Final answer:
Morphine decreases myocardial oxygen demand by reducing blood pressure, myocardial workload, stress, and pain.
Step-by-step explanation:
Morphine decreases myocardial oxygen demand by its effects on the cardiovascular system. It acts as a vasodilator, which means it widens the blood vessels and reduces peripheral vascular resistance. This causes a decrease in blood pressure and myocardial workload, resulting in a decrease in the oxygen demand of the heart.
Additionally, morphine has sedative and analgesic properties, which can help reduce stress and pain. By reducing stress and pain, the heart has to work less and requires less oxygen.
Overall, morphine decreases myocardial oxygen demand by reducing blood pressure, myocardial workload, stress, and pain.