DR. Shaheen Haroon Rashid Red Blood Corpuscles (RBCs) DR. Shaheen Haroon Rashid

Guyton and Hall “Textbook of Medical Physiology”12th ed. (413-420) At the end of the session the students should be able to: Describe the structure of RBCs Describe in detail erythropoiesis mechanism Describe the life-span of RBC (circulation and their breakdown) Explain the factors affecting erythropoiesis Guyton and Hall “Textbook of Medical Physiology”12th ed. (413-420)

Red blood cells RBCs Named red blood corpuscles as it not a true cell as it do not contain nuclei and other cell organells as mitochonderia Normal RBCs count: Adult male 5 -5.5 million/ mm3 Adult female 4.5 – 5 million/ mm3 Being higher in new born and person living at high altitude

Normal RBCs are circular and biconcave discs RBCs size and shape: Normal RBCs are circular and biconcave discs Diameter 7.8 micron Thickness 2.5 micron Volume 90 cubic micron Significance of RBCs shape Larger surface area Allow higher flexibility that allow RBCs to squeeze in small capillaries RBCs life span about 120 days

Structure of RBCs Like any cell surrounded by semi-permeable cell membrane Hb is its main content represent about 33% of RBC volume. Each RBC has 200 million Hb molecules K+ is the main intracellular cation Also contain Carbonic Anhydrase Enzyme that hydration of CO2 to carbonic acid No mitochondria so its energy derived from anaerobic glycolysis

Erythropoiesis

Erythropoiesis After birth During fetal life Def: formation of RBCs Sites of erythropoiesis After birth During fetal life Active (red) BM: In infancy & childhood red BM present nearly in all bones In adult red BM is restricted in ends of long bones, vertebrae, ribs, sternum, skull, pelvic bones 1) Yolk sac: in the first 6 w 2) Liver & spleen: from 6 w – 6 m 3) Bone marrow BM: from 6 m until after birth

Stages of haematopoiesis

Stages of Eythropoiesis PHSC developed under the effect of growth factor IL-3 to committed stem cell which then developed to CFU-E under the effect of erythropoietin CFU-E then developed to proerythroblast then to erythroblasts (basophil, polychromatophil, orthochromatic) Erythroblasts give normoblasts which lose their nucleus, and endoplasmic reticulum and transformed into reticulocytes which then become mature RBCs Reticulocytes represent less than 1% of RBCs in peripheral blood

Stages of Eythropoiesis

Factors affecting erythropoiesis O2 supply to the tissue = role of erythropoietin Nutritional factors: Dietary protein content Mineral ions Iorn Copper Cobalt Vitamins: vit B12. folic acid, and others Hormonal factors State of bone marrow State of liver

1) Tissue oxygenation & erythropoietin Decrease O2 supply to the tissue (Hypoxia) is the primary stimulus for erythropoiesis as in: Anaemia High altitudes Lung diseases Cyanotic heart diseases Hypoxia stimulate erythropoietin secretion that stimulate eythropoiesis in bone marrow

Erythropoietin A glycoprotein hormone (mw 34000 d) Source: Function: 90% from the kidney (renal tubular epithelium or endothelial cells of peritubular capillaries) and 10% form the liver (but mainly from the liver in fetal life). Function: Stimulates the production of proerythroblasts from stem cells Speeds up all stages of development of erythroblasts into mature RBCs Regulation (control of secretion): Hypoxia the main stimulus Adrenaline, noradernaline and some PGs Androgens Adenosine (adenosine antagonist decrease EPO secretion) Cobalt salts Clinical uses: Chronic renal failure Aplastic anaemia Anaemia with chronic diseases

Erythropoietin Mechanism Imbalance Start Normal blood oxygen levels Stimulus: Hypoxia due to decreased RBC count, decreased availability of O2 to blood, or increased tissue demands for O2 Imbalance Increases O2-carrying ability of blood Reduces O2 levels in blood 90% of EPO is renal Erythropoietin stimulates red bone marrow Kidney (and liver to a smaller extent) releases erythropoietin Enhanced erythropoiesis increases RBC count

2) Dietary factors A) proteins: Proteins of high biological value are essential for erythropoeisis (for the formation of globin part of Hb. Prolonged protein under nutrition lead to anaemia B) Minerals: Iron (Fe) is essential for formation of haeme part of Hb. Copper (Cu) Cu essential for erythropoeisis, transported in the plasma by ceruloplasmin (which catalyze the oxidation of ferrous iron to ferric) Co-factors in Hb synthesis Cobalt (Co) Stimulate erythropoeisis though stimulation of erythropoeitin secretion from the kdney enters in synthesis of Vit. B12 C) Vitamins: Vit B12, folic acid, others vit C Vitamin B12 & Folic acid; essential for DNA synthesis & maturation of bone marrow cells

Iron (Fe+2) Iron metabolism: Daily requirement: Iron is essential for formation of haeme part of Hb (also in other heme containing particles as myoglbin, cytochrome oxidase, catalase,perioxidase) decrease iron supply leads to iron deficiency anaemia Iron metabolism: The total body iron content is 4 – 5 gm. 65% in Hb, 15-30% stored as ferritin in RES in the liver, 4% in myoglobin, 1% in enzymes, 0.1% in transferrin in plasma. Normal serum iron level. (90-150 ug/dL) is bound to Transferrin Daily requirement: 0.6 mg / day for male 1.3 mg /day for female

Absorption of Iron Iron actively transported mainly in the upper small intestine (Duodenum & Jejunum) 1) Dietary Ferric (Fe3+) reduced to ferrous (Fe2+) 2) Fe2+ or Heme transported at brush border by different carrier proteins (IT, iron transporter & HT, heme transporter) 3) Intracellularly Fe2+ released from Heme by heme oxygenase Most of intracellular Fe2+ actively transported (AT) across the basolateral membrane to enter the blood. Some Fe2+ oxidize to Fe3+ & bound apoferritin forming ferritin Decrease Fe2+ Absorption Increase Fe2+ Absorption Oxalates & Phosphates Phytates & Tannin Vit C Gastric HCl Iron absorption occurs according to body needs when all apoferritin become saturated iron absorption from enterocytes is inhibited

Transport: In blood Fe2+ oxidize to Fe3+ & bound apotransferrin forming transferrin reach various iron tissues store. Storage: Excess iron in blood is stored in cells of RE system (liver mainly and spleen) it combind with apoferritin forming tissue ferritin

Feedback regulation of iron absorption The rate of iron absorption from GIT depend on the iron stores in the body: increased 5 times or more when iron stores in the body become depleted Greatly decrease when the body iron stores are saturated in the form of ferritin due to: The transferrin become fully saturated with iron (decrease the iron binding capacity of the blood) that leads to accumulation of ferritin in enterocytes that depress the active absorption of iron from the intestinal lumen The liver decrease the synthesis of apotransferrin required for iron absorption.

Iron Transport & Metabolism

III- Dietary vitamins Vitamin B12 & Folic acid; essential for DNA synthesis & maturation of bone marrow cells (maturation factors). Deficiency of B12 & Folic acid leads to failure of maturation of erythroblasts leading to formation of fragile larger cells with shorter life span (Macrocytic or Megaloblastic anaemia)

Vit B12 Source: animal source liver, meat, egg, fish, vegetables are poor for vit B12 Daily requirement: 5 µg. Absorption: Vit B12 combined with intrinsic factor (a glycoprotein secreted by parietal cells of gastric gland Intrinsic factor-vit B12 complex absorbed in the terminal ileum by pinocytosis Transport: Vit B12 is carried in the blood by PP transcobalamin to the site of storage or use Storage: Liver store large quantities of vit B12 (5 mg) that sufficient to supply vit B12 requirement for about 3 years Vit B12 deficiency: Megaloblastic anaemia Neurological manifestations

III- Role of Liver Healthy Liver is essential for normal erythropoiesis as it the site for: Storage of Vit. B12 & iron synthesis of 10% of EPO Chronic liver disease leads to anaemia

IV- Hormones Thyroid H Glucocorticoides Androgens All stimulate Erythropoiesis as they promote tissue metabolism

V- State of bone marrow Healthy bone marrow is essential for normal erythropoiesis Destruction of BM by irradiation, drugs toxins leads to aplastic anaemia

Life Cycle of Red Blood Cells

Life span and fate of RBCs: Erythrocytes live in the circulation for an average of 120 days. * As the cells grow older, they become more fragile and rupture during their passage through narrow spots in the circulation specially in the spleen. * The released Hb from ruptured RBC’s is phagocytized by the macrophage cells. * Inside the macrophage cells: Hb breaks into  globin + heme. Globin → amino acids Heme → iron + biliverdin → bilirubin biliruin (yellow, pigment excreted by the liver in bile).