A Friend in Need Is a Friend Indeed: A Case Study on Human Respiratory Physiology
Cliff, William H., Wright, Ann W., Journal of College Science Teaching
The dynamics of oxygen transport by the blood can be a particularly difficult topic for students to grasp. A directed case study about carbon monoxide poisoning was designed to help deepen student understanding of the solution chemistry of the oxygen-hemoglobin reaction and the role that hemoglobin plays in external respiration.
One of the greatest challenges students face in learning about the physiology of oxygen handling by the human body is developing a coherent mental model of how oxygen enters and leaves the blood and how it is transported from the lungs to the tissues. To do this successfully, students must combine their notions of how oxygen behaves free in the alveolar air, dissolved in the plasma, and in reaction with hemoglobin. This requires a facility with solution chemistry (including the concepts of partial pressure, solubility, and concentration) as well as an understanding of chemical equilibrium (including the concepts of mass action, reversible binding, affinity, and saturation). Integrating all of these chemical concepts into an understanding of human respiratory physiology can be a daunting task.
This case study (see Figure 1, page 38) was designed to help students strengthen their understanding of the transport of [O.sub.2] in the blood through an analysis of the pathophysiology of a common, real-world problem--CO poisoning. The case promotes learning about the chemistry of the oxygen-hemoglobin reaction by asking students to determine the extent that CO poisoning reduces the amount of oxygen delivered to the body tissues. It also helps students solidify their understanding of the role that hemoglobin plays in external respiration by prompting them to localize where CO poisoning disrupts this process.
The case was developed for use by undergraduates majoring in biology, nursing, and education who have had instruction in first-year, college-level sciences and are enrolled in a two-semester course in human anatomy and physiology. These students are typically sophomores who have successfully passed a year of general chemistry or a one-semester survey of chemistry for non-science majors.
The following learning objectives guided the design of the case and its use in class:
* Properly distinguish [P.sub.[O.sub.2]] (as a measure of the concentration of free oxygen in the plasma) from percent [O.sub.2] saturation (as a measure of the binding of oxygen to hemoglobin); be able to predict how changes in the function of hemoglobin might alter these measures in light of the mass action of the oxygen-hemoglobin reaction and the exchange of oxygen between the blood, alveolar air, and body tissues.
* Use the oxygen-hemoglobin dissociation curve to determine the amount of oxygen that is transported in the blood and delivered to the tissues; be able to predict how changes in the shape of the curve influence [O.sub.2] uptake into the blood and delivery to the body tissues.
* Explain the role played by hemoglobin in external respiration by determining how and where its dysfunction disrupts respiratory gas transfer in the body.
Although these objectives represent significant elements in the understanding of respiratory physiology, there are many other instructional purposes for which this case can be used by instructors.
The design of this case study is highly directive. The questions that accompany the case are written in a multiple-choice format in order to impose specific constraints on the pathway of student learning. This format also makes it easier to grade--a particularly useful feature in large-enrollment courses. The case could easily be rewritten so that one or more of the questions require free responses. It could also be expanded to address a greater range of cognitive and psychomotor learning objectives or designed to call for greater effort in clinical decision making and encompass further aspects of diagnosis, prognosis, and treatment prescription. …