This accounts for why cells specialize to become brain, bone, cartilage, muscle and fat cells, for example. The blood is a circulating tissue. Thus increasing levels of complexity and sophistication are seen as the human develops, but every part of the body can be defined by it's cellular components. The cells comprising a fledgling human grow very rapidly in the womb, in early childhood, and through puberty. In adulthood, new cells are only formed to replace those which have died from injury, old age or disease.
Cancer starts in a single cell which has become abnormal.
This cells produces millions, and eventually billions, of copies of itself. These clones fail to function as normal esophagus tissue, but instead divert resources from healthy cells to fuel their own growth. It is this ability to spread that makes cancer so dangerous. Patients rarely succumb to the growing tumors themselves. We also see interference with normal organ functioning, and a general debility arising from malnutrition, dehydration, and multiple metabolic disturbances.