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.
- No nucleus, cell organelles, or granules
- Maintained by spectrin, a cytoskeletal protein
- Leads to a large surface area to volume ratio, enabling rapid gas exchange
- Provides great flexibility, allowing for easy movement through narrow vessels and capillaries
- Contain hemoglobin, which leads to acidophilic cytoplasm
- Anisocytosis: the presence of RBCs of varying sizes
- Poikilocytosis: the presence of RBCs of abnormal shapes
|Overview of dysmorphic RBCs|
(teardrop cells, teardrop erythrocytes)
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| Sickle cells |
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| Schistocytes |
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| Macrocytes |
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| Echinocytes |
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|Target cells |
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Stomatocytes (mouth cells)
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|Degmacytes (bite cells)|| |
HALT when you see a Target! Associated etiologies of target cells are: H – Hemoglobinopathies (HbC/HbS), A – Asplenia, L – Liver disease, T – Thalassemia
Dacryocytes (teardrop cells) are formed via extramedullary erythropoiesis: “Don't cry, we can play outside to-marrow.”
RBCs with inclusion bodies
|Overview of erythrocyte inclusion bodies|
|Heinz bodies|| || |
|Pappenheimer bodies|| |
|Basophilic stippling|| || |
|Ringed sideroblasts|| |
|Howell-Jolly bodies|| |
|Iron granules|| || || |
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 chains are released and converted into amino acids.
Process: heme (red) → biliverdin (green pigment) → bilirubin (yellow pigment)
Heme is converted to biliverdin by heme oxygenase.
- Requires NADPH + H+
- Carbon monoxide (CO) is released.
- Iron is released, oxidized, and taken up by transferrin to be recycled.
- Biliverdin is converted to bilirubin by biliverdin reductase (requires NADPH + H+)
Heme breakdown is responsible for the color changes in hematomas.
- Unconjugated bilirubin (insoluble in water) is released into the blood by macrophages → binds to albumin and reaches the liver
Unconjugated bilirubin is converted into bilirubin via the enzyme UDP-glucuronosyltransferase in the liver.
- Bilirubin is conjugated with glucuronic acid → bilirubin diglucuronide = conjugated bilirubin (water soluble)
- Most conjugated bilirubin is excreted into the GI tract via bile, while some is released into the blood.
Conjugated bilirubin excreted in bile is broken down by GI bacteria into urobilinogen
- Most urobilinogen is converted to stercobilin → excreted in feces (brown color).
- Part of urobilinogen oxidates to urobilin → excreted in the urine (yellow color).
Indirect (unconjugated) bilirubin is insoluble in water.
2,3-bisphosphoglycerate mutase is vital for the formation of 1,3-bisphosphoglycerate (intermediate in glycolysis) → 2,3-BPG
- 2,3-BPG mutase is unique to erythrocytes and placental cells.
- 2,3-BPG binds to hemoglobin → conformational change → release of oxygen into tissue
- 2,3-BPG binds with greater affinity to deoxygenated hemoglobin than oxygenated hemoglobin.
- Mature RBCs are unable to generate energy (ATP) via the Krebs cycle because they lack mitochondria.
- Glucose is the main source of energy for RBCs.
Biochemical energy-producing pathways available in RBCs
- Glycolysis (catabolizes 90% of the available glucose) 
- Pentose-phosphate pathway
- Glutathione reduction (a reaction that converts oxidized glutathione to its reduced form)
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 “Hemoglobin variants” below for details).
- For more information about disorders of Hb, see “Anemia.”
- Heme is synthesized from protoporphyrin, a porphyrin.
Porphyrins: a group of intermediates in the heme synthesis pathway that is composed of four subunit rings
- Includes porphobilinogen and uroporphyrinogen
- Binding of a central ferrous iron (Fe2+) forms a porphyrin complex (i.e., heme).
- The steps of heme synthesis occur both in the cytoplasm and the mitochondria (the first and final steps occur in mitochondria).
|Overview of heme synthesis|
|Mitochondria|| || || |
|Cytoplasm|| || || |
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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 is an integral part of the Hb molecule.
- Tetramer consisting of four individual polypeptide subunits that bind heme; composed of amino acids that fold to form 8 alpha helices
- There are 6 types of globin chains.
- β, δ, γ, ε are encoded on chromosome 11.
- α and ζ are encoded on chromosome 16.
- The combination of subunits in the Hb molecule determines the type of Hb (e.g., embryonic, fetal, newborn, or adult Hb); each subunit is able to bind one O2 molecule
- Mutations in genes encoding globin results in Hb variants.
For genetic variants of hemoglobin patterns, see “Hemoglobin variants” below.
|HBA1 gene||HBA2 gene||HBZ1 gene||HBZ2 gene|
|α globin||ζ globin|
|Chromosome 11||HBB gene||β globin|| || |
|HBD gene||δ globin|| |
|HBG1 gene||γ globin|| || |
|HBE gene||ε globin|| || |
Sequence of switching from fetal to adult hemoglobin (γ-globin in HbF is replaced by β-globin in HbA, and α-globin is always present in both HbA and HbF): Gamma goes, Beta becomes, Alpha always.
|Overview of hemoglobin variants|
|Hemoglobin||Globin chains|| |
|Beta thalassemia||Alpha thalassemia||Sickle cells||Hemoglobin C|
|Minor (trait)|| |
|Sickle cell trait||Sickle cell disease||Hemoglobin SC disease (HbSC)||HbC carrier||HbC disease|
Oxygen and carbon dioxide transport
O2, CO2, and protons all bind Hb and influence one another's affinity to Hb, which is important for gas exchange.
- Hb releases CO2 more easily with increasing pO2 (Haldane effect).
- Hb binds CO2 more easily if the pO2 is low.
- Hb releases O2 easier with increasing H+(Bohr effect).
- CO2 is mainly carried in three forms in the body:
Oxygenation and deoxygenation of Hb
- Hemoglobin with oxygen bound to its heme component (oxygenated) → bright red blood
- Oxygenated hemoglobin has approximately 300 times higher affinity for oxygen compared to deoxygenated hemoglobin
- Exhibits positive cooperativity and positive allostery
- Hemoglobin with no oxygen bound to its heme component (deoxygenated) → dark red blood
- Blue to purple appearance of tissue during hypoxia → cyanosis
- Deoxygenated hemoglobin has a low affinity for oxygen → release of oxygen is promoted
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.
- For more details on the buffering mechanisms of the body, see “Compensation.”
- The O2 affinity of Hb is inversely proportional to the CO2 content and H+ concentration 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).
- 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
Oxygen-hemoglobin dissociation curve
- 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.
- Relationship between saturation and partial pressure of O2 is not linear
- Reflected in the sigmoidal shape (S-shape) of the curve, which begins flat, then rises steeply before plateauing.
- Due to positive cooperativity (the phenomenon by which, e.g., binding of one O2 molecule to a single heme increases the O2 affinity of hemoglobin, facilitating the binding of subsequent O2 molecules).
- Myoglobin is composed of a single polypeptide chain that is associated with a monomeric heme molecule.
- Therefore, it cannot show positive cooperativity and its curve is not sigmoidal.
- 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).
|Differences between the hemoproteins myoglobin and hemoglobin|
|Associated with|| || |
|Binds to|| || |
|Affinity for O2|| || |
|Function|| || |
Shift to the right of the oxygen dissociation curve
- ↓ Hb affinity for O2 → ↑ O2 dissociation from Hb → ↑ tissue oxygenation → ↑ P50 (the partial pressure of oxygen needed to maintain 50% of Hb saturated)
Causes of shift to the right include:
- ↑ 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 oxygen dissociation curve
- ↑ Hb affinity for O2 → ↓ O2 dissociation from Hb → ↓ tissue oxygenation → ↑ erythropoietin synthesis (due to renal hypoxia) → compensatory erythrocytosis
Causes of shift to the left include:
- ↓ Partial pressure of carbon dioxide (PCO2)
- ↓ Body temperature
- ↓ H+ (↑ pH)
- ↓ 2,3-BPG (e.g., due to mutations in the BPGM gene on chromosome 7) 
- ↑ CO
- ↑ Methemoglobin
- ↑ Fetal hemoglobin (HbF)
Oxygen transport and conditions that affect oxygenation
|Overview of factors that affect oxygenation|
|Anemia (e.g., due to chronic blood loss)||↓||↓|| |
Arterial oxygen content (CaO2)
- Definition: the sum of oxygen bound to hemoglobin and dissolved in plasma within arterial blood
Formula: arterial oxygen content (CaO2, mL of oxygen per 100 mL of blood) = (1.34 x Hb x SaO2) + (0.003 x PaO2)
- Hb (g/dL) = hemoglobin concentration
- SaO2 (%) = arterial oxygen saturation in hemoglobin
- PaO2 (mm Hg) = arterial oxygen saturation in plasma (partial pressure of oxygen)
- 1.34 (mL) = maximum oxygen binding capacity of 1 g of hemoglobin. Normal blood Hb concentration is approximately 15 g/dL. The oxygen-binding capacity of blood is, therefore: 15 g/dL x 1.34 mL ≈ 20 mL of oxygen per dL of blood.
- 0.003 = solubility coefficient of oxygen in plasma
- Arterial oxygen content is directly proportional to the Hb, SaO2, and PaO2.
Oxygen delivery (DO2)
- Definition: the rate at which oxygen is transferred from the lungs to the peripheral circulation
Formula: oxygen delivery (DO2, mL O2/min) = cardiac output (CO) x arterial oxygen content (CaO2)
- Oxygen delivery to an organ = organ blood flow rate x arterial oxygen content (CaO2). For example, renal oxygen delivery (renal DO2) = renal blood flow rate (RBF) x arterial oxygen content (CaO2).
- In an adult with a cardiac output of 5 L/min, the oxygen delivery is ∼ 1000 mL/min.
- Oxygen delivery is directly proportional to cardiac output and arterial oxygen concentration.
Carbon monoxide poisoning
- Formation of carboxyhemoglobin: hemoglobin that is removed from oxygen transportation because its oxygen binding site is occupied by carbon monoxide
- See “Carbon monoxide intoxication.”
- A form of hemoglobin that contains iron in its oxidized (ferric = Fe3+) rather than its reduced state (ferrous = Fe2+) and therefore cannot bind oxygen.
- Ferric iron binds less readily to oxygen compared to ferrous iron, leading to a decrease in blood oxygen saturation and total oxygen content (can lead to tissue hypoxia).
- Ferric iron has a high affinity for cyanide (therefore, in cyanide poisoning, amyl nitrite is given → Fe2+ turns into Fe3+→ formation of methemoglobin → binding of free cyanide in the blood to methemoglobin → less cyanide binding to cytochrome c oxidase)
- 0–3% of total hemoglobin: physiological
- > 3%: visible cyanosis (skin and membranes are brownish-blue or gray)
- > 20%: clinical symptoms of oxygen deprivation; brown blood ("chocolate-colored blood")
- > 70%: fatal
Causes of methemoglobinemia
Exposure to substances that increase methemoglobin levels (most common cause)
Nitrate/nitrite-containing compounds (e.g., drinking water, meat and cheese preservatives)
- These compounds are partially converted to nitrite in the gastrointestinal tract → nitrite oxidizes iron from its reduced state to its oxidized state, catalyzing the conversion of hemoglobin to methemoglobin.
- Drugs: nitroglycerin, sulfonamides, dapsone, inhaled nitric oxide, topical anesthetics such as lidocaine or benzocaine, and aniline derivatives
- Nitro and amino compounds of benzene
- Nitrate/nitrite-containing compounds (e.g., drinking water, meat and cheese preservatives)
- In newborns and infants, the enzyme activity of methemoglobin reductase (NADH-cytochrome b5 reductase) is low → ↑ risk of methemoglobinemia
- Congenital (hereditary) methemoglobinemia: autosomal recessive deficiency of methemoglobin reductase
- Glucose-6-phosphate dehydrogenase deficiency
- Exposure to substances that increase methemoglobin levels (most common cause)
- Dark brown (“chocolate”) discoloration of the blood
- Reduced oxygen saturation and total oxygen content with normal PaO2 on arterial blood gas analysis despite clinical signs of cyanosis
- Confirm with CO-oximetry.
- At lower concentrations, it is highly effective at reducing methemoglobin to hemoglobin.
- Contraindications: G6PD deficiency , concomitant use of serotoninergic drugs
- Reducing agents such as ascorbic acid (vitamin C) and riboflavin may sometimes help to reduce methemoglobin to hemoglobin.
- In acquired methemoglobinemia: Assess for pharmaceutical triggers and cease therapy with any agents thought to be the cause.
- Last resort for patients who do not respond to methylene blue: exchange transfusion
- Methylene blue
Because MetHb artificially increases pulse oximeter readings, oxygen concentration measured via pulse oximetry remains high (> 80%) even if methemoglobin levels are very high!
To remember that ferrous iron (Fe2+) is the reduced form of iron, think: “Joe reduced 2 ferocious animals from his iron cage.