How do epinephrine and norepinephrine act together to increase cardiac output?  

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Epinephrine and norepinephrine are both stress hormones that are produced in the adrenal medulla. When epinephrine and norepinephrine act together, they cause a constriction of the blood vessels leading to the nephrons of the kidneys. This effectively halts the function of the kidneys during acute stress response that triggers flight/fight/freeze...

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Epinephrine and norepinephrine are both stress hormones that are produced in the adrenal medulla. When epinephrine and norepinephrine act together, they cause a constriction of the blood vessels leading to the nephrons of the kidneys. This effectively halts the function of the kidneys during acute stress response that triggers flight/fight/freeze response. While this is occurring, blood pressure is increased, which in turn increases the cardiac output. The heart rate is also increased as both these hormones are cardio accelerators. This sudden heart rate acceleration is called tachycardia. As these hormones are released together in times of acute stress, there is often both an increased heart rate response as well as an increase in blood pressure through the sympathetic nervous system reaction.

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