Chapter 32: Red Blood Cells, Anemia, and Polycythemia

Written By: Razaullah Khattak (DPT Medical Professional)

Student Source Reference: Guyton and Hall Textbook of Medical Physiology

Chapter: 32

To read more check out Chapter 15:

https://mednotesbyrazaullah.blogspot.com/2026/08/chapter-15-vascular-distensibility-and.html

Red Blood Cells, Hemoglobin & Anemia

Red blood cells, also known as erythrocytes, are essential components of the circulatory system. Their primary function is to transport hemoglobin, which carries life-sustaining oxygen from the lungs to tissues throughout the body. Beyond oxygen transport, these cells house crucial enzymes like carbonic anhydrase for carbon dioxide transport and act as powerful acid-base buffers to maintain physiological blood pH.

Understanding the lifespan, structure, chemical synthesis, and regulatory pathways of red blood cells provides the foundation for hematology. When production or function is altered, conditions such as anemia or polycythemia develop. This comprehensive breakdown covers every structural feature, metabolic pathway, production stage, and clinical pathology associated with red blood cells strictly based on fundamental physiological principles.

Primary Functions of Red Blood Cells

• Erythrocytes transport hemoglobin throughout the circulatory system.
• Hemoglobin carries oxygen from lungs to peripheral tissues.
• Free plasma hemoglobin leaks into human tissue spaces.
• Glomerular membranes filter free plasma hemoglobin into urine.
• Intracellular hemoglobin prevents renal and tissue leakage loss.
• Carbonic anhydrase catalyzes carbon dioxide and water reactions.
• Carbonic anhydrase increases reaction speed several thousandfold.
• Carbon dioxide transports as bicarbonate ion in blood.
• Bicarbonate converts back to carbon dioxide inside lungs.
• Expelled carbon dioxide leaves body as metabolic waste.
• Hemoglobin acts as a powerful acid-base buffer.
• Erythrocytes provide most acid-base buffering in blood.

Shape, Size, and Physical Characteristics

• Normal red blood cells are flexible biconcave discs.
• Mean diameter of red cells is 7.8 micrometers.
• Thickness at thickest point measures 2.5 micrometers.
• Center thickness measures one micrometer or less.
• Average cell volume ranges between 90-95 cubic micrometers.
• Cell shape changes remarkably squeezing through narrow capillaries.
• Red cell membrane possesses significant excess surface area.
• Membrane deformation occurs without stretching or cellular rupture.

Concentration and Hemoglobin Content

• Healthy men average 5,200,000 red cells per cubic millimeter.
• Healthy women average 4,700,000 red cells per cubic millimeter.
• High-altitude residents maintain higher red blood cell counts.
• Cells concentrate hemoglobin up to 34 grams per 100ml.
• Metabolic limits prevent hemoglobin concentration above 34 percent.
• Deficient hemoglobin synthesis decreases red cell volume.
• Normal male blood averages 15 grams hemoglobin per 100ml.
• Normal female blood averages 14 grams hemoglobin per 100ml.
• One gram pure hemoglobin binds 1.34 milliliters oxygen.
• Male blood carries maximum 20 milliliters oxygen per 100ml.
• Female blood carries maximum 19 milliliters oxygen per 100ml.

Anatomic Sites of Erythropoiesis

• Yolk sac produces nucleated red cells during early embryogenesis.
• Middle trimester liver serves as primary erythropoiesis organ.
• Spleen produces red blood cells during middle trimester.
• Lymph nodes produce red cells during middle trimester.
• Bone marrow produces all red cells during late gestation.
• Marrow of all bones produces red cells until age five.
• Long bone marrow becomes fatty after age twenty.
• Proximal humeri and tibiae retain productive marrow past twenty.
• Membranous bones produce red cells after age twenty.
• Vertebrae, sternum, ribs, and ilia contain productive marrow.
• Membranous bone marrow productivity decreases with advancing age.

Genesis and Stem Cell Differentiation

• All blood cells derive from pluripotential stem cells.
• Pluripotential in hematopoietic stem cells reside in bone marrow.
• Stem cell populations diminish slowly with advancing age.
• Committed stem cells differentiate toward specific cell lines.
• CFU-E designates committed erythrocyte colony-forming stem cells.
• CFU-GM designates granulocyte and monocyte forming stem cells.
• Growth inducers control stem cell growth and reproduction.
• Interleukin-3 promotes growth of virtually all stem cells.
• Differentiation inducers cause stem cells to mature further.
• Low blood oxygen induces red cell differentiation pathways.
• Infectious diseases induce specific white blood cell production.

Stages of Red Blood Cell Maturation

• Proerythroblast is the first identifiable red cell precursor.
• CFU-E stem cells form large numbers of proerythroblasts.
• Proerythroblasts divide multiple times to form mature cells.
• Basophil erythroblasts stain with basic cellular dyes.
• Early erythroblast generations accumulate very little hemoglobin.
• Maturing erythroblasts concentrate hemoglobin to 34 percent.
• Erythroblast nucleus condenses and extrudes from cell.
• Endoplasmic reticulum reabsorbs during late erythroblast maturation.
• Reticulocyte stage contains remnants of basophilic material.
• Reticulocytes contain Golgi apparatus and mitochondria remnants.
• Reticulocytes enter blood capillaries by diapedesis mechanism.
• Basophilic material disappears within one to two days.
• Reticulocytes comprise less than one percent of erythrocytes.
• Regulation of Erythropoiesis and Erythropoietin
• Total red cell mass is regulated within strict limits.
• Regulation guarantees adequate tissue oxygenation without clogging flow.
• Tissue oxygenation is essential regulator of erythropoiesis.
• Anemia from hemorrhage stimulates rapid bone marrow production.
• X-ray damage causes hyperplasia in remaining bone marrow.
• High altitude exposure increases red blood cell production.
• Tissue hypoxia controls production rate rather than cell concentration.
• Cardiac failure increases red cell production via tissue hypoxia.
• Lung diseases cause hypoxia, increasing hematocrit and volume.
• Erythropoietin hormone stimulates red blood cell production.
• Erythropoietin is a glycoprotein weighing 34,000 Daltons.
• Hypoxia fails to stimulate production without active erythropoietin.

Renal Erythropoietin Secretion

• Kidneys produce ninety percent of circulating erythropoietin.
• Liver produces remaining ten percent of body erythropoietin.
• Interstitial fibroblast-like cells secrete erythropoietin in cortex.
• Tubular epithelial cells may secrete additional renal erythropoietin.
• Tissue hypoxia increases hypoxia-inducible factor-1 inside cells.
• HIF-1 acts as transcription factor for erythropoietin gene.
• HIF-1 binds hypoxia response element, increasing mRNA synthesis.
• Norepinephrine and epinephrine stimulate renal erythropoietin secretion.
• Prostaglandins stimulate renal erythropoietin hormone production.
• Bilateral nephrectomy causes severe anemia due to low erythropoietin.
• Liver erythropoietin maintains only one-third normal cell production.

Mechanism of Erythropoietin Action

• Hypoxia induces erythropoietin production within minutes to hours.
• Maximum erythropoietin production occurs within twenty-four hours.
• New circulating red cells appear five days after hypoxia.
• Erythropoietin stimulates proerythroblast formation from stem cells.
• Erythropoietin speeds erythroblast transition through maturation stages.
• Production rates decrease once tissue oxygenation returns normal.
• Bone marrow forms few red cells without erythropoietin.
• High erythropoietin increases red cell production tenfold.
• Nutritional Requirements for Cell Maturation
• Rapidly dividing erythroblasts require optimal nutritional support.
• Vitamin B12 and folic acid permit final maturation.
• Vitamin B12 aids thymidine triphosphate synthesis for DNA.
• Folic acid helps synthesize thymidine triphosphate building blocks.
• Deficiency impairs nuclear maturation and erythroblast cell division.
• Maturation failure produces oversized, irregular macrocytes.
• Macrocytes possess flimsy, fragile, easily ruptured cell membranes.
• Macrocyte functional oxygen transport capacity remains completely normal.
• Macrocytes exhibit shortened lifespans of one-third normal.

Pernicious Anemia and Vitamin B12 Absorption

• Defective intestinal absorption causes vitamin B12 maturation failure.
• Pernicious anemia features progressive gastric mucosal atrophy.
• Parietal cells secrete glycoprotein called intrinsic factor.
• Intrinsic factor protects vitamin B12 from GI digestion.
• Intrinsic factor binds ileal mucosal cell membrane receptors.
• Pinocytosis transports intrinsic factor and bound B12 combined.
• Absorbed vitamin B12 stores in large liver quantities.
• Daily B12 requirement ranges from 1-3 micrograms.
• Normal liver stores contain 1000 times daily requirement.
• Defective absorption requires 3-4 years causing anemia.

Folic Acid Metabolism and Deficiency

• Folic acid occurs in green vegetables, fruits, meats.
• Cooking easily destroys dietary folic acid content.
• Intestinal sprue impairs folic acid and B12 absorption.
• Sprue causes combined vitamin absorption failure anemias.

Chemical Synthesis of Hemoglobin

• Hemoglobin synthesis begins inside proerythroblast stage.
• Reticulocytes continue synthesizing hemoglobin for one day.
• Succinyl-CoA combines with glycine forming pyrrole molecules.
• Four pyrrole molecules combine forming protoporphyrin IX.
• Protoporphyrin IX combines with iron forming heme.
• Ribosomes synthesize globin polypeptide chains for heme.
• Each heme combines with globin forming hemoglobin chain.
• Subunit hemoglobin chains weigh approximately 16,000 Daltons.
• Four subunits bind loosely, forming whole hemoglobin molecule.
• Adult Hemoglobin A contains two alpha and beta chains.
• Hemoglobin A total molecular weight equals 64,458.
• Each hemoglobin molecule contains four iron atoms.
• One hemoglobin molecule transports four oxygen molecules.

Oxygen Transport and Sickle Cell Pathology

• Hemoglobin combines loosely and reversibly with oxygen molecules.
• Oxygen binds coordination bonds of iron atom loosely.
• Carried oxygen remains molecular rather than ionic form.
• Tissues receive easily released molecular oxygen.
• Sickle cell anemia replaces glutamic acid with valine.
• Valine substitution occurs within two beta chains.
• Low oxygen precipitates abnormal hemoglobin into long crystals.
• Elongated crystals reach lengths of 15 micrometers.
• Crystals puncture cell membranes, causing severe hemolysis.

Iron Metabolism, Transport, and Storage

• Total body iron content averages 4 to 5 grams.
• Hemoglobin contains 65 percent of body iron.
• Myoglobin contains 4 percent of body iron.
• Intracellular oxidation enzymes contain 1 percent body iron.
• Plasma transferrin carries 0.1 percent body iron.
• Ferritin stores 15 to 30 percent body iron.
• Absorbed iron binds apotransferrin, forming plasma transferrin.
• Transferrin delivers iron loosely to body tissues.
• Excess iron deposits inside liver parenchymal cells.
• Apoferritin binds iron cytoplasmically, forming storage ferritin.
• Apoferritin molecular weight equals approximately 460,000 Daltons.
• Hemosiderin represents highly insoluble storage pool clusters.
• Hemosiderin accumulates when iron exceeds apoferritin storage capacity.
• Transferrin binds receptors on bone marrow erythroblasts.
• Endocytosis transports transferrin-iron complexes into erythroblasts.
• Mitochondria receive iron directly for heme synthesis.
• Transferrin deficiency causes severe hypochromic anemia.
• Macrophages digest worn-out 120-day-old red cells.
• Macrophage-liberated iron enters ferritin storage pool.
• Males excrete 0.6mg daily iron into feces.
• Menstruating females lose average 1.3mg daily iron.

Intestinal Absorption and Regulation of Iron

• Liver secretes apotransferrin in into bile duct system.
• Bile delivers apotransferrin in into duodenal lumen space.
• Apotransferrin binds free iron, dietary hemoglobin, myoglobin.
• Bound transferrin attaches to intestinal epithelial receptors.
• Pinocytosis absorbs transferrin into intestinal epithelial cells.
• Epithelial cells release iron into blood plasma capillaries.
• Maximum intestinal iron absorption reaches few milligrams daily.
• Saturated apoferritin stores decrease intestinal iron absorption.
• Depleted iron stores increase intestinal absorption fivefold.

Life Span and Clearance of Erythrocytes

• Mature circulating red cells live average 120 days.
• Mature erythrocytes lack nucleus, mitochondria, endoplasmic reticulum.
• Cytoplasmic enzymes metabolize glucose, forming cellular ATP.
• Enzymes maintain membrane pliability and ion transport.
• Enzymes maintain ferrous state of hemoglobin iron.
• Aging red cells lose metabolic efficiency progressively.
• Structural fragility increases as cell processes wear out.
• Old red cells rupture passing through tight vascular spaces.
• Splenic red pulp trabeculae feature 3-micrometer spaces.
• Splenectomy increases numbers of circulating old cells.
• Macrophages phagocytize hemoglobin released from burst cells.
• Kupffer cells of liver digest free plasma hemoglobin.
• Macrophages release iron back into transferrin system.
• Porphyrin converts into bile pigment called bilirubin.
• Liver secretes plasma bilirubin into bile system.

Types and Etiologies of Anemia

• Anemia means deficient functional blood hemoglobin concentration.
• Rapid hemorrhage leaves low red blood cell concentration.
• Plasma fluid volume replaces within 1 to 3 days.
• Red cell concentration normalizes within 3 to 6 weeks.
• Chronic blood loss impairs intestinal iron absorption rates.
• Iron deficiency produces microcytic, hypochromic anemia cells.
• Bone marrow aplasia means total loss functional marrow.
• High-dose radiation damages bone marrow stem cells.
• Cancer chemotherapy causes severe bone marrow aplasia.
• Toxic chemicals like benzene cause aplastic anemia.
• Lupus autoimmune attacks destroy marrow stem cells.
• Idiopathic aplastic anemia has unknown primary etiology.
• Blood transfusions temporarily support severe aplastic anemia.
• Bone marrow transplantation provides definitive aplastic treatment.
• Gastric mucosa atrophy causes megaloblastic pernicious anemia.
• Surgical gastrectomy causes total loss intrinsic factor.
• Intestinal sprue impairs folic acid and B12 absorption.
• Megaloblasts exhibit fragile, irregular, oversized membrane structures.
• Hereditary spherocytosis forms small, spherical, fragile cells.
• Spherical cells rupture easily passing splenic pulp.
• Hemoglobin S precipitates during low oxygen states.
• Sickle cell crisis causes progressive, lethal tissue hypoxia.
• Rh-positive fetal cells trigger maternal anti-Rh antibodies.
• Antibody attack causes severe erythroblastosis fetalis hemolysis.
• Fetal marrow releases early nucleated blast forms.

Circulatory Effects of Anemia

• Severe anemia decreases blood viscosity to 1.5 water.
• Decreased viscosity reduces peripheral blood flow resistance.
• Tissue hypoxia causes peripheral blood vessel dilation.
• Cardiac output increases three to four times normal.
• Increased output partially offsets reduced oxygen delivery.
• Exercise creates severe tissue hypoxia in anemic patients.
• Acute cardiac failure ensues during strenuous anemic exercise.

Polycythemia Pathophysiology and Symptoms

• Secondary polycythemia results from sustained tissue hypoxia.
• High-altitude dwellers develop secondary physiologic polycythemia.
• Altitude polycythemia counts reach 6-7 million/mm3.
• Polycythemia vera stems from genetic hemocytoblasts aberrations.
• Polycythemia vera counts reach 7-8 million/mm3.
• Polycythemia vera hematocrit reaches 60 to 70 percent.
• Tumor-like excess production affects white cells and platelets.
• Total blood volume increases twice normal in polycythemia.
• High blood viscosity reaches ten times water viscosity.
• Increased viscosity decreases rate of venous return.
• Increased blood volume counteracts decreased venous return.
• Cardiac output remains near normal in polycythemia.
• Blood pressure regulation fails, causing hypertension eventually.
• Skin subpapillary venous plexus becomes engorged massively.
• Sluggish blood flow increases deoxygenated hemoglobin quantity.
• Polycythemia patients exhibit ruddy cyanotic skin tint.

Disclaimer: This content is for educational purposes only and not a substitute for professional medical advice.