Red blood cells have no nucleus, because most of their bulk is made up of hemoglobin, a compound that carries gases, such as oxygen and carbon dioxide. In fact, about a third of a red blood cell is dedicated to hemoglobin alone, so no room remains for a nucleus or many of the structures that other cells have.
Red blood cells look like flat disks that travel around the body in blood vessels. They carry oxygen to tissues and remove carbon dioxide from them. Hemoglobin plays an important role in red blood cells, because it carries the oxygen that nourishes cells and takes away carbon dioxide to prevent it from accumulating in cells.
In the mature form, red blood cells, or erythrocytes, do not have nuclei; however, they have not always been without nuclei. In their immature forms, red blood cells did have nuclei. An intermediate form of red blood cells, called a normoblast, expels its nuclei as the amount of hemoglobin accumulates in the developing blood cell. The immature red blood cell can still manufacture hemoglobin without the aid of a nucleus.
Because red blood cells do not have nuclei, they tend to live only about 120 days, which is a much shorter lifespan than that of other types of cells. Nuclei are important to cells, because they control what substances are made in the cell. Without the nuclei to replenish what is depleted in the cell, the erythrocyte will eventually die when it runs out of resources.