Final answer:
Tumors ensure a fresh blood supply through the process of angiogenesis, which involves the formation of new blood vessels from existing ones.
Step-by-step explanation:
Tumors ensure a fresh blood supply through a process called angiogenesis, which is the formation of new blood vessels from existing ones. In order to grow larger, tumors need more nutrients and oxygen, which can be provided by new blood vessels. The tumor cells release signals that promote the growth of blood vessels towards them, allowing for a continuous supply of fresh blood.