump is able to vary its power expenditure depending on circumstances. Here, one may introduce, in terms of the vital relations cause and effect in tension with intentionality, the initially startling concept that cardiac output from minute to minute is determined largely by factors external to the heart. The heart as a pump does not determine its own output; the requirements of the body tissues, communicated by the autonomic nervous system and hormones – neurohumeral regulation – dictate what the heart’s output must be. It is at this point that the balance in the blended space must be weighted heavily towards the cardiovascular rather than the mechanical pump frame; persistence of the mechanical pump analogy will obstruct elaboration of this sort of information. (Again, in contrast to the mechanical pump, the fact that the heart is responsible for its own power supply is far away from the blended space – it in fact provides for its own energy supply by means of the flow that it generates through the coronary arteries that supply the muscular walls of its own ventricles.)
Afterload is not an intuitively obvious concept, but makes sense when one considers that any pump has to eject against some sort of opposition to outflow – even if it is only the mass of the liquid being ejected. The size of the outlet, the viscosity of the liquid, the amount of liquid in the pipes into which the pump is ejecting, the stiffness of those pipes; all of these contribute to afterload. Here again there is a contrast with the mechanical pumping system; the ability of the piping to vary its calibre is the major determinant of cardiac afterload, and thus of blood pressure. (By analogy with Ohm’s law, Cardiac Output x Peripheral Vascular Resistance = Blood Pressure).
I hope to have shown in the foregoing how analogy in its broadest sense may contribute to effective teaching and learning, and its usefulness in negotiating the terminology-rich field of the health sciences. I have described various aspects of the pedagogic experience in terms of Fauconnier and Turner’s own terminology. I have elaborated considerably on some aspects of cardiovascular physiology, at the risk of providing unnecessary and tedious detail, in order to provide examples of how analogy, disanalogy and other ‘vital relations’ help make the transition by elaboration from the more obvious elements of the blended space to the less apparent, and from the area of dominance of the pump model to dominance of the cardiovascular system concept.
One can stretch – or even stray far from – the blended space to the extent that time and one’s learners allow. In my opinion, the risk greater than that of losing one’s audience is that of analogies in general: that the more easily grasped input dominates the blend, resulting in inappropriate understanding of the input to which the analogy was designed to lead. Elaboration is crucial to extend the concepts established in the blended space, and thereby to expand the capacity of the minds that one has led into and through the blend.
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Relationshipsbetweendifferent spacesintheheart-pump blend
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References:
Fauconnier, G. and Turner, M. (2002).The way we think. Conceptual blending and the mind’s hidden complexities. Basic Books, New York.
Ganong, W.F. (2005). Ch.29 The heart as a pump.In W.F. Ganong.Review of medical physiology (22 ed., pp. 565-576). Lange, New York.
Guyton A.C. and Hall, J.E. (2006). Ch.9 Heart muscle: the heart as a pump and function of the heart valves. In A.C. Guyton and J.E Hall.Textbook of medical physiology (11 ed., pp. 103-114). Elsevier, Philadelphia.
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