How Salt Concentration and Electrolytes Affect Oxygenation

1. The Oxygen-Hemoglobin Dissociation Curve - The Foundation

Effects of temperature (left) and pH (right) on the O2-Hb dissociation curve. Lower pH (7.2) and higher temperature (43°C) shift the curve rightward - Ganong's Review of Medical Physiology

2. Hydrogen Ions (H+) - The Most Direct Electrolyte Effect

• Rising H+ (falling pH) → shifts ODC to the right → Hb releases more O2 to tissues
• Falling H+ (rising pH) → shifts ODC to the left → Hb holds onto O2

HbO2 + H+ ⇌ HbH + O2

3. Bicarbonate (HCO3-) and CO2 - The Indirect Electrolyte Pathway

CO2 + H2O → H2CO3 → H+ + HCO3-

• The H+ produced drives the Bohr effect (see above), releasing O2 in the tissues
• The HCO3- exits the RBC in exchange for Cl- - this is the chloride shift (Hamburger shift), which maintains electrical neutrality across the RBC membrane

4. Chloride (Cl-) - Direct Allosteric Modulator

"The transition between the two structures [T and R state] is influenced by protons, carbon dioxide, chloride, and 2,3-bisphosphoglycerate (BPG); the higher their concentration, the more oxygen must be bound to trigger the transition." - Harper's Illustrated Biochemistry, 32e

5. 2,3-Bisphosphoglycerate (2,3-BPG) - The Major Organic Phosphate Electrolyte

• 2,3-BPG binds exclusively to deoxyhemoglobin (T state), inserting into the central cavity between the β chains
• This stabilizes the deoxy form and shifts the ODC significantly to the right
• The equilibrium: HbO2 + 2,3-BPG ⇌ Hb-2,3-BPG + O2

• The ODC shifts right
• O2 is released to tissues at a higher tissue pO2 (~10 mmHg higher)
• This is a key adaptive mechanism in conditions of prolonged O2 deprivation

• The ODC shifts left
• Hb binds O2 tightly and cannot easily release it to tissues
• This is a clinical concern with massive transfusion of old stored blood

6. Calcium (Ca2+), Potassium (K+), and Sodium (Na+)

7. Salt (Ionic Strength) and Osmolality Effects

• Very high salt concentrations (hypernatremia, hyperchloremia) raise osmolality, causing RBC shrinkage (crenation), which can concentrate intracellular 2,3-BPG and slightly affect O2 affinity
• Severe hypertonicity impairs RBC deformability, reducing capillary transit and tissue O2 delivery even if Hb saturation is maintained
• In isolated hemoglobin solutions (relevant to blood substitutes and lab physiology), increasing ionic strength (NaCl concentration) can modify cooperativity and O2 affinity by altering the electrostatic environment of the α-β subunit interfaces

8. Clinical Correlations

Summary of Curve-Shifting Factors

The Bohr Effect

The Core Concept

As pH falls (from 7.6 → 7.2 → 6.8), the ODC shifts progressively right - more O2 is unloaded at any given pO2 in the tissues. - Basic Medical Biochemistry, 6e

HbO2 + H+ ⇌ HbH+ + O2

Mechanism at the Molecular Level

• T state (tense/deoxy) - low O2 affinity, stabilized by salt bridges between subunits
• R state (relaxed/oxy) - high O2 affinity, salt bridges broken

1. When H+ rises in tissues → these groups become protonated (charged) → they form ionic salt bridges
2. Salt bridges stabilize the T (deoxy) conformation → O2 affinity drops → O2 is released

How CO2 Drives the Bohr Effect in Tissues

In the tissues: CO2 enters the RBC → carbonic anhydrase converts it to H2CO3 → dissociates to H+ + HCO3- → H+ binds HbO2 → O2 is released to tissues. - Basic Medical Biochemistry, 6e

1. Metabolically active cells produce CO2
2. CO2 diffuses into the RBC
3. Carbonic anhydrase catalyzes: CO2 + H2O → H2CO3 → H+ + HCO3-
4. The H+ produced binds to histidine residues on Hb → T state stabilized
5. O2 is released and diffuses into tissues
6. HCO3- exits the RBC in exchange for Cl- (the chloride shift) - maintaining electrical neutrality

• High pO2 → O2 loads onto Hb → conformational change to R state → H+ affinity drops
• H+ released → combines with HCO3- → forms H2CO3 → dissociates to CO2 + H2O
• CO2 is exhaled

Two Components of the Bohr Effect

Top panel: combined effect of rising PCO2 + falling pH (respiratory acidosis). Bottom panel: isolated pH effect at fixed PCO2. Most of the overall Bohr effect is pH-driven. - Medical Physiology (Boron & Boulpaep)

Magnitude of the Effect

• pH change from 7.4 → 7.2 shifts the ODC ~15% to the right (P50 rises from ~27 to ~31+ mmHg)
• At rest, the arterial-venous pH difference is only 0.03-0.05 units, so the Bohr effect contributes only an extra 2-3% unsaturation on top of the normal pO2-driven unloading
• During strenuous exercise with lactic acid production, pH drops further, and the Bohr effect becomes significantly more important for delivering O2 to working muscle
• The primary physiological benefit at rest is not increased O2 delivery per se, but rather the increased buffering capacity of deoxyhemoglobin for H+, which minimizes the fall in blood pH during CO2 metabolism - Fishman's Pulmonary Diseases

The Reciprocal Relationship: Bohr Effect and H+ Buffering

"The Bohr effect states that increased H+ causes a right shift of the O2-hemoglobin dissociation curve, causing hemoglobin to unload O2 more readily in the tissues; thus the H+ generated from tissue CO2 causes hemoglobin to release O2. In turn, deoxygenation of hemoglobin makes it a more effective buffer for H+." - Costanzo Physiology, 7e

Clinical Relevance

Summary

Factors Controlling Haemoglobin Formation

1. Role of Proteins

• Haem - the iron-containing porphyrin ring that binds oxygen
• Globin - the protein chains (alpha and beta) that form the scaffold of the molecule

2. Role of Minerals

(i) Iron - The Most Critical Mineral

• Iron makes up 0.33% of the weight of haemoglobin
• Normal blood contains 15 g of Hb per 100 mL
• So iron in 100 mL blood = 15 × 0.33/100 = ~0.05 g = 50 mg of iron per 100 mL blood

• RBC lifespan = 120 days
• So each day, 1/120 = 0.83% (~0.8%) of total RBCs are destroyed (by the spleen/liver via haemolysis of old cells)
• Total blood volume ≈ 6 litres
• 0.8% of 6 litres = 50 mL of blood is broken down daily
• Each 100 mL contains 50 mg of iron, so 50 mL contains ~25 mg of iron released daily

(ii) Copper - The Helper Mineral

1. Absorption - Copper is needed for the intestinal absorption of iron
2. Mobilization - Copper-containing protein ceruloplasmin (a ferroxidase enzyme made in the liver) oxidizes iron from Fe²+ (ferrous) to Fe³+ (ferric), which is the form needed for iron to be loaded onto transferrin (the blood transport protein for iron)
3. Utilization - Without ceruloplasmin, iron cannot be efficiently released from storage sites (liver, spleen) into the circulation for use in bone marrow

"Iron is absorbed in the ferrous (Fe²+) state but is oxidized to the ferric state by a ferroxidase known as ceruloplasmin (a copper-containing enzyme) for transport through the body." - Basic Medical Biochemistry, 6e

Summary Table

Why 1/120 is Used

A Simple Analogy

• Total RBC lifespan = 120 days
• Each day = 1 day out of that 120-day lifespan
• So 1/120th of all RBCs reach the end of their life and are destroyed daily by the spleen and liver

The Full Calculation in the Text

Treating Thalassaemia by Increasing HbF

Why HbF Helps in Thalassaemia - The Rationale

• β-thalassaemia is caused by reduced or absent β-globin chain synthesis
• Excess α-globin chains accumulate, precipitate inside RBC precursors, and destroy them (ineffective erythropoiesis)
• This causes severe haemolytic anaemia

• γ chains pair with the excess α chains → reduces the toxic free α-chain burden
• HbF carries oxygen effectively (actually with higher affinity than HbA)
• Anaemia is ameliorated and transfusion requirements fall

Approach 1: Pharmacological HbF Induction

(a) Hydroxyurea (Hydroxycarbamide)

• The oldest and most established HbF-inducing drug
• A ribonucleotide reductase inhibitor that causes mild myelosuppression → stimulates stress erythropoiesis → erythroid precursors reactivate γ-globin gene transcription (via poorly defined epigenetic mechanisms including DNA hypomethylation)
• Well-proven in sickle cell disease - reduces pain crises, acute chest syndrome, and transfusion needs
• Less effective in β-thalassaemia - because the HbF level needed to compensate in thalassaemia is much higher than in sickle cell disease. The clinical benefit is real but more limited

"Whereas the effect of pharmacologic treatments (particularly hydroxyurea) in sickle cell disease is clear, their benefit on the clinical course of β-thalassemia is presently limited, perhaps because of the higher level of HbF required in β-thalassemia to achieve clinical results." - Goldman-Cecil Medicine, International Ed.

(b) Hypomethylating Agents

• 5-Azacytidine and decitabine (DNA methyltransferase inhibitors)
• DNA methylation of the γ-globin promoter normally silences HbF after birth
• These drugs demethylate the γ-globin promoter → reactivate γ-globin transcription
• Effective at raising HbF but have significant toxicity concerns (myelosuppression, potential mutagenicity), limiting their routine clinical use

(c) Histone Deacetylase (HDAC) Inhibitors

• e.g., butyrate, valproic acid, vorinostat
• Histone deacetylation compacts chromatin around the γ-globin gene loci, silencing them
• HDAC inhibitors open up chromatin → allow transcription factors access → HbF induced
• Largely experimental; butyrate has been tested in clinical trials with modest results

Approach 2: Targeting BCL11A - The Key Molecular Switch

• Natural mutations that reduce BCL11A function → hereditary persistence of foetal haemoglobin (HPFH) - people with this are clinically normal even if they have β-thalassaemia or sickle cell, because HbF persists
• This means: silencing BCL11A = sustained HbF production = treatment

"BCL11A is a transcription factor that normally silences HbF expression. Deletion of BCL11A leads to persistence of fetal hemoglobin, and this gene is now the target of exciting clinical trials." - Fishman's Pulmonary Diseases

Approach 3: Gene Therapy - CRISPR/Cas9 Editing (Current Frontier)

How it works:

1. Patient's own haematopoietic stem cells (HSCs) are harvested from bone marrow
2. CRISPR-Cas9 is used to disrupt the erythroid-specific enhancer of BCL11A in those HSCs
3. Without this enhancer, BCL11A cannot be expressed in red cell precursors
4. BCL11A is silenced → γ-globin gene is de-repressed → HbF is permanently produced
5. The edited HSCs are transplanted back into the patient (after conditioning chemotherapy)

Results (from Harrison's Principles of Internal Medicine, 22e - 2025):

"CRISPR/Cas editing to downregulate BCL11A has resulted in increases in total hemoglobin ≥12 g/dL and HbF ≥10 g/dL, leading to transfusion independence in >90% of 52 patients aged 12-35 years with transfusion-dependent β-thalassemia."

Summary of All Approaches

Sickle Cell Anaemia and Its Relation to HbF

1. The Basic Defect

• Normal: Glutamic acid at position 6 of the β-globin chain (negatively charged, hydrophilic)
• Mutation: Valine substituted at position 6 (neutral, hydrophobic) - caused by A→T change in codon 6

2. The Sickling Process - Step by Step

A: Deoxygenated HbS tetramers → B: Aggregation → C: Long rigid polymer fibres → D: Sickle-shaped RBC → E: Vaso-occlusion in capillaries

1. In tissues (low pO2) → HbS deoxygenates → valine-6 hydrophobic patch exposed
2. HbS molecules stack into polymer fibres (a gel/crystal)
3. Fibres distort the RBC from flexible biconcave disc → rigid sickle shape
4. Sickled cells are inflexible → get stuck in small capillaries
5. Vaso-occlusion → tissue ischaemia and pain crises (the hallmark of SCA)
6. Sickled cells are also fragile → rupture easily → haemolytic anaemia

3. Why HbF Is the Key Relationship

When HbF is present alongside HbS inside the same RBC, HbF molecules interrupt and terminate the HbS polymer chains. They physically block polymerisation.

• If HbF > ~20% of total Hb in a cell → sickling is essentially prevented in that cell
• People with Hereditary Persistence of Fetal Haemoglobin (HPFH) who also carry HbS have mild or no disease because HbF protects every RBC
• The more HbF, the less sickling, the fewer crises

4. How HbF Is Raised in Sickle Cell Treatment

Hydroxyurea - The Established Drug

• Mechanism: Inhibits ribonucleotide reductase → mild myelosuppression → bone marrow "stresses" → stress erythropoiesis reactivates γ-globin gene → ↑ HbF production
• HbF increases heterocellularly (unevenly - some cells get more protection than others)
• Average HbF rises from ~5% baseline to 15-20% on treatment

"Hydroxyurea increases HbF levels, thus diluting HbS. Polymerisation of HbS is delayed and reduced in treated patients, so painful crises are not caused by sickled cells blocking capillaries." - Lippincott Illustrated Pharmacology

• ↓ Frequency of painful vaso-occlusive crises
• ↓ Acute chest syndrome episodes
• ↓ Hospitalisations and blood transfusions
• ↓ Mortality
• Recommended for all children and adults with SCA regardless of severity

CRISPR Gene Therapy (Casgevy) - The Curative Approach

• Same mechanism as described for thalassaemia - disrupts BCL11A erythroid enhancer in patient's own stem cells
• BCL11A cannot silence the γ-globin gene → permanent, high-level HbF in all RBCs
• Results: HbF rises to >40%, sickling essentially eliminated, no more pain crises in most patients
• Approved by FDA/EMA in 2023

Voxelotor - A Different Approach

• Does not raise HbF directly
• Instead binds to the α chain of HbS, stabilising it in the oxygenated (R) state
• Deoxygenated HbS concentration decreases → less polymerisation
• Approved for patients ≥12 years old

Crizanlizumab - Addresses Downstream Effects

• Monoclonal antibody against P-selectin
• Blocks adhesion of sickled RBCs to endothelium → fewer vaso-occlusive crises
• Does not affect HbS polymerisation directly

5. Sickle Cell + Thalassaemia - The Combined Condition

• Patient inherits one HbS allele (from one parent) and one β-thalassaemia allele (from the other)
• This is one of the sickle cell disease syndromes - can be as severe as HbSS (if β⁰-thal) or milder (if β⁺-thal, which allows some normal HbA production)
• HbF induction with hydroxyurea or CRISPR therapy is equally applicable here

Summary Table