A conceptual framework for homeostasis: development and validation

Abstract

We have developed and validated a conceptual framework for understanding and teaching organismal homeostasis at the undergraduate level. The resulting homeostasis conceptual framework details critical components and constituent ideas underlying the concept of homeostasis. It has been validated by a broad range of physiology faculty members from community colleges, primarily undergraduate institutions, research universities, and medical schools. In online surveys, faculty members confirmed the relevance of each item in the framework for undergraduate physiology and rated the importance and difficulty of each. The homeostasis conceptual framework was constructed as a guide for teaching and learning of this critical core concept in physiology, and it also paves the way for the development of a concept inventory for homeostasis.

there is growing consensus among life science faculty members that undergraduate biology education should help students develop a robust conceptual understanding of core concepts (4a, 6a). Core concepts or big ideas are those ideas that are central to the discipline and by definition are important and enduring. These ideas are used to explain and make predictions across a wide range of phenomena in a discipline (16). Each core concept integrates many different experimental findings and is the source of coherence for many theories in the discipline (12). Wiggins and McTighe (46) view big ideas as the building blocks from which meaningful patterns are constructed, connecting concepts within a discipline.

In the past decade, there have been many national and international efforts to identify and define the core concepts to guide undergraduate biology education (6a, 26, 29, 30, 33). The Vision and Change report (4a) reflects the consensus of over 500 faculty members from colleges, universities, and medical schools that five biology core concepts and six core competencies can help organize and lend meaning to the multitude of facts that students encounter in their undergraduate curriculum.

At a disciplinary level, a broad range of undergraduate and postgraduate physiology faculty members have agreed on a set of 15 core concepts for undergraduate physiology education (27), including homeostasis, structure/function, energy, flow down gradients, cell-cell communication, and others. The goals of physiology education are to help students develop deep conceptual understanding of key/critical core concepts and the ability to reason with and apply these concepts in real-world contexts. It is therefore incumbent on physiologists to create and use conceptual frameworks to help guide students in this task.

For many physiologists, homeostasis is the central core concept in physiology. For our purposes, homeostasis refers to the maintenance of a relatively stable internal environment by an animal in the face of a changing external environment and varying internal activity (31). This conceptual framework is limited to negative feedback mechanisms, because these are the types of homeostasis most often encountered in undergraduate physiology (31).

Conceptual Frameworks

A conceptual framework is a logical structure of a theory or model in a discipline (34, 49). Conceptual frameworks assist the student in developing and organizing sound explanations of core concepts (6); they are a way for students to organize generalized principles to make sense of them (49). Frameworks help learners “unpack” core concepts into a hierarchy of constituent ideas. This hierarchy of ideas helps students and instructors organize knowledge into logical structures, which, in turn, makes the knowledge more accessible for teaching and learning, easier to assess, and more meaningful (19, 38, 49). Frameworks help guide students to build connections between their prior knowledge and the new knowledge they are attempting to master, which, in turn, affords the new knowledge a greater chance of being integrated into existing cognitive frameworks (23, 34). Features of conceptual frameworks for student learning are shown in Table 1.

Table 1.

Applying conceptual frameworks: conceptual frameworks unpack core concepts into constituent ideas and reveal connections among these ideas as parts of a larger concept

A Conceptual Framework Can Serve Many Functions in:
Helping define and explain the discipline
Provides a hierarchical organizational structure of critical components and constituent ideas (18, 49).
Reveals connections between constituent ideas and provides a scaffold to make new connections as more complex ideas are introduced (34).
Integrates knowledge and understanding of interactions amongst core concepts (6, 18).
Helping students learn the discipline
Provides an important teaching and learning tool when the goal of learning is to use information rather than rote learning a list of facts as it gives a context and structure for the student (18).
Makes explicit the tacit knowledge or underlying assumptions of experts (faculty members) so that novices (students) can make sense of them and build a more transparent understanding (49).
Helping the instructor organize a course
Allows for the development of a learning progression using the frameworks: a lower level or simpler framework for an introductory course with additional information (constituent ideas) and more connections in graduate-level courses (11).
Can be used to inform and direct course, curriculum, and program design (19).

Conceptual Frameworks and Models

Both conceptual frameworks and physiological models are critical mental models for developing understanding. Physiologists use explicit models in their research and teaching (17, 49) to organize their understanding of complex concepts and processes, while their conceptual frameworks are well developed but implicit. Physiology education researchers and instructors recognize that both frameworks and models are explanatory and can be used to scaffold learning of core concepts for novices (21). During learning, students can construct and revise both frameworks and models based on new evidence or information.

Although frameworks and models are similar, they have distinct organization and purposes (49). Conceptual frameworks are descriptive and are organized hierarchically to describe components of a core concept and to identify the function of each component (Table 2). They are most often presented in a hierarchical, nested outline. Scientific models are abstract representations that can be used to explain phenomena and make predictions (17, 39). Mechanistic models of physiological processes are most often depicted as causal, flow diagrams. This report addresses the homeostasis conceptual framework, and a companion paper (31) has addressed a model for homeostasis for undergraduate physiology.

Table 2.

Comparison of conceptual frameworks and physiological models (17, 21, 31, 39, 49)

	Conceptual Framework	Physiological Model
Organization	Hierarchical	Causal
Format	Outline	Flow diagram
Type of representation	Descriptive	Abstract
Description	Phrases or sentences describing components, signals, and outputs	Words as labels on boxes or arrows representing components, signals, and outputs
Scaffolding	Can be scaffolded by adding constituent ideas to critical components in the hierarchical framework	Models are scaffolded by adding more complex components (e.g., gains in feedback loops) to a simpler, general model
Predictive	Not explicitly predictive	Can be used to make predictions
Expert to novice progression	Describes an appropriate scope of understanding for a particular stage on the novice to expert progression	Can be used to assess expert- or novice-level understanding

Conceptual Frameworks in Biology Education

Conceptual frameworks are a useful way to unpack core concepts. It is not surprising that some biology educators have produced them in conjunction with their efforts to define and assess student understanding of core concepts in biology, especially through the development of concept inventories. Khodor et al. (19) recognized the mutual interdependence of core concepts, conceptual frameworks, and the assessment of student learning in their description of the development of the biology concept framework. With the growth and proliferation of biology concept inventories (1, 5, 8, 10, 15, 20, 22, 36, 37, 40, 41, 45), it has become obligatory to identify the essential or core concepts within a particular domain of biology (4). However, most concept inventories appear to be constructed from lists of core concepts or learning goals and not from the hierarchical array of nested concepts that is characteristic of a conceptual framework. There are noteworthy exceptions. For example, the EvoDevo concept inventory was derived from a comprehensive, three-tiered framework of core concepts of developmental biology and evolution (18).

The Homeostasis Conceptual Framework

The purpose of this article is to present an undergraduate-level, hierarchical conceptual framework for the core concept of homeostasis and to describe the validation of that framework within a community of physiology faculty. The development of the homeostasis conceptual framework (HCF) began by reviewing the efforts of others to situate homeostasis as a core concept in physiology (14) and define its place within the general theory of organismal biology (49). Following the examples of other conceptual frameworks in biology (18, 19), we adopted a tiered structure that included the following three levels of hierarchy:

• The core concept for this framework is homeostasis.

• The critical components are aspects of the concept that are essential for building an accurate mental model of this core concept.

• The constituent ideas are elements that are necessary to develop a working understanding of each critical component of the core concept of homeostasis for undergraduate physiology education.

This conceptual framework is limited to the concept of homeostasis maintained by negative feedback mechanisms because these are the mechanisms most often encountered in undergraduate physiology (31). Furthermore, this framework specifically addresses homeostasis at the level of the organism (e.g., an animal) and does not address cellular or ecosystem homeostasis (31).

We deliberately limited the scope of the conceptual framework to include only critical components and constituent ideas underlying homeostasis that second- and third-year undergraduate students in the life sciences would be expected to understand. Even though the choice of this target demographic circumscribed the breadth and depth of concepts included in the framework, we were confident that the framework also included all of the concepts needed for a meaningful understanding of homeostasis by K–12 students as well as graduate and professional students.

METHODS

Faculty Involvement in the Construction of the HCF

For construct validation, the development of the HCF has been intentionally situated within a community of faculty members who teach physiology. Three groups of faculty members were involved with the creation and iterative revisions of the HCF: a project team, survey respondents, and conference participants.

The project team consisted of six faculty members (the authors of this article) from four different types of institutions of higher education; medical schools, liberal arts colleges, a research university, and a community college. The members of this project team have from 20 to >40 yr of experience teaching physiology as well as experience in physiology education research, science education, scholarship of teaching and learning, and biology faculty development. The project team has been working together for 4 yr to create and revise the framework.

The survey respondents are physiology faculty members who responded to online surveys concerning versions 2 and 3 of the HCF (HCF-2 and HCF-3) during the development of the framework (see Table 3). This group of faculty members provided extensive feedback, in a quasi-Delphi approach, to help vet and edit the conceptual framework as it was developed. Survey respondents were recruited from a broad range of institution types (2-yr colleges to professional schools) across the United States and one institution from the United Kingdom. This group of faculty members is acknowledged at the end of this article. Many of these physiology faculty members initially assisted in the identification of the 15 core principles for undergraduate physiology (27) and continue to be involved in the development of the homeostasis concept inventory.

Table 3.

Process of development of versions of the homeostasis conceptual framework

Participants	Timeframe	HCF Version
Project team	Summer-fall 2011	Produced HCF-1
Survey 1 respondents	Fall 2011	Vetted HCF-1
Project team	Winter 2012	Produced HCF-2
Conference participants	Spring 2012	Vetted HCF-2
Project team	Summer 2013	Produced HCF-3
Survey 2 respondents	Winter 2013	Vetted HCF-3
Conference participants	Spring 2013
Project team	Fall 2013	Produced the final HCF

HCF, homeostasis conceptual framework. HCF-1, HCF-2, and HCF-3 are versions 1, 2, and 3 of the HCF, respectively.

The conference participants are physiology faculty members from the United States and Canada who participated in and gave feedback during workshops, symposia, or poster sessions at national or regional professional meetings where members of the project team presented the most current version of the framework. These meetings included the Human Anatomy and Physiology Society (2012 and 2013), Experimental Biology (which includes the annual meeting of the American Physiological Society, 2012 and 2013), National Association Research in Science Teaching (2012 and 2013), North East Physiology Group, and Northwest Biology Instructors Organization (2012 and 2013).

Process of Development of the HCF

One of the first steps in the development of a conceptual framework is to determine what aspects of thinking about the topic are important to faculty members (4). This process was informed by the Delphi method used by Streveler et al. (44) when their research team identified fundamental concepts in thermal and transport sciences (35, 43). The Delphi technique specifically addresses and provides a method to avoid the problems associated with reliance on a single expert or a self-selected group when creating a “disciplinary standard.” The Delphi method requires that a large and varied group of physiology faculty “experts” be queried. Therefore, feedback on the HCF was solicited from survey respondents and conference participants several times over a 2-yr period (2011–2013), and this feedback was used to guide revisions. The process used to develop the HCF is shown in Table 3.

The conceptual framework was initially developed through several virtual and in-person roundtable discussions among the project team. The project team used the earlier unpacking of homeostasis into seven constituent ideas (26) as a starting point and relied heavily on their experience, prior publications (27, 29, 30, 32), and review of major undergraduate physiology textbooks to further refine the initial framework. At that time, we evaluated how 11 commonly used textbooks in undergraduate human anatomy and physiology (n = 4), human physiology (n = 2), animal physiology (n = 2), and introductory biology (n = 3) introduce and apply the core concept of homeostasis (available at http://physiologyconcepts.org/publications/). The textbooks were especially informative when selecting the appropriate terms to use in the framework. The end result of this initial phase of work was version 1 of the HCF (HCF-1).

HCF-1 was evaluated via online survey 1 (SurveyMonkey) to determine which items were “not important” or “essential” for students to understand (27). The following text preceded the Likert queries, to provide the survey respondents with our perspective on homeostasis in multicellular organisms:

Organisms maintain a relatively stable internal environment while living in a changing external environment. This process involves a negative feedback system that requires a sensor, a controller and effector(s).

Survey respondents were recruited from the 70 faculty members who responded to our core concepts survey (27). The 38 respondents ranked each element on a 5-point Likert scale from “not important” (1) to “essential” (5). If fewer than 30% of faculty members responding to this survey ranked an item as “essential,” that item was omitted from the next draft (HCF-2). Some items were also rewritten based on the comments of these respondents.

HCF-2 was taken to workshops and meetings in the spring of 2012, and comments were solicited from individual faculty members at poster sessions and talks and from small-group discussions at workshops. Based on the comments of these conference participants, the project team revised HCF-2 and created HCF-3 during the summer of 2012. That winter, all items (i.e., critical components and constituent ideas) in HCF-3 were assessed in online survey 2 for both importance and perceived difficulty (35). Respondents were instructed to first rank each item on a 5-point Likert scale from “not important” (1) to “essential” (5) and then rank their perception of the relative difficulty their students have in understanding each item on a 5-point Likert scale from “no one understands” (1) to “everyone understands” (5). We considered 3.0 to be the demarcation between easy and difficult items.

The purpose of survey 2 was twofold: to determine if each of these items was deemed important by physiology faculty members outside of the project team and to determine which of these items faculty members perceive are difficult for students to understand. The link to survey 2 was sent to 20 faculty members who were selected from survey 1 respondents and conference participants. Selection of these 20 faculty members was based on the type of institution (to have all institution types represented) and strong interest in this project (because this survey required twice as much time as survey 1). Fifteen faculty members provided usable responses to survey 2, including six faculty members who had responded to survey 1. Based on results from this survey as well as feedback from conference participants in 2013, the project team made further revisions to the framework to produce the final HCF.

Validation

Validation of the HCF required evidence that the critical components and constituent ideas within the framework satisfactorily describe homeostasis (i.e., represent a concept domain) (9) and that the framework contains elements appropriate for the development of conceptual understanding by undergraduate students. Validation is critical for the development of a conceptual framework that guides the construction of a concept inventory. Three types of validity were addressed for the HCF: face validity, content validity, and construct validity.

Face validity: clear and unambiguous for undergraduate students.

For this conceptual framework to be valid and useful, it must be easy to understand and unambiguous. The expression “face validity” has many meanings“ (9), and we focused on the ease of understanding of the framework as written. For face validation, faculty members involved with vetting the HCF were asked ”would you change the wording“ of the HCF and does it communicate the constituent ideas using accepted terms. In addition, 17 undergraduate students at 2- and 4-yr institutions revealed terms that were unfamiliar or confusing during read-aloud interviews of early versions of homeostasis concept inventory questions. We edited the wording of the concepts to reflect our understanding of the language students use to describe them.

Content validity: accurate and appropriate for undergraduate physiology.

The two online surveys of faculty members described above were used to test the claim that the framework addressed the undergraduate physiology domain and was accurate and complete. For content validation, we asked faculty members for feedback regarding the organization, comprehensiveness, and appropriateness of the HCF for undergraduate physiology.

Construct validity: important and relevant for undergraduate physiology.

Construct validity assures that the critical components and constituent ideas are important and relevant for student understanding of the concept of homeostasis. The expert judgment of the project team, survey respondents, and workshop participants was solicited regarding the importance of all items in the framework. Both online surveys tested the claim that this framework contains important ideas for undergraduate understanding of homeostasis.

RESULTS

Faculty Involvement

Through the two online surveys and several presentations at conferences and meetings, a broad range of faculty members were involved in the Delphi process used to create the HCF. In all, written or online feedback was received from 50 different physiology faculty members from 21 states, 2 Canadian provinces, and the United Kingdom on all versions of the HCF (see Table 4; some faculty participants gave feedback more than once). The conference participants and survey respondents teach at all major types of institutions of higher education (Table 4). In addition to those participants that gave written feedback, ∼30 conference participants who attended workshops or poster sessions in spring 2012 on HCF-2 and ∼90 conference participants who attended a workshop, symposium, or poster presentation on HCF-3 in spring 2013 had the opportunity to give oral feedback on these versions of the HCF.

Table 4.

Types of academic institutions of the faculty survey respondents and conference participants

Institution Type	Survey 1 (2011) HCF-1	Human Anatomy and Physiology Society (2012) HCF-2	Survey 2 (2013) HCF-3
2-yr community college	12	21	3
4-yr college granting only BS/BA degrees	4	3	1
4-yr institution granting BS/BA and some graduate degrees	10	6	6
Research university	5	4	3
Professional (medical) school	7	2	2
Number of faculty participants	38	36	15

Faculty survey respondents and conference participants teach physiology at different types of academic institutions in the United States, United Kingdom (n = 1), and Canada (n = 2). Faculty members who responded to surveys and those that participated in the workshop at the Human Anatomy and Physiology Society 2012 annual meeting were asked to name the institution where they teach. Participants at the other workshops were not asked for their institutional affiliation.

Results of Survey 1 on HCF-1

Greater than 80% of the 38 faculty respondents ranked all 5 critical components of HCF-1 as essential to the understanding of homeostasis for undergraduates (Table 5; percentages of ”essential“ responses are also shown). Of the 22 constituent ideas, 12 constituent ideas were ranked as essential by >50% of the respondents. The 5-point Likert response data from the survey are available at http://physiologyconcepts.org/publications/.

Table 5.

Survey 1 data on the “importance” of HCF-1 items

	Percentage of Faculty Members Who Ranked the Item as “Essential”
i. The organism maintains a more or less stable internal environment.	92
A. The organism's internal environment differs from its external environment.	66
B. The external environmental variables may change.	55
C. As the external environment changes, homeostatic processes maintain a more or less stable internal environment.	76
D. Many variables of the internal environment are maintained stable in order to sustain cell function.	76
E. Only a limited number of physiological variables are regulated.	14
I. A substantial change to a regulated variable will result in a physiological response to restore it toward to its normal range	84
A. The regulated variable is usually held stable by a negative feedback system.	79
B. The process of homeostasis requires a sensor, a control center, and an effector or effectors (the components of a negative feedback system).	82
C. In most instances, the compensatory response can only partially correct the disturbance to the regulated variable.	16
II. Homeostatic processes require a sensor (“what can't be measured can't be regulated”) [84%]	84
A. Sensors detect a the regulated variable and respond by transducing that stimulus into a different physiological signal.	61
B. A physiological system may use a variety of types of sensory receptors.	47
C. Sensory receptors may be in different, distant locations in the body.	29
III. Homeostatic processes require a control center.	84
A. The control center includes an integrator/comparator and a manipulator and directs effector(s).	37
B. The integrator/compartor receives a signal from the sensor.	50
C. The integrator/compartor integrates information from multiple sources regarding the normal range of the regulated variable.	42
D. Physiological systems have mechanisms to establish the normal range for a regulated variable (a so-called set point).	50
E. The integrator/compartor determines the difference between the signal from the sensors the normal range of the regulated variable.	40
F. The manipulator control center uses the this difference to change the activity of the effectors.	42
G. The control center may be distributed (different parts in different physical places).	13
IV. Homeostatic processes require effectors.	91
A. Effectors can be cells or tissues, typically muscles (smooth muscle, cardiac muscle, skeletal muscle) or glands.	82
B. The manipulated variables are the result of the action of the effectors.	53
C. Altering the manipulated variables (e.g., heart rate) results in changes in the regulated variables (e.g., blood pressure).	71
D. The values of the manipulated variables can fall within a large physiological range.	32

Based on feedback from faculty members, some words were added (in bold) and some words were omitted (strikethrough).

The results of the importance survey data from physiology faculty members in October 2011 led to the changes to HCF-1 shown in Table 5. The project team determined that if <30% of faculty members (∼11 faculty members) ranked an item as “essential,” that item would be omitted from or rewritten in the next version. Based on feedback from faculty members, some words were added (in bold in Table 5) and some words were omitted (strikethrough in Table 5). Four of twenty-two constituent ideas in HCF-1 fell into this category. Those four items were deemed accurate by respondents but were classified as not essential for undergraduate physiology. For example, the following is a constituent idea in HCF-1: “In most instances the compensatory response can only partially correct the disturbance to the regulated variable.” Only 16% of the faculty members ranked this component idea as “essential.” It was interesting to note that the faculty members at 2-yr colleges and medical schools were in agreement and none of them ranked this idea as “essential.” Therefore, this accurate idea was omitted from subsequent versions of the HCF. Three other constituent ideas were also deleted from the framework at this stage for similar reasons, as indicated by the strikethroughs in Table 5. Although removed from our HCF for second-year undergraduates, these items could be appropriate for a HCF in upper-division, graduate, or professional school courses.

Fifteen faculty members had substantive suggestions when asked “what changes would you suggest?,” which resulted in changes to the wording of the HCF (bold or strikethrough wording in Table 5). For example, the phrase “manipulated variable” in HCF-1 was omitted because most faculty members in the field did not use that term and, in student interviews, many students didn't know what this term meant.

Input From Conference Participants on HCF-2

Based on feedback from conference participants on HCF-2 in spring of 2012, a few new component ideas were added for clarity (for example, “The control center is part of the endocrine and/or nervous system”). The project team also continued to refine the wording of each item. By the end of 2012, HCF-3 had an expanded number of constituent ideas that resulted in a 30-item framework (Table 6).

Table 6.

Survey 2 data on “importance” and “difficulty” of HCF-3 items

HCF-3	Relative Importance*	Relative Perceived Difficulty†
I. The organism maintains a stable internal environment in the face of a fluctuating external environment.	5.00	3.31
A. The organism's internal environment differs from its external environment.	4.73	3.29
B. The external environmental variables may change.	4.60	3.14
C. A limited number of variables (i.e., regulated variables) of the internal environment are maintained stable via homeostatic processes in order to sustain cell function (if these variables change too much cells can not function normally and may die).	4.64	3.36
D. Not all variables that remain within a normal range over time are homeostatically regulated variables (e.g., blood hematocrit or testosterone).	3.57	2.93
E. Depending on the particular system, the regulated variable may be kept within a very narrow range or within a much wider range.	3.87	3.36
F. Homeostatic (i.e., regulatory) mechanisms operate all the time to determine the value of the regulated variable (they do not turn “on” or “off”; they are not like a “light switch,” they are like a “volume control knob”).	4.73	3.07
II. A substantial change to a regulated variable will result in a physiological response to restore it toward its normal range	4.87	3.14
A. The regulated variable is held stable by a negative feedback system.	5.00	3.29
B. Not all negative feedback systems are homeostatic.	3.53	3.38
C. The process of responding to a perturbation requires an action by a sensor, a control center, and an effector (the components of a negative feedback system).	4.93	3.14
D. The sensor, control center, and effectors may be physically far from or near to each other in the body and can even exist in the same cell.	4.33	3
III. Homeostatic processes require a sensor inside the body (“what can't be measured can't be regulated”).	4.67	2.86
A. Sensors detect the regulated variable and respond by transducing that stimulus into a different signal.	4.20	3
B. Sensors respond within a limited range of stimulus values.	3.73	3.14
C. Sensors generate an output whose value is proportional to the magnitude of the input to the sensor (i.e., the stimulus).	3.80	3.14
D. Sensors are constantly active (not just active when the regulated variable is not at the set point value).	4.27	2.93
E. An organ system may use a variety of types of sensors (e.g., chemoreceptors, baroreceptors, mechanoreceptors, etc.) to regulate variables associated with that organ system.	4.40	3.14
IV. Homeostatic processes require a control center (which includes an integrator).	4.86	3.07
A. The control center is part of the endocrine and/or nervous system.	4.33	2.86
B. The integrator receives a signal from the sensor.	4.67	2.71
C. The integrator is a component of the control center.	4.53	2.93
D. Physiological systems have a normal range for a regulated variable (a so-called set point).	4.93	3.08
E. The integrator determines the difference between the signal from the sensors and the set point (i.e., the normal range of the regulated variable).	4.53	2.86
F. The value of the difference (between the signal from the sensor and the set point) is used by the control center to calculate a change in the signals going to the effectors (i.e., targets).	4.20	2.93
G. It is possible in some circumstances and in some systems for the set point to change.	3.93	3.14
V. Homeostatic processes require target organs or tissues, i.e., “effectors.”	4.87	3.15
A. Physiological targets or effectors are cells, tissues, or organs (unlike “effector molecules” in biochemistry).	4.71	2.86
B. The action of the targets (i.e., effectors) cause physical or chemical changes that alter the regulated variable.	4.67	3
C. Effectors result in changes in nonregulated variables that in turn alter the regulated variable (e.g., the regulated variable, blood pressure, can be changed by altering heart rate and peripheral resistance, which are not regulated).	4.27	3.29

Likert scale rankings were assigned values from 1 to 5, and these were averaged for comparison of items within the framework to assess relative importance and difficulty.

^*Relative importance is the average of 5-point Likert scale from “essential” (5) to “not important” (1).

^†Relative difficulty is the average of a 5-point Likert scale from “everyone understands” (5) to “no one understands” (1).

Results of Survey 2 on HCF-3: Importance

The fifteen faculty respondents of survey 2 unanimously scored the first critical component as essential (score of 5) and gave the four other critical components an average score of 4.6 or higher. This reinforced the previous survey results indicating the importance of all five of the critical components. Average scores for the constituent ideas ranged from 3.53 to 5.00, confirming the value of including these items in the framework.

The HCF

The validated, final version of the conceptual framework for homeostasis is shown in Table 7.

Table 7.

Conceptual framework for homeostasis¹

H1. The organism maintains a stable internal environment in the face of fluctuating external environment.

H1.1. The organism's internal environment differs from its external environment.

H1.2. As the external environment changes, homeostatic processes maintain a more or less stable internal environment.

H1.3. If homeostatic variables change too much, cells cannot function normally and may die.

H1.4. A limited number of variables (i.e., regulated variables) of the internal environment are maintained stable via homeostatic processes in order to sustain cell function.3

H1.5. Some variables remain within a normal range over time but are not homeostatically regulated variables (e.g., blood hematocrit or testosterone).

H1.6. Depending on the particular system, the regulated variable may be kept within a very narrow range or within a much wider range.

H1.7. Homeostatic (i.e., regulatory) mechanisms operate all the time to determine the value of the regulated variable.

H1.8. Homeostatic mechanisms depend on resources in the external environment, which may limit the ability of the negative feedback to restore a variable to its normal range.

H2. Any change to a regulated variable (a perturbation) that results in an error signal will result in a physiological response to restore the regulated variable toward to its normal range.

H2.1. The regulated variable is held stable by a negative feedback system.

H2.2. Not all negative feedback systems are homeostatic.

H2.3. The process of responding to a perturbation requires an action by a sensor, a control center, and an effector (the components of a negative feedback system).

H2.4. The sensor, control center, and effectors may be physically far from or near to each other in the body and can even exist in the same cell.

H3. Homeostatic processes require a sensor inside the body (“what can't be measured can't be regulated”).

H3.1. Sensors detect the regulated variable and respond by transducing that stimulus into a different signal.

H3.2. Sensors respond within a limited range of stimulus values.

H3.3. Sensors generate an output whose value is proportional to the magnitude of the input to the sensor (i.e., the stimulus).

H3.4. Sensors are constantly active (not just active when the regulated variable is not at the set point value or outside of a “normal” range).

H3.5. An organ system may employ a variety of types of sensors (e.g., chemoreceptors, baroreceptors, mechanoreceptors, etc.) to regulate variables associated with that organ system.

H4. Homeostatic processes require a control center (which includes an integrator).

H4.1. The control center is part of the endocrine and/or nervous system.

H4.2. The integrator receives a signal from the sensor.

H4.3. The integrator is a component of the control center.

H4.4. Physiological systems have a normal range for a regulated variable (a so-called set point).

H4.5. The integrator continuously determines the difference between the signal from the sensors the set point (i.e., the normal range of the regulated variable).

H4.6. The value of the difference (between the signal from the sensor and the set point) is used by the control center to calculate a change in the signals going to the effectors (i.e., targets).

H4.7. It is possible in some circumstances and in some systems for the set point to change.

H5. Homeostatic processes require “effectors” or target organs or tissues.

H5.1. Physiological targets or effectors are cells, tissues, or organs (unlike “effector molecules” in biochemistry).

H5.2. The action of the effectors or targets is the physiological response that results in physical or chemical changes in the internal environment.

H5.3. The response of the effectors determines the value of the regulated variable.

H5.4. The response of the effectors can result in changes in nonregulated variables, which, in turn, alter the regulated variable.4

Organisms maintain a relatively stable internal environment while living in a changing external environment and varying internal activity. Variables that are homeostatically regulated are necessary for the survival of cells and the life of the organism. Homeostasis is a dynamic process that involves a negative feedback system(s) that requires a sensor, a controller (integrator), and effector(s) (targets). Although other mechanisms may contribute to homeostasis (including feedforward processes), this conceptual framework is limited to the concept of homeostasis via negative feedback mechanisms, because these are most often encountered in undergraduate physiology. This framework specifically addresses homeostasis at the level of the organism (e.g., an animal) and does not address cellular or ecosystem homeostasis. ¹This is the final version of the HCF. Previous versions are shown in Tables 5 and 6. ²From Ref. 31. ³The following are widely recognized and clearly established as homeostatically regulated variables in humans: ions (H⁺, Ca²⁺, K⁺) in extracellular fluid (not blood), blood CO₂, O₂, blood glucose, blood pressure, blood osmolarity, and core body temperature (31). ⁴For example, blood pressure is a regulated variable that can be changed by altering heart rate and peripheral resistance, which are not homeostatically regulated.

Validation

Several undergraduate physiology textbooks were examined by this project team, and the terms commonly used in these textbooks, and that were acceptable to faculty respondents (28, 31), were then used in this conceptual framework (Table 7). Flesch-Kincaid readability was grade level 12.0, as assessed by MS Word.

Results of Survey 2 on HCF-3: Difficulty

When scoring the 30 items on a Likert scale of a “very difficult” (1) to “very easy” (5), the average faculty score for 12 of the 30 items was ≤3.0, which is the demarcation between easy and difficult items. Therefore, on average, faculty members thought that 12 of the items were challenging for students to master, whereas 18 items were classified on average as easier for students to learn. Only one critical component, “Homeostatic processes require a sensor inside the body (‘what can't be measured can't be regulated'),” received an average difficulty score below 3.0. Faculty members indicated that “Homeostatic processes require a control center (which includes an integrator)” was the most challenging for students to master, as five of the seven constituent ideas associated with that component had average scores below 3.0.

Figure 1 shows the relationship between the importance of an item and its difficulty. All items were considered important (>3.0) and ranged in difficulty from an average of 2.86 to 3.38 (Table 6). Some items were thought essential and relatively easy, for example, component H2.1 (Table 7), “The regulated variable is held stable by a negative feedback system,” is essential (5 on a 5-point scale, 5/5) and relatively easier to understand (3.29/5). Other items were judged important and more difficult, e.g., component H3 (Table 7), “Homeostatic processes require a sensor inside the body․ ․ ․ ,” is important (4.67/5) and relatively difficult to understand (2.86/5).

Fig. 1.

Perception of difficulty and importance based on survey 2 results from 15 physiology faculty members on version 3 of the homeostasis conceptual framework (HCF-3; Table 6). Faculty members used a 5-point Likert scale to assess the importance and perceived difficulty of the items in HCF-3. Component H3 is a critical component (Table 7), whereas component H2.1 is a constituent idea (Table 7). The y-axis shows difficulty, which can be thought of “ease of understanding” with “easy” at the top and more difficult below. The x-axis represents importance, where 5.0 is essential and all items are above 3.0.

DISCUSSION

The HCF

A broad and diverse community of physiology faculty members participated in the development and validation of the HCF for undergraduate physiology.

The framework is arranged to bring out the logical relations and essential cohesion between the critical components and constituent ideas of homeostasis. The framework begins with an overview of homeostasis that presents a broadly recognized definition of this core concept and unpacks it into a hierarchy of critical components and constituent ideas. The overview also identifies important boundaries to the framework (e.g., the limitation of negative feedback mechanisms and to an organismal level of analysis) that help specify the knowledge domain that is being addressed.

The first critical component (component H1) develops the central principle of a stable internal environment. This principle underscores the important contribution that Claude Bernard's theory of the milieu intérieur has made to our understanding of the functioning of multicellular organisms (7, 31). The framework proceeds with a second critical component (component H2) that defines the role of a regulated variable within a normal range. This second critical component makes explicit its dependence on the general model of negative feedback (31, 32). The last three critical components (components H3–H5) unpack the essential elements of a physiological regulatory system in the cause-and-effect order within a regulatory pathway: sensor → control center → effector (31).

Each critical component is stated as a requirement to show its necessity for the proper operation of a homeostatic system. Where appropriate, the underlying theory is highlighted (e.g., “what can't be measured can't be regulated”) and clear distinctions in related concepts are made (e.g., regulated vs. nonregulated variables). Throughout, there is an attempt to minimize specifics (i.e., discrete facts, particular entities, and examples). However, in some cases, these were added to provide clarity (e.g., examples of sensors and the endocrine and nervous systems) or enhance definition (e.g., effector biomolecules).

Importance Versus Perceived Difficulty

All of the critical components were judged to be essential for physiology students to gain a meaningful understanding of homeostasis. Although all items in the final framework (HCF) were deemed important by the project team and survey respondents, some items are more important than others (Fig. 1 and Table 6). Furthermore, faculty members recognized that some of the constituent ideas are more difficult for students to master (Fig. 1 and Table 6). Early testing and student interviews with questions for a homeostasis concept inventory confirmed that some ideas are easier for students than others (25, 48). Given the limited time available in undergraduate physiology courses and the ever-increasing content in textbooks, these data suggest that classroom instruction should focus on items that are both important and difficult for students to understand. As the homeostasis concept inventory is validated and tested (24), actual (vs. perceived) student difficulty with aspects of the conceptual framework can be verified.

Validation

The results of faculty surveys and Flesch-Kincaid readability test demonstrated that the language of this conceptual framework is clear and unambiguous and thus has face validity. Faculty surveys and feedback from conferences were used to refine the framework through multiple revisions to ensure content validity (that it is accurate and appropriate) and construct validity (that it is important and relevant) for undergraduate physiology.

Alignment With Identified Core Concepts for Undergraduate Biology

The HCF presented here aligns with and builds on reports calling for reform in undergraduate life science education. Participants at three earlier National Science Foundation-funded meetings on Conceptual Assessment in Biology (26, 29) identified homeostasis as a core principle in biological science. The HCF explicitly addresses the first of the medical school competencies (competency M1) in the Scientific Foundations for Future Physicians report (6a) and one of the competencies for undergraduate “premed” education (competency E7). Furthermore, the HCF aligns with the core concept of “systems” and the core competency of “modeling” in the Vision and Change report (4a).

Limitations

This conceptual framework was developed within specific boundaries, which narrowed the focus but allows for adaptation and expansion. As presented, the framework is constrained in four ways.

First, this framework is focused on undergraduate physiology. This focus was informed by the authors' experience with undergraduate physiology education and by the input from many other physiology faculty members. There were aspects of homeostasis that were not included in the framework because they were considered not critical for student understanding at this level. The framework can be adapted for other levels (graduate and medical education) by including these and other, additional constituent ideas.

Second, although other mechanisms may contribute to homeostasis, this conceptual framework is limited to the concept of homeostasis via negative feedback mechanisms, because these are most often encountered in undergraduate physiology (31). Even though Woods and Ramsay (47) have called for “research that encompasses a broad view of homeostasis that goes beyond simple feedback circuit models” and encompass anticipatory processes, we believe this broader view of homeostasis is beyond the scope of this framework. It is, however, critical that students understand that negative feedback is not the sole biological control mechanism and that they do not equate homeostasis and negative feedback (31, 47).

Third, this framework specifically addresses homeostasis at the level of the organism (e.g., an animal) and does not address cellular or ecosystem homeostasis. The term “homeostasis” has been used to describe maintaining equilibrium at other levels of biological organization, e.g., inside individual cells and within ecosystems. For example, one instance of “cellular homeostasis” is the “maintenance of a constant volume in the face of extracellular and intracellular osmotic perturbations” mediated by phosphorylation and dephosphorylation reactions within intracellular signaling pathways (42). On a larger scale, environmental homeostasis “suggests there are certain differentiated processes or mechanisms operating within the Earth system that are able to oppose perturbations in such ways as to reduce their impacts” (13). In distinction, the HCF for undergraduate physiology describes homeostasis at the level of the individual organism, often mediated by more than one organ system (31).

Finally, this framework is written from the perspective of animal physiology and mammalian physiology in particular. Although this framework applies to animal physiology in general, this work has been primarily informed by teaching and learning in mammalian (including human) physiology.

Applications of the HCF

There are several practical applications of the HCF for guiding in teaching, learning, and curriculum development (see Table 8).

Table 8.

Applications of the HCF for teaching and learning

The HCF Can Be Used
By departments to
Inform and direct physiology course design.
Provide a hierarchical organizational structure for undergraduate physiology, with additional constituent ideas for graduate level physiology.
By faculty members to
Reveal connections between critical components (e.g., sensors and effectors) and constituent ideas in a hierarchical organizational structure that makes the relationships between these ideas explicit.
Make explicit the tacit knowledge or underlying assumptions of experts (faculty members) so that novices (students) can make sense of them and build a more transparent understanding. For example, some regulated variables are kept within a very narrow range, whereas others have a much wider normal range.
Focus active learning exercises on those ideas that are essential and difficult to master.
Constructively align student's learning outcomes with their assessment.
By students to
Construct their understanding and build an accurate mental framework of homeostatic processes.
Use as a framework to identify homeostastic processes as they are encountered across different organ systems.
Distinguish between homeostastic and nonhomeostastic physiological processes.

Conclusions

The original purpose of the framework was to form a basis for developing a homeostasis concept inventory for undergraduate physiology. However, a validated conceptual framework for undergraduate homeostasis is in and of itself a valuable tool for physiology teaching and learning. The process of rigorous validation was critical to insure that the items of this framework were clear and unambiguous (face validity), accurate and appropriate (content validity), and important and relevant (construct validity) to undergraduate physiology. Furthermore, the development of this framework became a powerful way for a large and diverse community of physiology faculty members to reflect on how we, as faculty members, think about this important core concept. This process can and likely should be applied to other core concepts in physiology.

GRANTS

This work was supported by National Science Foundation Grant DUE-1043443.

DISCLAIMERS

Any results, conclusions or recommendations expressed in this article are those of the authors and do not necessarily reflect the views of the National Science Foundation.

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the author(s).

AUTHOR CONTRIBUTIONS

Author contributions: J. McFarland, M.P.W., J. Michael, W.H.C., A.W., and H.I.M. conception and design of research; J. McFarland, M.P.W., J. Michael, and A.W. performed experiments; J. McFarland, M.P.W., J. Michael, W.H.C., and A.W. analyzed data; J. McFarland, M.P.W., J. Michael, W.H.C., A.W., and H.I.M. interpreted results of experiments; J. McFarland and M.P.W. prepared figures; J. McFarland, M.P.W., and A.W. drafted manuscript; J. McFarland, M.P.W., J. Michael, W.H.C., A.W., and H.I.M. edited and revised manuscript; J. McFarland, M.P.W., J. Michael, W.H.C., A.W., and H.I.M. approved final version of manuscript.