Blood cells are produced through hematopoiesis, a process that occurs primarily in the red bone marrow and begins with unspecialized hematopoietic stem cells. These stem cells undergo differentiation, allowing them to develop into specialized blood cells such as erythrocytes (red blood cells), leukocytes (white blood cells), and platelets (thrombocytes). The production of red blood cells is regulated by the hormone erythropoietin (EPO), which is released by the kidneys when oxygen levels in the blood decrease. Adequate levels of iron, vitamin B12, and other nutrients are required for the formation of healthy red blood cells and proper hemoglobin production. Blood composition can also be examined through measurements such as hematocrit, which represents the percentage of blood volume made up of red blood cells. When blood is centrifuged, it separates into three layers: plasma at the top, the buffy coat containing leukocytes and platelets in the middle, and erythrocytes at the bottom. Understanding how blood cells are produced, regulated, and measured helps scientists and healthcare professionals diagnose diseases and monitor overall health.