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A CHAPTER and Physiology Introduction to the Chapter Homeostasis, the constancy of the extracellular milieu, is the central unifying concept in physiology. Homeostasis is maintained by negative feedback control mechanisms. These two concepts are both abstract and unfamiliar to students and thus often difficult to grasp. The following “thought experiment” should help make both these concepts more concrete. First, make clear that cells are the basic units that make up organisms, and that although their internal workings are complex, individual cells are entirely dependent on the rest of the body to supply their needs. Then, tell the students that cells do best when the fluid that surrounds them provides an approximately constant set of conditions. Tell them that an essential tool of modern biotechnology is the ability to grow cells outside the body under artificially maintained conditions. Ask the class to imagine that they have been hired as biotechnologists to set up a system for growing cells. Guide their thinking by pointing out that cells must be kept in an aqueous environment. Ask the students if the aqueous solution must have any special properties. Go to the board and develop a list of conditions in the cell surroundings that must be kept relatively constant. They should easily come up with temperature, oxygen, and nutrient levels. You could coax them to include pH, removal of carbon dioxide and other metabolic wastes, osmotic pressure, particular ions, vitamins, and so on. Pick a parameter that should be held reasonably constant, such as the level of glucose, in the culture medium. Since cells consume glucose, it must be replaced as fast as it is consumed. What will be needed to accomplish this constancy of the cell’s environment (homeostasis)? First, a probe is required that measures glucose levels (a sensor or receptor); next, a controller that takes the reading from the probe and compares it to the preset desired level (the set point); finally, a dispenser that adds glucose to the bath in the amount needed to restore the desired level (the effector). Tell them that these components form a negative feedback loop. It is designated “negative,” not because it is bad, but because this arrangement will remove or negate the difference between the set point and the actual level. The term “feedback” is used because the error is fed back from the controller to the effector. To make the point that disease often results from a failure of homeostatic regulation, point to diabetes mellitus in which blood levels of glucose are far too high because some component of the negative feedback loop is faulty. Chapter Learning Outcomes 1.1 Describe the universal characteristics of living things. 1.2 Define anatomy and physiology, and describe macroscopic and microscopic anatomy. 1.3 Explain the relationship between structure and function. 1.4 Describe various types of cells in the human body and explain the basic principles of the cell theory. 1.5 Define histology and explain the interrelationships among the various types of tissues. 1.6 Identify the 11 organ systems of the human body, and describe the major functions of each. 1.7 Explain the concept of homeostasis and discuss the roles of negative feedback and positive feedback in maintaining homeostasis. 1.8 Use correct anatomical terms to describe superficial and regional anatomy. 1.9 Use correct directional terms and sectional planes to describe relative positions and relationships among body parts. 1.10 Identify the major body cavities and the subdivisions of each. Teaching Strategies 1. Encouraging Student Talk a. At the beginning of this topic, have students discuss the following question in pairs: “In what ways does an office building exhibit, or fail to exhibit, the characteristics shared by all living things?” Randomly select some students to report on their pair discussions. Students frequently reveal naïve ideas about the characteristics shared by living things (for example, that living things must think, talk, or eat). Ask students if they can
2. Lecture Ideas & Points to Emphasize a. Discuss some examples to stress the relationship between anatomy (structure) and
b. Use a variety of examples to demonstrate different types of feedback loops. Begin with the examples in Module 1.7, then expand to discuss how negative vs. positive feedback is demonstrated at multiple scales in the human body (for example, blood sugar regulation, childbirth, bone growth/resorption based on levels of pressure applied to the bone, cancerous cell division, and so on). Ask students whether they think these examples show negative or positive feedback. Emphasize that the response in a feedback loop directly influences the original stimulus. c. Note that homeostatic set points change from moment to moment and throughout life. Ask students how they think various internal set points might change when we exercise or when we contract an infection. d. Discuss the need for a common language of anatomy by pointing out the impacts of communication on the outcomes for patients. Show news articles or other reports discussing the correlation between poor communication and medical errors. Tell students they will begin practicing this new language right away, so that they can try to avoid miscommunications in their careers. e. Sectional anatomy is challenging for students to visualize. It is wise to explicitly acknowledge this difficulty. Start by stating that the process of sectioning reduces a three-dimensional object to a two-dimensional artifact. Have them do several drawing exercises to reinforce the visualization process. Elaborate on Module 1.9, pointing out how it takes thought and imagination to identify familiar bones and organs in these sections. Radiologists struggle with this sort of anatomical problem on a daily basis. Use examples of common items such as pasta shapes (compare spaghetti to elbow macaroni and lasagna noodles). Encourage students to draw the image that would result from various planes of sections. Point out that sections of the human body produce two parts: A sagittal section produces a right and a left piece; a transverse section produces a superior and an inferior piece; a frontal section produces an anterior and a posterior piece. Use these terms to give them practice in hearing and using these directional tags. f. Body cavities are lined with membranes that secrete a serous (watery) fluid. Draw an example on the board, such as the heart in the pericardial cavity or the lung within the pleural cavity. Label and describe the difference between the parietal and visceral membranes. Illustrate that the parietal membrane lines the cavity that contains the organ, while the visceral membrane covers the surface of the organ (viscera, organs within a body cavity), and the cavity is the space between the visceral and parietal membranes. Ask them to guess what function the watery fluid might serve. Help them to understand its function of by showing YouTube videos or animations of the vigorous motions of a beating heart within the thoracic cavity surrounded by the slowly moving lungs. g. When discussing the organization of the text, point out that major module headings are complete sentences. This helps to prepare the reader for the information that follows. h. Share tips that former students have given for succeeding in your class, or discuss the tips in the table at the start of Section 1. Note that there are many different ways to succeed in an A&P course, and that students should experiment with a wide variety of study techniques and memorization strategies to find the tools most useful to them. Many students continue employing ineffective study strategies because they have never been encouraged or taught how to develop other tools. Point out that many resources are available to help them develop study strategies that work for them. They can use coloring books, YouTube videos, mnemonic associations, analogies, concept mapping, diagramming, and any other techniques that produce the results they desire. Numerous books exist specifically to help A&P students study the material (for example, Get Ready for A&P). Note that trying to explain the material to others (classmates, friends, family members, coworkers) is a technique that works well for almost every student. 3. Making Learning Active a. As an alternative to a traditional lecture on feedback loops, have students demonstrate the components and actions of positive vs. negative feedback loops in front of the class. Use string to connect a thermometer, a thermostat from a hardware store, a hand fan, and a matchbox. Randomly select a student to hold each prop. Call on other students to help label the props as receptor, control center, effector, afferent pathway, and
4. Analogies a. Use viruses as a model for understanding the characteristics of life, the cellular level of organization, and the existence of “gray areas” in biology. Briefly describe the characteristics and behavior of viruses. Note that most biologists consider viruses nonliving, since they lack cells and rely on hosts to perform most of the characteristics of living things. However, viruses share certain similarities with cells, and even clearly “alive” things rely on external influences to fulfil all the characteristics of life. Students can
b. As an analogy for homeostasis, imagine a circus performer perched on a board balanced on a ball. As the performer balances, he never holds perfectly still; his outstretched arms are continuously moving up and down, and his weight is continuously shifting from one leg to the other. He relies on input from several senses (the sensors) to stay upright (the set point) by using his muscles (the effectors) to remain balanced. That is, he uses his feedback control system to negate the balancing errors; thus, this constitutes a negative feedback loop. If his muscles were fatigued and the compensating movements occurred too late, this would work against him. The weakness of the effectors would lead to a positive feedback loop: his efforts to keep things in balance lead to the opposite outcome, a worse disturbance, and a fall. Often, this is the course of disease. c. As an analogy to the internal environment for cells (interstitial fluid), have the students imagine fish living in an aquarium. State that fish are living units like cells, complex on the inside, but helpless on the outside. They need certain things in their environment to be held within a satisfactory range by external mechanisms. Review what these factors are and relate them to body function. For example, adding oxygen and removing carbon dioxide; pumping fluids; making nutrients available; keeping osmotic pressure right; holding the pH in range; removing ammonia (nitrogen waste); suppressing bacterial growth; and so on. Of course, the keeper of the fish is the sensor, the control center, and the effector all in one. 5. Demonstrations a. Ask students to consider the changes that would happen to their muscles if they
b. To demonstrate the importance of a common anatomical language, ask students how many different ways they can think of to describe the location of the fingers compared to the location of the belly button, or the location of the spine compared to the heart. Similarly, you can place a dot on an outline of the human body and ask students to describe its location. Students should realize that many common terms are ambiguous (below, above, up, down) depending on the position of the body and each individual’s interpretations of those words. Similarly, colloquial terms for the body cavities (gut, chest, stomach, and so on) can lead to confusion. This means we must have a common vocabulary and means of positioning the body in anatomy. c. The ability to infer function from form (or form from function) is a useful skill for A&P courses. To demonstrate this, you can show pictures of antique devices or surgical tools that students are likely unfamiliar with. Though they might not know exactly what the items are supposed to be used for, students will still be able to guess many reasonable functions for those devices/tools. Point out that students should take a similarly creative stance when examining a new organelle, tissue, or organ. They can make predictions about form and function from the outset and then use the book to check their predictions against the actual forms and functions of cells or body parts. 6. Applications a. Almost any disease or condition would provide an excellent opportunity to apply basic concepts of homeostasis and feedback loops. Without delving into the detailed physiology of specific organ systems, students could discuss the general physiological
7. Common Student Misconceptions & Problems a. Students often have difficulty understanding the broad cellular and chemical characteristics shared by all living things. Instead, students tend to use gross external characteristics to describe life. They are likely to identify features such as talking, thinking, showing emotions, and having a nose/mouth/arms/legs as the common characteristics of life. Help students recognize the need for more broad definitions of life, since many living things—including living humans—lack those less broad characteristics. In
b. Students should understand that homeostasis is a dynamic process, constantly fluctuating within normal limits. The literal meaning of homeostasis is “staying the same,” but we never are physiologically in a state of “unchanging sameness” unless we’re dead! For this reason, it is important that students be aware of “normal range” when learning physiological parameters like pH of blood, RBC count, or heart rate. Only when a controlled variable moves out of range will a compensatory response be made. c. Students similarly often assume that any sustained deviation from a “normal” set point or range must involve positive feedback or a harmful alteration to the body’s internal environment. Fever, in particular, is assumed to be positive feedback, since it involves an increase in body temperature beyond the normal range. In fever, though, the set point changes and the body performs negative feedback around that new, higher set point. Should the fever continue to go inappropriately higher and higher, that would indeed represent unhealthy positive feedback. Point out the many reasons set points might have to change from moment to moment or throughout the life of an individual and how negative feedback still tends to be used at those altered set points. d. Given the informal uses of the words “positive” and “negative,” it is often difficult for students to identify positive vs. negative feedback when given an applied scenario. Some students assume positive feedback is good for us and negative feedback is bad. Even more commonly, students think positive feedback involves an increase in some variable in the body while negative feedback involves a decrease in a variable. Provide specific examples where uninterrupted positive feedback is quite harmful to the body (e.g., excessive clotting of blood). Similarly, explicitly describe examples of negative feedback where the responding variable changes to be higher (e.g., when our body temperature drops and we must raise it to maintain homeostasis). Finally, you can
e. Students sometimes confuse the ways we use the names of body cavities and surface regions with the ways we use directional terms. If asked to describe the location of the navel using anatomical terms, they might refer to the figures in Modules 1.8–1.10 and describe it as “gluteus to the neck” or “ventral of the medial.” Give students plenty of opportunities to practice these unfamiliar terms by describing various points on the body using a directional term in relation to a body region. f. The mediastinum is not a cavity, but rather a region within the thoracic cavity. The mediastinum contains the pericardial cavity and is flanked by the pleural cavities. 8. Terminology Aids a. Encourage the students to find the anatomical landmarks and regions on their own bodies while they study the terms in the text. Relative anatomical directions are more easily mastered if they are studied in pairs. Try this: Give one of a pair of relative anatomic terms in class. Point to it. Say it. Now ask them to say out loud the word it contrasts with (e.g., anterior posterior) and point to it. Encourage the use of the text in class. Lead them through a few examples, and then recommend that they do them again and again at home until they have mastered these first pieces of anatomical lingo, the first tools in their professional toolbox. b. The word parietal (the wall side) sounds like, but doesn’t come from perimeter (outer boundary). Contrast this with visceral, related to viscera, meaning the “guts.” c. “Serous” sounds like “serum”—the watery component of blood. d. Point out that the combining form–stat in thermostat, is also found in the word
e. Use the examples of “inferior” and “superior” to make the point that often in anatomy and physiology, common words are used in a specialized sense, different from their common sense meaning. 9. Incorporating Diversity & the Human Side of A&P a. Efforts to name and describe the parts of the human body have spanned thousands of years and crossed continents. Such efforts, which continue through today, demonstrate the fundamental need for a common anatomical language in the medical sciences. In introducing anatomical nomenclature, you might reference the long arc of human history devoted to the language of anatomy. African papyri dating to the 16th century BC provide some of the first written lists of anatomical terms. Egyptian healers made early accounts of anatomical features, ailments, and treatments. Additional major works came between the first few centuries BC–AD from Greek physicians, like Herophilus and Galen. Many Greek physicians borrowed from Egyptian knowledge and in fact performed their studies in Egypt due to cultural prohibitions on dissections and other such studies in Greece. Numerous Middle Eastern physicians, like Avicenna and Ibn al-Nafis, contributed works in the following centuries. In modern times, health scientists have attempted to standardize terminology in works such as Nomina Anatomica and Terminologia Anatomica, which reported terminological updates as recently as 1998. As knowledge of A&P has changed, new discoveries inspired debate over the terms used to describe the body. Certainly many more names and publications could be added to the list above, which could provide the backdrop to this content or form the basis for further student research. References/Additional Information: http://www.nlm.nih.gov/onlineexhibitions.html http://www.nlm.nih.gov/dreamanatomy/da_timeline_anatomy.html http://puffin.creighton.edu/museums/greiner/ http://www.mnsu.edu/emuseum/prehistory/egypt/dailylife/medicine.html http://www.med.wayne.edu/imsa/Islam%20and%20Medicine.html
Mary was excited when she first felt regular contractions, signalling that she would soon meet her new baby. Few physiological activities in the human body are so outwardly awe-inspiring as the birth of a child, particularly for the child’s parents! This process usually occurs vaginally in a process referred to as “labor.” Mary had read that during labor, her body would go through a number of changes to allow the baby to safely exit her uterus and vagina. She knew that labor would begin with the baby’s head stretching the exit to her uterus, called the cervix. Nerve cells in her body would sense this stretching and transmit the message to her brain. Her brain would then produce hormones to stimulate contractions in her uterus, which would further stretch her cervix. That process, which Mary knew had just started in her body, would continue until her cervix was stretched wide enough for her baby to exit. While Mary hoped her child would be born vaginally, as is normally the case, she also
Questions 1. Identify the components of the feedback loop involved in the process of labor. Would this example constitute negative or positive feedback, and why? 2. Create a series of labeled diagrams to interpret the surgical procedures described above for a Caesarean section. In your diagrams, indicate the locations of the cuts made and show the relative locations of the uterus, bladder, and rectum. 3. Given the location of a Caesarean section surgery, what are the names and characteristics of the membranes you think surgeons must cut through in order to view the uterus? Suggested Answers 1. This would be a positive feedback loop, since the response to the stimulus serves to exaggerate the change in conditions. Stretching of cervix by baby = stimulus; uterine nerve cells = receptors; brain = control center; uterine muscles = effectors. 2. Diagrams show a transverse (left/right) incision on the belly below the hipbones. Uterus is deep (behind) bladder and superficial to (above) rectum. Transverse (left/right) cut is in uterus near edge of bladder. 3. The peritoneal membranes—serous membranes—would be cut. Copyright © 2011 Pearson Education, Inc. Yüklə 49,56 Kb. Dostları ilə paylaş:
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