Our cells need oxygen and nutrients to survive. The cells of very small organisms can obtain what they need from the environment by simple diffusion, in which nutrients and gases pass from areas of high concentration to low concentration through cell membranes. But diffusion isn’t very useful over distances greater than a millimeter or so. Larger animals, including humans, need a vascular system, a way for the oxygen and nutrients to get from the outside environment to every cell in the body (and for carbon dioxide and waste products to get back out to the environment).
Cells grow and multiply, and they die. Their metabolic rates can change, which changes their requirements for oxygen and nutrients. The vascular system can adapt to the body’s changing needs by changing the amount of blood it delivers to the tissues. The formation of new blood vessels, a process called angiogenesis, needs to be highly regulated, and poorly regulated angiogenesis can have detrimental effects. Disease can occur if there are not enough blood vessels to feed the tissues or, conversely, if blood vessels grow too exuberantly.