What Do Epinephrine and Norepinephrine Do to Blood Pressure and Heart Rate?

Epinephrine generally increases heart rate, cardiac contractility and systolic blood pressure. Norepinephrine primarily increases vasoconstriction and therefore raises both systolic and diastolic blood pressure; the resulting baroreceptor reflex may limit or reduce heart rate.

The Mechanism

Epinephrine and norepinephrine are catecholamines involved in communication between the endocrine, nervous and cardiovascular systems. Epinephrine is released mainly as a hormone from the adrenal medulla, whereas norepinephrine acts mainly as a neurotransmitter released by sympathetic neurons, although the adrenal medulla also releases some norepinephrine.

Their effects can be explained using:

$$\text{Cardiac output} = \text{heart rate} \times \text{stroke volume}$$ $$\text{Mean arterial pressure} \approx \text{cardiac output} \times \text{total peripheral resistance}$$

During exercise, increased sympathetic activity and reduced parasympathetic activity raise heart rate, while catecholamines help redirect blood towards active muscles and maintain arterial pressure. The baroreceptor reflex is adjusted rather than switched off.

A common misconception is that norepinephrine simply increases heart rate because it stimulates $\beta_1$ receptors. Its direct cardiac effect can increase heart rate, but its strong blood-pressure increase activates the baroreceptor reflex, so the net heart-rate response may be small or negative.

Exam Technique

For an IB SEHS explain question, connect receptor action to cardiac output or total peripheral resistance, then connect that variable to blood pressure. For a compare question, emphasize epinephrine's stronger effect on heart rate and norepinephrine's stronger vasoconstrictor effect.