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kweigl1 kweigl1
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10 years ago
I need help with these 2 questions, from PhysioEX 8.0, Cardiovascular Physiology & will appreciate any help you can offer.
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6. Explain why the SA node generates action potentials at a frequency of approximately 100 beats per minute even though the average resting heart rate is 70 beats per minute?


9. When the heart is externally stimulated just after the start of the contraction cycle, why does this have no affect on heart rate?
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9. When the heart is externally stimulated just after the start of the contraction cycle, why does this have no affect on heart rate

This is because once a contraction has started, the action potential has already fired, stimularing the muscle fibers. Once they are fired in the heart, there is: depolarization, followed by a plateau, a rapid repolarization, and then a small refractory period. It is only after the refractory period can the heart fire again, because the Na+ and K+ balances must return to the state they were just before depolarization.
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biolovebiolove
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6. Explain why the SA node generates action potentials at a frequency of approximately 100 beats per minute even though the average resting heart rate is 70 beats per minute?

The average heart rate is lower than the sinoatrial node's ability to generate action potentials due to the vagus nerve and the parasympathetic nervous system. Your brain limits your heart rate inside your body to approximately 70 beats per minute. This allows your cardiovascular centres in your brain a little bit more latitude as far as controlling your heart rate. If you were to remove a beating heart from someone's body you would find that the heart is no longer being suppressed by the parasympathetic nervous system and it will accelerate to the same pace as the sinoatrial node. The SA node generates action potentials approximately 100 times per minute due to the physiology of the heart, its own permeability to sodium and calcium via its channels, and a host of other things. Anywhere between 80 to 100 beats per minute is normal for the SA node.
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