• Clinical science

Erythrocyte morphology and hemoglobin

Summary

Erythrocytes, or red blood cells (RBCs), are the most common blood cells. Normal RBCs have a biconcave shape and contain hemoglobin but no nucleus or organelles. Dysmorphic RBCs (e.g., sickle cells, target cells) have an altered form and are often a sign of an underlying condition. Hemoglobin (Hb) is composed of heme and globin subunits and is responsible for transporting oxygen and carbon dioxide throughout the body. Hb can undergo conformational changes (e.g, depending on its oxygenated state), which influence how it binds and releases O2 and CO2. Deficient Hb (anemia), genetic Hb variants (e.g., HbS, HbC), and certain substances (carbon monoxide, nitrates that form methemoglobin, cyanide) affect the affinity for and ability to transport O2, resulting in decreased tissue oxygenation.

Erythrocyte morphology

Normal morphology

  • Biconcave shape: large surface area-to-volume ratio for rapid gas exchange
  • Content

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

Small, spherical, no central pallor

Elliptocytes (ovalocytes)

Oval or elliptical
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 or more semicircular portions removed ("bitten off") from the cell margin


RBCs with abnormal content

Content Appearance Cause
Heinz bodies Denaturated hemoglobin
  • Small
  • Red or pink
Pappenheimer bodies Iron
  • Small, blue or purple granules
Basophilic stippling RNA
  • Small, basophilic granules throughout cytoplasm
Ringed sideroblasts Iron
Howell-Jolly bodies DNA (nuclear remnants)
  • A typically small, basophilic spot, located toward the periphery of the cell

Metabolism of erythrocytes

The lifespan of RBCs is about 120 days. After 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

Globin chains are released and converted into amino acids.

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 the enzyme UDP-glucuronosyltransferase in the liver.
    • Bilirubin is conjugated with glucuronic acidbilirubin 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 into 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.

Energy production

References:[1][2]

Hemoglobin synthesis

Overview

Hb is a circulating globular protein composed of a heme moiety with a central iron ion and four 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 state of oxygenation), which influence how it binds and releases O2 and CO2.
  • Deficient or defective Hb can ultimately affect the transport of O2 (see Hb variants for details).
  • For more information about disorders of Hb, see the learning card on anemia.

Heme synthesis

Location Reaction Enzyme Clinical significance
Mitochondria

Glycine + succinyl CoAδ-aminolevulinic acid (ALA)

ALA synthase

Sideroblastic anemia due to:

Cytoplasm

Two molecules of δ-ALA combine → porphobilinogen

ALA dehydratase

Sideroblastic anemia due to lead poisoning (inhibits ALA dehydratase)

Porphobilinogenhydroxymethylbilane

Porphobilinogen deaminase

Enzyme deficiency causes acute intermittent porphyria.

HydroxymethylbilaneUroporphyrinogen III

Uroporphyrinogen-III synthase -

Uroporphyrinogen III → coproporphyrinogen III

Uroporphyrinogen decarboxylase

Enzyme deficiency causes porphyria cutanea tarda.

Coproporphyrinogen III (in cytoplasm) → protoporphyrinogen III (in mitochondria)

Coproporphyrinogen III oxidase

-
Mitochondria

Protoporphyrinogen III → protoporphyrin IX

Protoporphyrin oxidase

-

Protoporphyrin binds iron (Fe2+)heme

Ferrochelatase

Sideroblastic anemia due to:

Sideroblastic anemia is due to ineffective heme synthesis, which may be congenital (X-linked defect in the δ-ALA synthase gene) or acquired (e.g., vitamin B6 deficiency, or lead poisoning leading to sequential inhibition of δ-ALA dehydratase and ferrochelatase).

Globin

  • Globin is an integral part of the Hb molecule.
  • Tetramer consisting of four individual subunits that bind heme
    • Composed of amino acids that fold to form 8 alpha helices
  • There are 6 types of globin chains.
  • The combination of subunits in the Hb molecule determines the type of Hb (e.g., embryonic, fetal, newborn, or adult Hb).
  • Mutations in genes encoding globin results in Hb variants.

Hemoglobin patterns

For genetic variants of hemoglobin patterns, see hemoglobin variants below.

Chromosome 16
HBA1 gene HBA2 gene HBZ1 gene HBZ2 gene
α globin ζ globin
Chromosome 11 HBB gene β globin
  • Portland 2 (ζζββ): embryonic Hb
HBD gene δ globin
HBG1 gene γ globin
  • HbF (fetal hemoglobin): ααγγ
    • Adult: ∼ 0.8–2%
    • Newborn: ∼ 80%
    • Synthesized in
    • Has ↓ binding of 2,3-bisphosphoglycerate (2,3-BPG) → affinity for O2↑ O2 extraction from the maternal circulation (via the placenta)
  • Portland 1 (ζζγγ): embryonic Hb
HBG2 gene
HBE gene ε globin
  • Gower 2 (ααεε): embryonic
  • Gower 1 (ζζεε): embryonic Hb

References:[3][4]

Hemoglobin variants

Hemoglobin Globin chains Beta thalassemia Alpha thalassemia Sickle cells Hemoglobin C
Minor (trait)

Major

(Cooley's anemia)

Silent carrier

(minima)

Trait

(minor)

Hb H disease

(intermedia)

Hb Bart disease

(major)

Sickle cell trait Sickle cell disease Hemoglobin SC disease (HbSC) HbC carrier HbC disease
HbA ααββ Absent Normal ↓↓ Absent Absent Absent Absent
HbA2 ααδδ ↑↑ Normal ↓↓ Absent Absent Absent
HbF ααγγ ↑↑ Normal ↓↓ Absent Normal Normal Normal Absent
HbH ββββ Absent Absent Absent ↑↑ ↑↑ Absent Absent Absent Absent Absent
Hb Bart γγγγ Absent Absent Absent Absent ↑↑ Absent Absent Absent Absent Absent
HbS ααββ Absent Absent Absent Absent Absent Absent ↑↑ Absent Absent
HbC ααββ Absent Absent Absent Absent Absent Absent Absent Absent ↑↑

Oxygen and carbon dioxide transport

Overview

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

Oxygenation and deoxygenation of Hb

  • Oxyhemoglobin: hemoglobin with oxygen bound to its heme component (oxygenated) → bright red blood
  • Deoxyhemoglobin: hemoglobin with no oxygen bound to its heme component (deoxygenated) → dark red blood; blue to purple appearance of tissue during hypoxia → "cyanosis"

Bicarbonate buffer system

  • RBCs carry carbonic anhydrase, which converts HCO3- and H+ to H2O and CO2 in the following steps: HCO3 + H+ H2CO3 ⇄ H2O + CO2
  • Ultimately, excess H+ during acidic states is eliminated through conversion to CO2, which can be exhaled.
  • During basic states, the bicarbonate buffer system can reverse 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. HCO3- accounts for 50% of the blood buffer capacity.
  • For more details on the buffering mechanisms of the body, see compensation in acid-base disorders.

Bohr effect

  • The O2 affinity of Hb is inversely proportional to the CO2 content and pH of blood.
  • 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 + CO2Hb-COO- + H+ + O2

Haldane effect

  • The CO2 affinity of Hb is inversely proportional to the oxygenation of Hb.
  • When Hb is deoxygenated (typically in peripheral tissue), uptake of CO2 is facilitated.
  • When Hb is oxygenated (in high pO2, for example, in the lungs):
    • Oxygenated Hb has a decreased affinity for CO2. . → CO2 that is bound to Hb is released in the pulmonary arteries to diffuse into the alveoli (the O2-Hb dissociation curve is shifted to the left).
    • Hb releases bound H+ → ↑ H+ shifts equilibrium to CO2 production (see equation above) → CO2 is exhaled in lungs

References:[3][4]

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 mitochondriaaerobic metabolism
  • Transport of O2 in blood

Conditions that affect oxygenation and oxygen transport

Overview

CaO2

SaO2

PaO2

O2-Hb dissociation curve

Anemia (e.g., due to chronic blood loss)

Normal

Normal

Normal

CO poisoning (carboxyhemoglobin) and methemoglobinemia

Normal Left-shifted
Cyanide poisoning Normal Normal Normal Normal
Polycythemia Normal Normal Normal
High-altitude exposure Normal Right-shifted

Carbon monoxide poisoning

Methemoglobinemia

Because MetHb artificially increases pulse oximeter readings, oxygen concentration measured via pulse oximetry remains high (> 80%) even if methemoglobin levels are very high!

References:[5][6]

Clinical significance

  • 1. 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.
  • 2. Siems WG, Sommerburg O, Grune T. Erythrocyte free radical and energy metabolism. Clin Nephrol. 2000; 53(1 Suppl): pp. S9–17. pmid: 10746800.
  • 3. 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.
  • 4. Kaplan. USMLE Step 1 Lecture Notes 2018: Biochemistry and Medical Genetics. New York, NY: Kaplan; 2017.
  • 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 09/02/2019
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