prob_set6_solns

prob_set6_solns - prob_set6 Page 1 of 3 SOLUTIONS TO...

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SOLUTIONS TO PROBLEM SET #6 1. Normally depolarization sweeps over the heart from rostral to caudal, synchronizing contraction as it goes. Therefore, contraction begins at the atria and moves through the ventricles to the apex of the heart, and this coordination is crucial to moving blood efficiently into and out of the heart. Following depolarization, the heart cells become refractory. This period, during which the heart cells cannot depolarize again, prevents depolarization from reversing its direction and beginning to travel back toward the atria. ADDED NOTE: In certain pathological conditions, some myocardial fibers escape from their refractory period while other (damaged) neighboring myocardial fibers are still depolarized. Currents flowing in the depolarized regions can then reinvade the newly non-refractory regions, a condition called "re-entry." As a result, part of a ventricle depolarizes and contracts independently of the rest of the heart. In extreme cases, small portions of the heart depolarize and repolarize at random (fibrillation). Asynchronous depolarization produces asynchronous contraction, and asynchronous contraction cannot move blood out of the heart and into the vessels. 2. See a diagram of the cardiac cycle (in Lectures 12-14 in the course outline or on page 479 of your text) to check the correlation between the EKG and mechanical events in the heart. Actually, although electrical events are normally correlated with particular events, the EKG cannot tell you anything directly about the mechanical behavior of the heart; it only reflects the electrical events. 3. PLEASE NOTE: Electrocardiology is a broad and complicated field. Internists and cardiologists spend years learning to interpret abnormal EKGs, and they use much more complicated methods than just looking at a Lead II record. (Now a lot of EKG analysis is done by computers, which can rapidly and efficiently evaluate many different parameters of the records, but cardiologists still must understand and interpret EKG results.) BIPN 100 cannot teach you to be a cardiologist. Nonetheless, some records are sufficiently abnormal and show sufficiently obvious patterns that you are likely to recognize the abnormalities and to be able to make an informed guess about the underlying physiological problem.
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