Erythrocyte

The structure and composition of red blood cells reflects their highly specialized function: to deliver the maximum quantity of oxygen possible to tissues and aid in the removal carbon dioxide, a waste product of cellular respiration, and urea. The interior of a red blood cell contains a massive concentration of hemoglobin, roughly one-third by weight (30-34 g/dL for an adult). This
extraordinary hemoglobin capacity has been achieved, in part, by the adoption of an unusually simplified cell structure. Mature red blood cells are devoid of the intracellular
organelles found in other eukaryotic cells (eg, nucleus, lysosome, Golgi apparatus, mitochondria). As a consequence, enucleated red blood cells are unable to reproduce.

Red blood cells possess an extensive cytoskeletal network responsible for maintaining their biconcave configuration. Their unusual shape enhances the exchange of oxygen and carbon dioxide between erythrocytes and tissues in
two ways. First, their disc-like configuration possesses a much higher ratio of surface area to volume than more spherical geometries.

Second, it enables red blood cells to fold over and squeeze through narrow capillaries whose diameter is smaller than that of the erythrocyte itself. By minimizing the distance to be traversed, these factors promote efficient gas exchange between capillary walls and the rapidly moving (up to 2 mm/s) erythrocytes.

The 120 day lifespan of a normal red blood cell requires that nearly 1% of the roughly 30 trillion erythrocytes in a typical individual must be replaced daily. This equates to a rate of production of ∼2 million new red blood cells per second. Newly formed red blood cells retain portions of the ribosomes, endoplasmic reticulum, mitochondria, etc that were present in their nucleated precursors. During the ≈24 hours required to complete the transition to a mature erythrocytes, these nascent red blood cells, called reticulocytes, retain the capacity to synthesize polypeptides under the direction of vestigial mRNA molecules.

Red blood cells lack mitochondria, and hence the enzymes of the TCA cycle, electron transport chain, β-oxidation pathway, or ATP synthase. This renders them incapable of utilizing fatty acids or ketone bodies as metabolic fuel. Consequently, red blood cells are completely reliant of glycolysis to generate ATP. Glucose enters red blood cells by facilitated diffusion, a process mediated by the glucose transporter (GLUT1), also known as glucose permease