Final answer:
The adrenal medulla hormone that dilates or relaxes the sphincter of smooth muscle by beta 1 receptors, causing an increase in heart rate and myocardial contractility, is epinephrine.
Step-by-step explanation:
b. Epinephrine is the adrenal medulla hormone that dilates or relaxes the sphincter of smooth muscle by beta 1 receptors, causing an increase in heart rate and contractility of the myocardium.
To better understand this, let's consider the effects of epinephrine and norepinephrine. Epinephrine primarily acts on beta 1 receptors, which are found in the heart. When epinephrine binds to these receptors, it increases heart rate and the force of contractions, resulting in an overall increase in cardiac output. On the other hand, norepinephrine primarily acts on alpha receptors, which constrict blood vessels and increase blood pressure. Therefore, the correct answer is a. Epinephrine.