• Clinical science

Erythrocyte morphology and hemoglobin

Summary

Erythrocytes, or red blood cells (RBCs), are the most common blood cells. They are filled with hemoglobin (Hb), which is responsible for transporting oxygen throughout the body. If Hb's binding site for oxygen is blocked, this transport mechanism will be impaired, resulting in decreased oxygen transportation and therefore tissue oxygenation. This can occur, for instance, after inhaling carbon monoxide or exposure to substances that increase methemoglobin levels such as nitrates. Particular disorders or abnormalities often involve characteristic changes to RBC morphology. For example, the presence of schistocytes is an important factor for diagnosing hemolytic uremic syndrome. For this reason, the microscopic analysis of RBCs, either in a blood smear or in urine samples, is a valuable tool for diagnosing erythrocyte pathologies.

Hemoglobin synthesis

Overview

Hb is a circulating globular protein composed of a heme moiety with a central iron and 4 subunits of globin.

  • The main function of Hb is to take up O2 from the lungs and deliver it to tissues.
  • It can undergo conformational changes (e.g, depending on its oxygenated state), which influence how it binds and releases O2 and CO2.
  • Deficient or defective Hb can ultimately affect the transport of O2.
  • For more information about disorders of Hb, see the learning card on anemia.

Heme synthesis

Heme is synthesized from protoporphyrin, a porphyrin (see profile below). The steps of heme synthesis occur both in the cytoplasm and the mitochondria (first and final step occur in mitochondria).

Location Reaction Enzyme Clinical significance
Mitochondria
Cytoplasm
  • Coproporphyrinogen III (in cytoplasm) → protoporphyrinogen III (in mitochondria)
  • Coproporphyrinogen III oxidase
Mitochondria
  • Protoporphyrinogen III → protoporphyrin IX
  • Protoporphyrin oxidase
  • Protoporphyrin binds iron (Fe2+) → heme

Sideroblastic anemia (with possible basophilic stippling) has different etiologies, of which three are part of heme synthesis: X-linked defect in the δ-ALA synthase gene, vitamin B6 deficiency and lead poisoning (and sequential inhibition of δ-ALA dehydratase and ferrochelatase).

Porphyrins: derivatives include protoporphyrin

  • Composed of 4 rings with a central iron
  • O2 binds to ferrous iron (Fe2+)
  • Iron binds to globin via histidine residues

Globin

Globin is an integral part of the Hb molecule. It is composed of amino acids that fold to form eight alpha helices. Throughout the lifespan of an individual, different types of globin chains are present in Hb, particularly during embryonic, fetal, and early life.

Chromosomes Hb genes Globin chains Time of physiologic expression
Chromosome 11 HBB β adult
HBD δ adult
HBE ε embryonic
HBG1 γ fetal
HBG2
Chromosome 16 HBA1 α fetal + adult
HbA2
HBZ1 ζ embryonic
HBZ2
  • Embryonic Hb: ζ and ε
  • Fetal Hb
    • HbF: α2γ2
    • Synthesized in
    • Has ↓ binding of 2,3-bisphosphoglycerate (2,3-BPG) → ↑ affinity for O2 → ↑ O2 extraction from the maternal circulation (via the placenta)
  • Adult Hb: synthesized in bone marrow
  • Alpha thalassemia: Defective α chain production results in tetramer formation → tetramer have heightened affinity for O2 → decreased O2 release
    • HbH: 4 β-chains form a tetramer
    • Hb-Barts: 4 γ-chains form a tetramer
  • Beta thalassemia: Defective β-chain formation results in increased HbA2 and HbF formation.

See “Hemoglobin patterns” for more information.

References:[1][2]

Carbon dioxide transport

Overview

  • O2, CO2, and protons all bind Hb and influence each other's affinities to Hb, which is important for gas exchange.
  • CO2 is mainly carried in three forms in the body:

Bicarbonate buffer system

  • RBCs carry carbonic anhydrase, which converts HCO3- and H+ to H2O and CO2 in the following formula: HCO3− + H+ ⇄ H2CO3 ⇄ H2O + CO2
  • Ultimately, excess H+ during acidic states can be eliminated by being converted to CO2, which can be exhaled.
  • During basic states, the reversal can occur so that CO2 is converted to HCO3− + H+.
    • Chloride shift: Excess intracellular HCO3− produced this way is released into the plasma in exchange for Cl-.
  • This phenomenon makes HCO3- the most important buffer in the body and accounts for 50% of the blood buffer capacity.
  • For more details on the buffering mechanisms of the body, see compensation in acid-base disorders.

Haldane effect

When Hb is oxygenated (in high pO2, for example, in the lungs), it releases bound H+ → ↑ H+ shifts equilibrium to CO2 production (see equation above) → CO2 is exhaled in lungs

Bohr effect

High CO2 and H+ concentrations (from tissue metabolism) cause decreased affinity for O2. → O2 that is bound to Hb is released to tissue (the O2-Hb dissociation curve is shifted to the right).

  • HbO2 + H+ ⇄ H+Hb + O2
  • HbO2 + CO2 ⇄ Hb-COO- + H+ + O2

References:[1][2]

Oxygen-hemoglobin dissociation curve

  • Description
    • The O2-Hb dissociation curve shows the arterial partial pressure of O2 (PaO2) in relation to the percentage saturation of Hb, i.e., the binding affinity of Hb for O2.
    • The binding affinity of Hb is influenced by external factors that may lead to a left or right shift of the O2 dissociation curve (ODC).
  • Shift to the right of the ODC → ↓ O2 affinity for Hb → ↑ O2 dissociation from Hb → ↑ tissue oxygenation
    • Causes of shift to the right
      • ↑ Partial pressure of carbon dioxide (↑ PCO2)
      • ↑ Body temperature (e.g., fever)
      • ↑ H+ (↓ pH)
      • ↑ 2,3-BPG (generated by 2,3-BPG mutase during erythrocyte glycolysis)
      • ↑ Exercise
      • ↑ Altitude
  • Shift to the left of the ODC → ↑ O2 affinity for Hb → ↓ O2 dissociation from Hb → ↓ tissue oxygenation
    • Causes of shift to the left
      • ↓ Partial pressure of carbon dioxide (PCO2)
      • ↓ Body temperature
      • ↓ H+ (↑ pH)
      • ↓ 2,3-BPG
Differences between the hemoproteins myoglobin and hemoglobin
Myoglobin Hemoglobin
Associated with 1 heme (monomeric) 4 hemes (tetrameric)
Binds to 1 oxygen molecule 4 oxygen molecules
Affinity for O2 Very high (hyperbolic oxygen-myoglobin dissociation curve) High (sigmoidal curve)
Function
  • Storage of O2 in muscle
  • Transport of O2 to mitochondria → aerobic metabolism
  • Transport of O2 in blood

Metabolism of erythrocytes

The lifespan of RBCs is about 120 days. At this time, macrophages in the reticuloendothelial system of the bone and the spleen phagocytose RBCs. They are then broken down and their parts recycled.

Globin metabolism

Hemoglobin metabolism

Process: heme (red) → biliverdin (green pigment) → bilirubin (yellow pigment)

  1. Heme is converted to biliverdin by heme oxygenase.
  2. Biliverdin is converted to bilirubin by biliverdin reductase.

Heme breakdown is responsible for the color changes in hematomas!

Bilirubin metabolism

  1. Unconjugated bilirubin (insoluble in water) is released into the blood by macrophages → binds to albumin and reaches the liver.
  2. Unconjugated bilirubin is converted into bilirubin via enzyme UDP-glucuronosyltransferase in the liver.
    • Bilirubin is conjugated with glucuronic acid → bilirubin diglucuronide = direct bilirubin (water soluble)
    • Most direct bilirubin is excreted into the GI tract via bile, while some is released into the blood.
  3. Bilirubin diglucuronide excreted in bile is broken down by GI bacteria to urobilinogen

2,3- bisphosphoglycerate shunt

  • 2,3-bisphosphoglycerate mutase is vital for the formation of 1,3-bisphosphoglycerate (intermediate in glycolysis) → 2,3-BPG
  • 2,3-BPG binds to hemoglobin → conformational change → oxygen is released into local tissues
  • 2,3-BPG binds with greater affinity to deoxygenated hemoglobin than oxygenated hemoglobin (allosteric effector)

Energy production

References:[3][4]

Erythrocyte morphology

Dysmorphic RBCs

Morphology Associated Conditions
Dacryocytes
(teardrop cells, teardrop erythrocytes)		 Teardrop-shaped
Sickle cells
(drepanocytes)		 Sickle-shaped
Schistocytes
(fragmentocytes)

Fragmented

  • Microangiopathic hemolytic anemia (e.g., HUS, DIC, TTP)
  • Mechanical damage: artificial cardiac valves, extracorporeal circulation
Macrocytes
(megalocytes)

Large, spherical

Spherocytes

Spherical

Echinocytes
(burr cells)

Smooth, rounded, and evenly spaced cytoplasmic projections

Target cells
(codocytes)

Bullseye appearance

Acanthocytes
(spur cells)

Thorny projections

Stomatocytes

Slit-like central pallor most often caused by changes in membrane permeability

Degmacytes (bite cells) One more semicircular portions removed ("bitten off") from the cell margin

RBCs with abnormal contents

Content Appearance Cause
Heinz bodies Denaturated hemoglobin
  • Small
  • Red or pink
Basophilic stippling RNA
  • Small, basophilic granules
Howell-Jolly bodies DNA
  • Small basophilic spots
Pappenheimer bodies Iron
  • Small blue or purple granules

Hemoglobin variants

Hemoglobin undergoes conformational changes as it travels in the blood, and takes up oxygen in the lungs to deliver it to tissues. Also, changes in iron state (from Fe2+ to Fe3+) causes a change in the structure of hemoglobin within the RBC. The affinity and ability of hemoglobin to carry oxygen are dependent on its configuration.

Oxyhemoglobin and deoxyhemoglobin

Carboxyhemoglobin

Methemoglobin and methemoglobinemia

Oxygen concentration measured via pulse oximetry will remain high (> 80%) even if methemoglobin levels are very high!

References:[5][6]

Clinical significance

  • 1. Le T, Bhushan V,‎ Sochat M, Chavda Y, Zureick A. First Aid for the USMLE Step 1 2018. New York, NY: McGraw-Hill Medical; 2017.
  • 2. Kaplan. USMLE Step 1 Lecture Notes 2018: Biochemistry and Medical Genetics. New York, NY: Kaplan; 2017.
  • 3. Van Wijk R. The energy-less red blood cell is lost: erythrocyte enzyme abnormalities of glycolysis. Blood. 2005; 106(13): pp. 4034–4042. doi: 10.1182/blood-2005-04-1622.
  • 4. Siems WG, Sommerburg O, Grune T. Erythrocyte free radical and energy metabolism. Clin Nephrol. 2000; 53(1 Suppl): pp. S9–17. pmid: 10746800.
  • 5. Schuerholz T, Irmer J ,Simon TP, Reinhart K,Marx G. Methemoglobin level as an indicator for disease severity in sepsis. Crit Care. 2008; 12(2): p. 448. doi: 10.1186/cc6669.
  • 6. Ohashi K, Yukioka H, Hayashi M, Asada A. Elevated methemoglobin in patients with sepsis. Acta Anaesthesiologica Scandinavica. 1998; 42(6): pp. 713–716. doi: 10.1111/j.1399-6576.
last updated 03/26/2019
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