Overview
This lecture covers hematopoiesis, the process of blood cell formation, focusing on red blood cell production (erythropoiesis), the regulation by erythropoietin, the roles of bone marrow and spleen, and hemoglobin structure and recycling.
Hematopoiesis and Erythropoiesis
- Hematopoiesis is the process of making all blood cells: red blood cells (RBCs), white blood cells (WBCs), and platelets.
- Erythropoiesis is the formation of RBCs specifically.
- RBC production is stimulated by the hormone erythropoietin, primarily released by the kidneys.
- Hypoxia (low oxygen) in kidneys triggers erythropoietin release to increase RBC production.
- Causes of hypoxia include lung disease, high altitude, exercise, blood loss, and RBC destruction.
- Excessive RBC production increases blood viscosity, raising stroke or heart attack risk.
Blood Cell Formation and Stem Cells
- All blood cells originate in bone marrow from pluripotent stem cells.
- Fetal stem cells can differentiate into any cell type; adult (pluripotent) stem cells can become any blood cell.
- In fetuses, blood cells are made in the yolk sac, liver, and spleen; after birth, mainly in red bone marrow.
- Infants have red marrow in most bones; adults have red marrow mainly in femur, hip, vertebrae, sternum, ribs, and skull.
Stages of Red Blood Cell Development
- Pluripotent stem cell (with nucleus) commits to blood cell lineage.
- Colony-forming unit receives erythropoietin signal to become red cell.
- Erythroblast builds hemoglobin and ejects nucleus.
- Reticulocyte contains many ribosomes for protein synthesis.
- Mature erythrocyte (RBC) has no nucleus and circulates for ~120 days.
Hemoglobin Structure and Function
- Hemoglobin (Hb) is a complex quaternary protein in RBCs that binds oxygen.
- One hemoglobin molecule carries four oxygen molecules; a single RBC has 250 million hemoglobin molecules.
- Oxyhemoglobin: Hb bound to oxygen; Carbaminohemoglobin: Hb bound to COâ‚‚; Carboxyhemoglobin: Hb bound to CO (toxic).
- RBCs and hemoglobin are recycled by the spleen, with iron and amino acids retained.
Breakdown and Recycling of Red Blood Cells
- Spleen breaks down old RBCs and hemoglobin into heme and globin.
- Globin is broken into amino acids for reuse.
- Heme is split into iron (stored) and bilirubin (excreted in bile).
- The liver processes bilirubin; failure leads to jaundice.
Spleen Functions
- Spleen is the main site for destruction of old blood cells.
- Spleen can release stored blood during emergencies.
- Also filters pathogens and debris from blood via phagocytosis.
- It is possible to live without a spleen, as the liver can compensate.
Key Terms & Definitions
- Hematopoiesis — formation of all types of blood cells.
- Erythropoiesis — process of red blood cell production.
- Erythropoietin — hormone from kidneys stimulating RBC production.
- Pluripotent Stem Cell — stem cell in bone marrow that can become any type of blood cell.
- Hemoglobin (Hb) — oxygen-carrying protein in RBCs.
- Oxyhemoglobin — hemoglobin bound to oxygen.
- Carbaminohemoglobin — hemoglobin bound to carbon dioxide.
- Carboxyhemoglobin — hemoglobin bound to carbon monoxide.
- Bilirubin — yellow pigment from heme breakdown, excreted in bile.
- Jaundice — yellowing of skin/eyes due to bilirubin buildup.
- Phagocytosis — process of cells engulfing and digesting debris or pathogens.
Action Items / Next Steps
- Review the five stages of red blood cell maturation, focusing on beginning and end stages.
- Learn main nutrients required for RBC production: amino acids, iron, folic acid, vitamin B12.
- Study the roles of spleen and liver in RBC recycling.