There are 4.2 to 6 million erythrocytes, or red blood cells, per microliter of blood. These small cells are flattened biconcave discs with depressed centers. The erythrocyte plasma membrane is associated with proteins such as spectrin, forming a flexible cytoplasmic meshwork. This protein net enables erythrocytes to twist, turn, become cup-shaped, and regain their biconcave shape while passing through narrow capillaries. Additionally, erythrocytes can form stacks like dinner plates, allowing smooth passage through narrow capillaries without obstructing the blood flow. Mature erythrocytes lack nuclei and cell organelles, creating internal space for protein molecules. The available space is mainly occupied by oxygen-carrying hemoglobin molecules, constituting about 97% of the cytosolic proteins. Erythrocytes also lack mitochondria and generate their ATP by anaerobic processes. So, erythrocytes do not use the oxygen attached to hemoglobin, making them efficient oxygen carriers. Their flat and biconcave shape provides 30% more surface area relative to volume compared to other spherical cells. As a result, the cytoplasmic hemoglobin is close to the surface, facilitating swift exchange of gasses.