Morphine primarily dilates which type of blood vessels?

Morphine primarily dilates veins, which is a key pharmacological effect of this opioid analgesic. By causing venodilation, morphine decreases the venous return to the heart, leading to reduced preload. This effect can be particularly beneficial in conditions such as heart failure or acute pulmonary edema, where reducing the volume of blood returning to the heart can alleviate symptoms and decrease cardiac workload.

While morphine does have some effects on arterial blood vessels, its principal action is more pronounced in venous dilation. This selective effect is significant in clinical practice, especially when managing pain and associated symptoms in critically ill patients. The other types of blood vessels listed do not demonstrate the same response to morphine. Specifically, while arteries may undergo some dilation due to a variety of mechanisms related to opioid action, the predominant and clinically notable impact remains on the veins.