There are between 4.2 and 6 million erythrocytes, also known as red blood cells, in every microliter of blood. These cells are small, flattened biconcave discs with centers that are depressed.

The erythrocyte plasma membrane is associated with proteins such as spectrin, which forms a flexible cytoplasmic meshwork. This meshwork allows erythrocytes to twist, turn, become cup-shaped, and regain their biconcave shape as they pass through narrow capillaries. Additionally, erythrocytes can form stacks similar to dinner plates, enabling them to move smoothly through narrow capillaries without obstructing blood flow.

Mature erythrocytes lack nuclei and cell organelles, providing space for cell protein molecules. This space is primarily occupied by hemoglobin molecules, which carry oxygen and constitute about 97% of the cytosolic proteins.

Erythrocytes do not contain mitochondria and generate their ATP through anaerobic processes. As a result, they do not consume the oxygen bound to hemoglobin, making them highly efficient oxygen carriers.

Their flat and biconcave shape provides 30% more surface area relative to volume than spherical cells, positioning the cytoplasmic hemoglobin close to the surface to facilitate a swift exchange of gases.