Erythrocytes, leucocytes, and platelets are collectively called formed elements and they constitute nearly 45% of the blood. Erythrocytes, or red blood cells (RBCs), are the most abundant of all the cells in blood. A healthy adult man has, on average, 5 million to 5.5 million RBCs per mm of blood. RBCs are formed in the red bone marrow in adults. RBCs are devoid of a nucleus in most mammals and are biconcave in shape. They have a red-colored, iron-containing complex protein called hemoglobin, hence the color and name of these cells. A healthy individual has 12-16 grams of hemoglobin in every 100 ml of blood. These molecules play a significant role in the transport of respiratory gases. RBCs have an average lifespan of 120 days after which they are destroyed in the spleen (the graveyard of RBCs).
The process of red blood cell (RBC) formation is called erythropoiesis. Blood cell formation is also called hematopoiesis or hemopoiesis. The main function of RBCs is to transport oxygen around our body. Blood cells are divided into three groups: red blood cells (erythrocytes), white blood cells (leukocytes), and blood platelets (thrombocytes). This takes place at hematopoietic stem cells in the red bone marrow of the humerus, femur, sternum, rib, vertebra, and pelvis. These stem cells divide by mitosis, and some of the daughter cells mature into erythrocytes while the others remain as stem cells. The entire maturation phase takes around four days and proceeds step by step. In fact, all blood cells are divided from these stem cells. RBCs are formed and destroyed at a breathtaking rate; 200 billion RBCs are destroyed and replaced.
In the early weeks of embryonic life, primitive, nucleated red blood cells are produced in the yolk sac. During the middle trimester of gestation, the liver is the main organ for production of RBCs, but a reasonable number is also produced in the spleen and lymph nodes. Then, during the last month or so of gestation and after birth, RBCs are produced exclusively in bone marrow. All bone marrow is red initially, but after a certain age, some bone marrow, such as the humerus and femur, become fatty (yellow). In adults, 2.4 million RBCs are produced each second. In a normal adult, the red cells of about half a liter (almost one pint) of blood are produced by the bone marrow every week. RBCs have a life span of approximately 100-120 days. After they have completed their life span, they are removed from the bloodstream by the spleen.
As the kidney's main function is to monitor blood volume and oxygen content of blood, it releases erythropoietin (EPO) as necessary. When oxygen concentrations in the blood are low, erythropoietin is released from the kidney, operating in a negative feedback mechanism to maintain red blood cell homeostasis.
There are few hormones that secret kidney (Thyroxin). The main job of thyroxin is to increase the ATP production of cells. That means that cell need more oxygen to make more ATP then they need more RBC. So Thyroxin stimulates the release of EPO from the kidney. Men have more Thyroxin then female because Testosterone stimulates Thyroxin. So men have more RBC in their blood than women because Testosterone stimulated Thyroxin which stimulates EPO.
As RBC is mature and aged, the metabolic system of the RBC become progressively less active and the cell become more fragile. Many of the RBC are self-destroyed in the spleen where they squeeze through the red pulp of the spleen.
As the erythrocytes are destroyed, the iron-containing moiety of hemoglobin is conserved and the pigmentary part is converted into bile pigment. Hemoglobin is phagocytosed by macrophages in many parts of the body (Kupffer cells in the liver, macrophages of the spleen and bone marrow). It releases the iron and porphyrin portions.
The porphyrin portion of hemoglobin is converted into biliverdin. The biliverdin is then converted into bilirubin, which is transported by albumin to the liver where it undergoes conjugation with glucuronyl transferase, forming conjugated bilirubin. The conjugated bilirubin is then passed into the duodenum through the hepato-pancreatic ducts.