What causes a right shift in the oxygen-hemoglobin dissociation curve?

Right shift is caused by increased CO2, decreased pH (acidosis), increased 2,3-BPG, increased temperature, and vigorous exercise. The mnemonic CADET helps: CO2, Acid, 2,3-DPG, Exercise, Temperature. Right shift means lower O2 affinity, which helps release oxygen to working tissues.

What is the difference between HbF and HbA oxygen affinity?

Fetal hemoglobin (HbF) has lower affinity for 2,3-BPG than adult HbA. Because 2,3-BPG normally reduces O2 affinity, HbF (which ignores it) has higher O2 affinity. This allows HbF to pull oxygen from maternal HbA across the placenta.

Why does carbon monoxide cause a left shift on the dissociation curve?

Carbon monoxide binds hemoglobin 200 times more strongly than oxygen and causes an allosteric conformational change that shifts the remaining unbound subunits into the R-state (high affinity). This left shift means the remaining bound O2 is held too tightly and cannot be released to tissues.

What is the P50 of normal adult hemoglobin?

Normal adult hemoglobin has a P50 of approximately 26 mmHg. This is the partial pressure of oxygen at which hemoglobin is 50% saturated. Right shift increases P50 (less affinity); left shift decreases P50 (more affinity).

What is the significance of pO2 60 mmHg on the dissociation curve?

At a pO2 of 60 mmHg, hemoglobin is approximately 90% saturated. This sits at the 'knee' of the sigmoid curve where small drops in pO2 cause large drops in saturation. Saturation above 90% ensures adequate oxygen delivery to the brain, heart, and kidneys.

Hemoglobin & O₂ Dissociation

Cooperative binding · Bohr effect · P50 · HbF vs HbA · Toxic hemoglobins · Sickle cell

Hemoglobin T-state to R-state conformational change animation

Sickle cell anemia blood smear - crescent shaped RBCs

The Numbers That Win Points

pO2 = 60 mmHg= 90% saturation (the knee)

P50 (normal HbA)= 26 mmHg

CO vs O2 affinityCO binds 200x stronger

HbA: 2alpha + 2beta98% of adult Hb

HbF: 2alpha + 2gammaleft shift (ignores 2,3-BPG)

Cooperative Binding: T-state to R-state

Hemoglobin starts in T-state (tense, low affinity). Each O2 that binds shifts the whole tetramer toward R-state (relaxed, high affinity). This allosteric switch is WHY the curve is sigmoid.

T-state: All four subunits locked. Low O2 affinity. This is deoxygenated hemoglobin in peripheral tissues.

O2-Hemoglobin Dissociation Curve

The S-shape is cooperative binding made visible. Click each curve to see what shifts it clinically.

Structure: The Hemoglobin Tetramer ▼

Hemoglobin is a quaternary protein: 4 globin chains, each hugging one heme group, each heme carrying one iron (Fe2+) that grabs one O2. Total capacity: 4 O2 per molecule.

Type	Subunits	Context	Curve
Hb A	2alpha + 2beta	98% adult	Normal
Hb A2	2alpha + 2delta	2% adult	Normal
Hb F	2alpha + 2gamma	Fetal / neonates	Left shift
Hb S	2alpha + 2beta(E6V)	Sickle cell	Normal P50; polymerizes when deoxy

Alpha chains are present in ALL types. The beta/gamma/delta chain difference defines each variant. Hb A = Alpha Beta = Adult.

The key board fact: Myoglobin is a MONOMER (1 chain, 1 heme). It has a hyperbolic curve (not sigmoid) because it cannot do cooperative binding. Highest O2 affinity of any oxygen-binding protein. It is the muscle oxygen reserve.

Right Shift: CADET, Face Right ▼

Right shift means lower O2 affinity: hemoglobin gives up oxygen more easily to tissues. P50 rises above 26 mmHg.

CADET (face right for right shift):
CO2 up · Acidosis (H+ up) · DPG (2,3-BPG) up · Exercise · Temperature up

The Bohr effect is the pH/CO2 piece: when CO2 rises in tissues, H+ rises, hemoglobin binds H+ and releases O2 to the tissues that are actually working. This is purposeful physiology, not a failure.

2,3-BPG is produced in RBCs during glycolysis. It stabilizes the T-state (deoxy conformation), shifting the curve right. Chronically hypoxic patients (high altitude, chronic anemia) upregulate 2,3-BPG as an adaptation to deliver more O2 despite lower Hb levels.

From the Attending

The board favorite: a patient with chronic anemia, Hgb 7, but surprisingly "comfortable." The question is asking you to explain why. The answer is compensatory 2,3-BPG elevation causing a right shift. Low Hb does not always equal incapacitation - the curve adapts.

Quick self-test: List all five CADET right-shift factors before revealing.

Left Shift: HbF, CO, Methemoglobin, Alkalosis ▼

Left shift means higher O2 affinity: hemoglobin holds onto O2 too tightly and refuses to release it to tissues. P50 falls below 26 mmHg.

FALCO: Fetal Hb · Alkalosis · Low temperature · CO poisoning · Other (methemoglobin)

Agent	How it shifts left	Board tell
HbF	Gamma chains ignore 2,3-BPG; HbF stays in R-state (high affinity)	Neonate; shifts to HbA by 6 months
CO	200x O2 affinity; allosteric lock into R-state for remaining subunits	Cherry red skin; SpO2 falsely normal
MetHb	Fe3+ cannot bind O2; remaining Fe2+ sites are locked in high-affinity state	Cyanosis with normal PaO2; chocolate blood
Alkalosis	Low H+ reduces Bohr effect; Hb holds O2 more tightly	Hyperventilation patient, CXR normal

CO poisoning board trap: Pulse oximetry reads falsely normal (cannot distinguish HbO2 from HbCO). The patient is dying with a 99% "saturation." Always order a co-oximeter ABG when CO poisoning is suspected, not a standard pulse ox.

Methemoglobin board trap: SpO2 is low (78-85%) but PaO2 on ABG is normal. Treatment: methylene blue 1-2 mg/kg IV. It reduces Fe3+ back to Fe2+ via NADPH pathway. Watch for G6PD-deficient patients: methylene blue fails (they cannot generate NADPH), and exchange transfusion is required instead.

Fetal Hemoglobin and the Placental Gradient ▼

In the placenta, maternal and fetal blood run side by side but do not mix. Fetal hemoglobin needs to pull oxygen off maternal HbA - a steeper left-shift is the mechanism.

HbF gamma chains bind 2,3-BPG poorly. Because 2,3-BPG normally stabilizes the T-state (reduces affinity), HbF without BPG binding stays in R-state: high affinity. Maternal HbA, which does bind BPG, has lower affinity. O2 moves from mother to fetus along this gradient.

HbF = BPG-independent = always R-state = high affinity = left shift. This CANNOT release O2 to tissues efficiently - fine for a fetus getting O2 from mom, fatal for an adult needing tissue delivery.

Hydroxyurea induces HbF production in sickle cell patients. HbF chains cannot participate in HbS polymerization (requires beta chains with valine at position 6). More HbF = fewer sickling events. Trade-off: bone marrow suppression (check CBC).

Physiologic timeline: HbF peaks at birth and falls to adult levels (less than 1%) by 6 months. Neonates with sickle cell disease are asymptomatic at birth because of high HbF. Symptoms emerge as HbF wanes after 6 months - the classic dactylitis presentation.

Sickle Cell Disease: Mechanism to Crisis ▼

The mutation: Beta-globin position 6 - glutamic acid (hydrophilic) replaced by valine (hydrophobic). One hydrophobic swap destroys a career.

When O2 is present: valine gets pushed toward the protein exterior by conformational change. RBC stays round. When O2 falls: valine sinks hydrophobically into adjacent HbS molecules, forming rigid polymers. Crescent shape, vessel occlusion.

Triggers of sickling: hypoxia, acidosis, dehydration, fever, infection, cold exposure, and high altitude. The board will present one of these as the precipitant.

Complication	Underlying process	Key board fact
Dactylitis	Vaso-occlusion in hand/foot vessels	First manifestation after 6 months
Acute chest syndrome	Pulmonary vaso-occlusion + infection	Most common cause of death
Aplastic crisis	Parvovirus B19 infects erythroid progenitors	Reticulocyte count falls to near zero
Autosplenectomy	Repeated splenic infarcts by age 6	Howell-Jolly bodies on smear
Splenic sequestration	Sudden pooling in spleen	Rapid fall in Hgb, shock
Avascular necrosis	Femoral/humeral head infarction	Hip pain with normal plain film early

Management: Acute crisis: O2, IV fluids, analgesics (opioids for severe pain). CVA: exchange transfusion (not tPA, not aspirin - this is mechanical, not thrombotic). Aplastic crisis: supportive care, transfusion if severe; Parvovirus infection is self-limited. Prophylaxis: pneumococcal, meningococcal, Hib vaccines; penicillin prophylaxis through age 5.

From the Attending

Aplastic crisis: the classic board setup is sickle cell child with "fever and rash" 2-3 weeks ago, now profoundly anemic. The rash was slapped-cheek from Parvovirus B19. Reticulocyte count is the key lab. If it is near zero, the marrow has shut down - not sickling, not sequestration. Know your clues.

O2 Content vs pO2 vs Saturation: The Triple Confusion ▼

O2 content (mL O2/dL blood) = (1.34 x Hgb x SaO2) + (0.003 x PaO2)

The 0.003 term (dissolved O2) is tiny. Almost all O2 is carried by hemoglobin. This means:

Anemia: Hgb is low, O2 content is low, but PaO2 is NORMAL. This is NOT hypoxemia. The lungs are fine.
CO poisoning: PaO2 is normal (dissolved O2 is unaffected). SpO2 is normal (pulse ox fooled). O2 content is catastrophically low (Hb occupied by CO).
Polycythemia: Hgb is high, O2 content is high. Useful at altitude.

clinical medicine love to present CO poisoning with a "normal" pulse ox and ask why the patient is dying. O2 content equation is the answer.

clinical Walkthrough

VIGNETTE 1 OF 6

Walkthrough Complete

Review your misses, then remix and try again.

Hemoglobin & O2 Dissociation

The Numbers That Win Points

clinical Walkthrough

Walkthrough Complete

Hemoglobin & O₂ Dissociation