The human body is crossed by a very dense network of blood vessels, which carry blood through to every tissue and organ and distribute oxygen and nutrient to every single cell in the body. This intricate system includes both larger veins and arteries and smaller capillaries, which are only a few micrometers wide. The accumulation of all of these vessels is known as the vascular system.
Like all great systems, the vascular system can be used both for good and evil. While it can take much needed-nutrients anywhere in the body, it can also be exploited by cancer cells to spread throughout. Cancer can arise anywhere, but because of the design of the human body nowhere is very far away from a blood vessels. Cancer cells that have acquired the ability to move start moving around the local environment where the cancer is situated. For example, melanoma cells start crawling through the layers of the skins, getting progressively deeper until they encounter the capillaries that are embedded in the deep layers of the tissue. Similarly, breast cancer cells move around within the breast, eventually finding one of the capillaries that normally serves the mammary glands. Interestingly, tumors can actually grow their own blood vessels for the purposes of feeding the large tumor mass that would otherwise run out of oxygen and sugar and die. This also means that cancer cells moving away from the main body of a tumor are nearer to blood vessels than they would be in a healthy tissue.
The traditional view of this system is that cancer cells just randomly move around the space they have access to until they run into a blood vessel. However, this is not necessarily always true. Recent evidence suggests that cancer cells are able to “sniff out” blood vessels by recognizing the chemical signals that are produced by the cells lining the sides of capillaries. This means that cancer cells can move directly towards blood vessels. When they encounter them, they squeeze in the spaces between the cells lining the blood vessel and enter the bloodstream. It’s commonly thought that cancer cells do this following the same molecular strategies that immune cells use when they r-enter blood vessels after having taken care on an infected areas. It’s also interesting to note that cancer-originated blood vessels have been shown to be “leaky”, making entering them much easier. Since these vessels are closer to the tumor than regular capillaries, cancer cells that acquire the ability to grow their own vessels give themselves a double advantage, both feeding themselves and providing a convenient stepping stone to spread throughout the body.