Erythrocytes are red blood cells. Erythro means red while cyte mean cell. Red blood cells are responsible for carrying oxygen to the tissues and carbon dioxide away from the tissues. Red blood cells have an unusual shape. They look like a donut, they are thicker on the outside of the cell and have a concave center. This shape allows the red blood cell to have a large surface area to volume ration which allows them to absorb and release oxygen quickly. It also lets red blood cells to form stacks that go smoothly through narrow blood vessels. The shape also allows the red blood cells to bend and flex when entering small capillaries.
Red blood cells do not have a nucleus or ribosomes. Because they lack both of these organelles they cannot replicate or make proteins or enzymes. Because of this they can't repair damage and they have a short life span. They also have a low energy demand so they get their energy through anaerobic metabolism of glucose from the plasma around them instead of stealing oxygen that it is carrying to burn glucose.
Erythrocytes is another name used for red blood cells present in blood. These absorb oxygen present in the environment and supply it to all parts of the body. They also absorb from all parts of the body carbon dioxide generated as a waste product for its removal from the body. Red blood cells areshaped like flat, round disks that are thicker on the edges as compared to the center.
One very unusual feature of red blood cells is absences of nucleus. While most types of cells have a nucleus that control many of the cell functions, a mature red blood cell has no nucleus. Also the cell membrane of red blood cells very flexible. This allows them to squeeze through tiniest of the blood vessels.