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Iteration is at the heart of science and engineering
Students should learn iteratively in both science and the engineering design process, as called for by the Next Generation Science Standards (NGSS Lead States 2013; see box, p. 55). This article describes how to use the iterative pedagogical approach in a biology lesson in which students develop and revise a model for cardiopulmonary circulation and then build a prototype through iterative designs of an artificial heart valve.
Developing and revising models for getting oxygen to cells
Developing initial models
This lesson on the circulatory and respiratory systems was taught in a unit on human body systems and how they interact to maintain homeostasis. Students were asked to consider how necessary materials (e.g~, oxygen, food, water) get to cells deep within the body. Students first viewed images of liver cells and then created mi- tial models (through words and pictures) individually in response to the following question: 110w do you think your liver's hepatocytes get oxygen molecules from the air?"
Then, in groups of two or three, students combined their ideas into small-group models. These models var- ied widely in their levels of detail and understanding. Initial conceptions of circulation were vague.
For example, while most students indicated that air entered the lungs and oxygen entered the bloodstream, few groups could explain how the lungs expanded or how gas exchange occurred with the circulatory system. Some models showed circuitous routes of blood travel- ling through the body.
No students used the diaphragm or alveoli in their explanations.
Revising the models
The teacher circulated among the groups, asking challenging questions, and then shared a "starter kit" with images of eight key structures of the circulatory and respiratory systems: diaphragm, lungs, trachea, bronchi, bronchioles, alveoli, vessels (capillaries/arteries/veins), and heart. The groups were then asked to revise their models with a stepby-step explanation for how liver cells get oxygen and how the body cells expel carbon dioxide using all eight structures. At the end of the session, a whole-group discussion allowed students to examine other groups' models and the teacher to gauge their understanding, particularly concerning the role of the alveoli in the diffusion of gases into and out of the blood stream. Incomplete ideas about circulation remained. For...