Ralph is looking at a blood vessel under a microscope as part of his study about the cardiovascular system. he notices that the vessel contains blood and has valves. what is the blood vessel that he is observing?

Ralph is looking at a vein (likely a muscular vein).

Explanation:

Of all three great types of vessel present in vertebrates (arterial, venous, lymphatic), only veins and lymphatic vessels may normally contain valves.  This is an essential feature of vessels which present low pressure flow, since it ensures that such flow is unidirectional.

Since blood cells are not typically found in lymphatic vessels, the vessel in question can be assumed to be a vein.

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