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BGYC34_physioex_lab_6_assigment_2013 (1).doc

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University of Toronto St. George
Michael Reid

1 BGYC34; First Assignment (2013) PhysioEx Lab 6 (Cardiovascular Physiology) Due Date: Friday, January 25, 2013 Perform the experiments in PhysioEx lab 6 (Cardiovascular Physiology). Provide the results requested, and answer the questions, for each of the following activities. There are a total of 25 marks. Note: The activity names and numbers listed below correspond to those in PhysioEx version 9.0. If you are using an older version of PhysioEx the exact activity names and numbers may not match. In this case, identify which activities in the older versions correspond to the ones listed below and place the appropriate data in the corresponding section. Activity 1: Investigating the Refractory Period of Cardiac Muscle Results: Briefly describe what happened when you applied the electrical stimuli to the heart. (2 marks) When single stimuli (shocks) were delivered in succession to the frog heart, the oscilloscope showed a “doublet” or a double peak, which contained an extrasystole or an extra contraction of the ventricle and then a compensatory pause. When multiple stimuli were applied to the frog heart, the oscilloscope showed extrasystoles being produced more frequently. Question 1: Explain why the larger waves seen on the oscilloscope represent ventricular contraction. (1 mark) A ventricular contraction is of greater force then an atrial contraction because it sends blood throughout the entire body (systematic circulation), whereas the contraction of the atrium sends blood to the lungs and the rest of the heart (pulmonary circulation). Activity 2: Examining the Effect of Vagus Nerve Stimulation Results: Briefly describe what happened when you electrically stimulated the vagus nerve. (2 marks) When the vagus nerve was electrically stimulated at a rate of 50 stimuli/sec the heart stopped beating for about 3 seconds, and this was illustrated on the oscilloscope as a pause (a straight line) in the contractile activity of the frog heart. Question 2: Explain how possible mechanisms of vagal escape would differ in an isolated heart preparation such as the one used in this experiment versus a heart in a live animal/human. (3 marks) 2 Vagal escape is characterized as the resumption of the heartbeat after stimulation of the vagus nerve has caused it to stop, which can be result of sympathetic reflexes or initiation of the rhythm by the Purkinje fibers. In order for vagal escape to occur the strong stimulation of the vagus must be maintained in order for the ventricles to begin beating again. In a heart of a live animal/human, the stimulation is maintained by the sympathetic and parasympathetic nervous systems because they remain connected to the heart through the vagus nerve. In the isolated heart however, the heart is not connected to the nervous systems and the stimulation must be indirectly maintained (direct electrical stimulation to the vagus nerve) in order for vagal escape to occur. Activity 3: Examining the Effect of Temperature on Heart Rate Results: Provide the heart rate values recorded when the heart was bathed with Ringer’s solution of different temperatures. (2 marks) Solution Heart Rate (beats/min) 23 °C 60 5 °C 50 32 °C 70 Question 3: Homeothermic animals such as humans have a thermoneutral zone (i.e., a range of external environmental temperatures) in whichinternal body temperature remains at a constant level while simultaneously maintaining metabolic rate (i.e., energy expenditure) constant. If the environmental temperature drops below the thermoneutral zone, humans can still keep internal body temperature constant (as long as it isn’t too cold). However, in this case, metabolic rate does not simultaneously remain constant but, rather, it increases. a) In this case (below the thermoneutral zone), what would happen to heart rate (in an intact living human)? Explain your answer. (2 marks) When the environmental temperature drops below the thermoneutral zone the body loses more heart to the cold air. In response your cardiovascular system reduces blood flow to your skin surface and extremities by constricting your blood vessels. Your caloric needs rise, you use up more glucose and burn more fat
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