Erythropoiesis

Question 1

Describe the components of the erythron system.

Answer 1

• The red blood cells (RBCs) are made from stem cells in the bone marrow of the adult and are released into the circulation as immature RBC called reticulocytes
  
  • Reticulocytes mature into normal RBCs
• Aged RBC, as well as aborted RBCs in the bone marrow, are removed by macrophages in several tissues and are digested to useful products, amino acids (aa), Fe+3, and the waste product bilirubin derived from Hemoglobin, (Hb)
  
  • The aa and Fe+3, are recycled, and bilirubin is excreted in bile and its metabolic products in the gut are excreted in feces

Question 2

How are the various blood compartments measured?

Answer 2

• The volumes of whole blood (Vb), plasma, (Vp), and red blood cell compartment (Vc), are measured by isotopic dilution
  
  • To measure Vc, RBCs can be labeled with 51Chromium
  • A fixed volume of packed RBC is labeled and injected into a subject and, at suitable times, drawing a blood sample and isolating a fixed volume of packed RBC will be useful in determining Vc
• After measuring the radioactivity isolated, the isotopic dilution formula allows the calculation of the RBC compartment volume
  
  • Compartment Volume = Total label injected into compartment/(Label recovered/unit volume)
• Similarly, to measure plasma volume, Vp, inject 131I-albumin, which will be diluted by the albumin normally found in plasma
  
  • Knowing Vc and Vp allows the calculation of whole blood, Vb
  • **Vb = Vc + Vp **

Question 3

What are some standard numbers for red blood cell volume (Hgb, RBC, Hct)? What is anemia and polycythemia?

Answer 3

• Determined on a sample of peripheral blood:
  
  • Blood Hemoglobin Concentration, (Hgb), in g/100 ml of whole blood, g %
    
    • 13.6-17 in men
    • 12-15 in women
  • Red Cell Count, (RBC), in millions per cubic mm. of whole blood, 106/mm3.
    
    • 4.3-5.9 in men
    • 3.5-5.0 in women
  • Hematocrit (Hct), in ml of packed red cells per 100 ml of whole blood, ml %.
    
    • 39-49 in men
    • 33-43 in women
• **Anemia **a decrease in total circulating Hgb
• Polycythemia an increase in RBC

Question 4

What are the normal values for MCV, MCH, and MCHC?

Answer 4

• Mean corpuscular volume, MCV, in µm3 per cell:
  
  • MCV = 10 x Hct/RBC
  • Normal is 82-96
• Mean corpuscular hemoglobin, MCH, in pg. per cell (10 -12 g/cell):
  
  • MCH = 10 x Hgb/RBC
  • Normal is 27-33
• Mean corpuscular hemoglobin concentration, MCHC, in pg/100 µm3:
  
  • MCHC = 100 Hgb/Hct
  • Normal is 33-37

Question 5

What are the defintions of normocytic, normochromic, macrocytic, hypochromic, and microcytic?

Answer 5

• RBC with normal MCV are normocytic
• RBC with normal MCHC are normochromic
• Cells with abnormally high MCV, macrocytic
• Cells with abnormally low MCHC, hypochromic
• Cells with abnormally low MCV, microcytic

Question 6

What are the major categories of anemia?

Answer 6

• Anemias may be, at first, broadly analyzed and classified according to MCV and MCHC, for example:
  
  • Normocytic, normochromic anemia
  • Microcytic, hypochromic anemia
  • Macrocytic, normochromic anemia

Question 7

What is the effect of hemorrhage on hematocrit and whole blood measures?

Answer 7

• At first a large, abrupt hemorrhage loses Vc, Vp and Vb, all in proportion, and the Hct shows no immediate change
  
  • But then, within minutes to 2-3 days, blood volume regulation restores Vb by increasing Vp above normal
• Note that Vc has not increased but the increase in Vp lowers the hematocrit
  
  • After about 3 days Vc increases for 4-5 weeks back to normal, the excess of Vp being gradually eliminated

Question 8

Describe the broad cycle of regulation of erythropoiesis. What is the major regulator and its properties?

Answer 8

• The blood levels of oxygenated hemoglobin are monitored in the kidney
  
  • A drop in arterial blood hemoglobin concentration below normal levels leads to increased release of eythropoietin (EPO)
    
    • ​stimulates the synthesis of red blood cells (RBC) and Hgb
  • Conversely, an increase in arterial blood hemoglobin concentration leads to decreased release of EPO and to decreased formation of new RBCs
• Regulation of erythropoiesis is effected by stimulation of RBC formation and Hb synthesis, not by degradation of new or old RBCs

Question 9

Describe the major stages of erythrocyte maturation.

Answer 9

• The stem cells in the bone marrow may be uncommitted stem cells at rest, or may be committed stem cells.
  
  • The committed stem cells are pluripotent and will enter one of the various hemopoietic pathways after stimulation by appropriate humoral signals
• Cells entering the erythrogenic pathway are stimulated by erythropoietin, EPO, to proliferate and mature
• Stages:
  
  • Stem cells
  • Pronormoblast
  • Basophilic normoblast
  • Polychromatic normoblast
  • Orthochromatic normoblast
  • Reticulocyte
  • Mature red cell

Question 10

What is the duration of the stages of RBC development? How much of the blood is usually reticulocytes? What does a high count suggest?

Answer 10

• Stem cell - unknown
• Nucleated stages - 3.0 days
• Reticulocyte stages - 2.75 days
  
  • In the bone marrow: 0-2.75 days
  • In the circulation: 0.5-1.5 days
• Mature red blood cells, RBC - 118-119 days
• Normal reticulocyte count is about 0.5-1.5%.
  
  • A high reticulocyte count suggests a high rate of erythropoiesis whereas a low count suggests insufficient erythropoiesis

Question 11

Describe the structure of a red blood cell and its major metabolic pathways.

Answer 11

• The red blood cell is a biconcave disc, with a mean diameter of 8 µM, a thickness of 2 µM and a volume of 90 fL
  
  • It lacks a nucleus or mitochondria
  • The RBC membrane is a phospholipid bilayer that is fixed to an intracellular protein network
  • The membrane allows the RBC to change shapes to allow flexible navigation through the microvasculature
• Intracellular energy requirements are supplied by glucose metabolism
• The main metabolic pathways are the Emden-Meyerhof pathway and the Hexose monophosphate shunt
  
  • RBCs depend on glucose to generate NADH (to maintain Fe+2), ATP (for ion transport) by the glycolytic pathway and NADPH (to maintain membrane protein SH groups) by the pentose phosphate shunt

Question 12

What is the primary role of the red blood cell? How does it do this?

Answer 12

• The primary function of the RBC is oxygen transport
• Hemoglobin is the protein that makes up 33% of the content of a RBC
  
  • Hemoglobin is a tetramer that contains four heme groups each bound to a globin monomer
  • Each heme group can reversibly bind one oxygen molecule
  • Therefore each hemoglobin molecule is able to bind 4 oxygen molecules

Question 13

Describe the globin chains of the various hemoglobin molecules.

Answer 13

• Globin chains are assembled by the cytoplasmic ribosomes
• Globin chains change from from the fetal period to adulthood
• The final globin molecule is a tetramer of two α-globin chains and two non-α globin chains
• The genes for the globin chains are on Chromosomes 11 and 16.
  
  • non-α globins: Chromosome 11
    
    • β, ε ,δ (delta), γ (gamma)
  • α-globins: Chromosome 16
    
    • α (alpha) & ζ (zeta)

Question 14

What are the different types of hemoglobins in the embryo, fetus, and adult?

Answer 14

• Embryo
  
  • Hb Gower 1 – ζ2ε2
  • Hb Portland - ζ2γ2
  • Hb Gower 2 - α2ε2
• Fetus
  
  • Hb F - α2γ2
• Adult
  
  • Hb A - α2β2
  • Hb A2 - α2δ2
  • Hb F - α2γ2
• In the adult, 95-98% of the hemoglobin is Hemoglobin A (2α globin chains and 2 β globin chains)
  
  • 1.5-3.5% of the hemoglobin is Hemoglobin A2 (2α globin chains and 2 δ chains

Question 15

Describe the degradation of hemoglobin and what happens when this goes wrong.

Answer 15

• When macrophages lining the blood vessels of the bone marrow, spleen, and liver recognize senescent or severely damaged RBCs, they entrap them by phagocytosis and digest these cells
  
  • Hemoglobin, Hb, is digested to reusable amino acid (from globin) and Fe+3
• Oxidation of the heme ring gives rise to linear biliverdin which is reduced to bilirubin, a toxic waste product
  
  • Bilirubin is released into the plasma where it is transported as a soluble complex with albumin
  • The hepatic cells extract bilirubin from the plasma, conjugate it with glucuronic acid
  • The soluble bilirubin diglucuronide is secreted into the bile which is discharged into the duodenum through the biliary tract
  • Intestinal bacteria chemically alter the bilirubin to urobilinogen and this to other products
  • Bilirubin is present in the normal circulation at concentrations of 0.5-1.0 mg/DL (deciliter)
• Abnormal increases in plasma bilirubin, hyperbilirubinemia, commonly cause a yellow pigmentation of the skin (jaundice) and of the sclera (icterus)

Question 16

What are the determinants of red blood cell oxygen transport. How doe these factors affect the hemoglobin dissociation curve?

Answer 16

• Tissue oxygen supply is a function of the number of RBC perfusing the tissues and the hemoglobin oxygen-carrying capacity
• The Hemoglobin Oxygen-dissociation curve has a unique sigmoid shape
  
  • It allows adjustment of oxygen delivery to match tissue metabolism
• As physiologic changes occur changes occur in the dissociation curve which allows for increased or decreased oxygen delivery
• The affinity of hemoglobin for oxygen is influenced by:
  
  • temperature
  • pH (Bohr effect)
  • CO2 concentration
  • level of RBC 2,3-BPG (biphosphoglycerate)
• Changes in these parameters can either shift the curve to the right or left
  
  • The middle line represents the normal state
  • The dotted line to the right, represents a right shift of the curve
  • A right shift in the curve can occur when the pH drops, with higher temperatures or with higher levels of 2,3 BPG
  • This results in a release of oxygen from the RBC, and therefore a reduction in the saturation of hemoglobin