Two adaptations that have been made by the red blood cell are the lack of a nucleus and organelles, or small, specialized bodies in the cell. This allows for more room in the blood cell for hemoglobin.
Hemoglobin is a molecule that bonds with oxygen and delivers it to the cells. Hemoglobin also gives red blood cells their red color. The development of a cell specialized in carrying oxygen throughout the body allows oxygen to be utilized more efficiently by an organism.
Red blood cells don't use the oxygen they carry because they lack organelles, such as mitochondria. Because they have no nuclei, they have no DNA or RNA and can't divide or repair themselves well. They live about 100 days before they are destroyed.
Other adaptations of red blood cells are their donut shape and flexibility. These adaptations allow them to squeeze through tiny capillaries.
Red blood cells also release adenosine triphosphate, or ATP when they find themselves in very narrow blood vessels. ATP causes the vessels to open up. Red blood cells also produce hydrogen sulfide, which signals the blood vessels to relax.
When the body is invaded by a pathogen, red blood cells attack it with free-radical molecules, which destroy the pathogen's cell walls.