Blood is a specialized fluid tissue circulating through the cardiovascular system, made up of liquid plasma and formed elements. The formed elements include erythrocytes for oxygen transport, leukocytes for immune defense, and thrombocytes for blood clotting. In early embryonic life, blood cell formation begins in the yolk sac. As the fetus grows, this process shifts to the liver and spleen, and finally settles in the bone marrow by the time of birth. In adults, active bone marrow is found mainly in the axial skeleton such as the vertebrae, sternum, ribs, and pelvis. All these mature blood cells trace their lineage back to a single common ancestor known as the pluripotent hematopoietic stem cell. This remarkable cell possesses the unique ability to self-renew and to differentiate into specialized blood cell lines depending on the chemical signals it receives.
Haemopoiesis is the continuous, regulated process of blood cell production and maturation. Stem cells in the bone marrow commit to one of two main developmental pathways:
This entire system responds dynamically to the body's changing needs. For instance, an infection triggers a surge in white blood cell production, while low oxygen levels stimulate red blood cell creation.
Erythropoiesis is the specific developmental pathway that creates red blood cells. Starting from a committed proerythroblast, the cell gradually decreases in size, condenses its nucleus, synthesizes haemoglobin, and finally ejects its nucleus before entering the circulation as a reticulocyte.
The master regulator of this pathway is erythropoietin (EPO), a glycoprotein hormone produced mainly by the kidneys. When renal cells detect low oxygen tension—such as during high altitude living, chronic lung disease, or severe anaemia—they secrete EPO into the bloodstream to accelerate red cell production in the bone marrow.
Haemoglobin synthesis takes place primarily within developing red blood cells in the bone marrow, specifically starting in the proerythroblast stage and continuing through the normoblast stages. The process requires two main components: globin protein chains and heme rings.
Heme synthesis is a complex metabolic pathway that begins in the mitochondria, moves to the cytosol, and finishes back in the mitochondria. The rate-limiting step is catalyzed by the enzyme ALA synthase. Iron must be actively transported into the developing cell and inserted into the protoporphyrin ring by the enzyme ferrochelatase to complete the heme molecule. Meanwhile, globin chains are synthesized on ribosomes in the cytoplasm according to genetic instructions. Any disruption in iron supply, protein synthesis, or porphyrin production halts successful haemoglobin assembly, resulting in smaller, paler red blood cells.
Normal adult haemoglobin, known as HbA, is a complex protein made up of four folded polypeptide subunits. Each subunit consists of a globin chain attached to a heme group containing an iron atom in the ferrous state. HbA contains two alpha chains and two beta chains.
The primary physiological job of haemoglobin is to bind oxygen in the lungs where partial pressure is high, and release it in active tissues where oxygen is scarce. This efficient delivery is made possible by cooperative binding: when one oxygen molecule binds to a heme group, it changes the shape of the protein, making it easier for the remaining three oxygen molecules to attach. Conversely, when the first oxygen is released in a hypoxic tissue, the remaining molecules dissociate more readily.
Disorders of haemopoiesis and haemoglobin lead to significant clinical conditions. Anemia is defined as a reduction in the oxygen-carrying capacity of the blood, often resulting from iron deficiency, chronic blood loss, or bone marrow failure. Patients typically present with pallor, fatigue, and shortness of exertion.
Haemoglobinopathies are genetic disorders resulting from structural alterations in the globin chains. The most prevalent example in West Africa is sickle cell disease, caused by a single point mutation in the beta-globin gene that substitutes valine for glutamic acid. Under low oxygen tension, abnormal HbS molecules polymerize, distorting the red blood cell into a rigid, sickle shape that can occlude small blood vessels and cause painful vaso-occlusive crises.