A 33-year-old woman is brought to the emergency department 30 minutes after being rescued from a fire in her apartment. She reports nausea, headache, and dizziness. Physical examination shows black discoloration of her oral mucosa. Pulse oximetry shows an oxygen saturation of 99% on room air. The substance most likely causing symptoms in this patient primarily produces toxicity by which of the following mechanisms?

Inhibition of mitochondrial complex V

Degradation of 2,3-bisphosphoglycerate

A 2-day-old boy is examined on day of discharge from the newborn nursery. He was born at 39 weeks by vaginal delivery to a primigravid mother. The pregnancy and delivery were uncomplicated, and the baby has been stooling, urinating, and feeding normally. Both the patient’s mother and father have no known past medical history and are found to have normal hemoglobin electrophoresis results. Compared to adult hemoglobin, the infant’s predominant hemoglobin is most likely to exhibit which of the following properties?

Decreased affinity for 2,3-bisphosphoglycerate

More likely to form hexagonal crystals

More likely to cause red blood cell sickling

Lower affinity for binding oxygen

Increased affinity for 2,3-bisphosphoglycerate

An investigator is conducting a study on hematological factors that affect the affinity of hemoglobin for oxygen. An illustration of two graphs (A and B) that represent the affinity of hemoglobin for oxygen is shown. Which of the following best explains a shift from A to B?

Decreased serum 2,3-bisphosphoglycerate concentration

Increased hemoglobin γ-chain synthesis

A 30-year-old woman presents to clinic for a routine checkup. She reports that she is in good health but that she felt short of breath on her hiking and skiing trip to Colorado the week prior. She explains that this was the first time she has gone that high into the mountains and was slightly concerned for the first few days because she felt chronically short of breath. She reports a history of childhood asthma, but this experience did not feel the same. She was on the verge of seeking medical attention, but it resolved three days later, and she has felt fine ever since. What other listed physiological change results in a physiologic alteration similar to that which occurred in this patient?

Increase in concentration of 2,3-bisphosphoglycerate in blood

Increase in partial pressure of water in air

Increase in concentration of dissolved carbon dioxide in blood

A scientist is working on creating synthetic hemoglobin that can be used to replace blood loss in humans. She therefore starts to study the behavior of this artificial hemoglobin in terms of its ability to bind oxygen. She begins by measuring the affinity between this synthetic hemoglobin and oxygen in a purified system before introducing modifications to this system. Specifically, she reduces the level of carbon dioxide in the system to mimic conditions within the lungs and plots an affinity curve. Which of the following should be observed in this artificial hemoglobin if it mimics the behavior of normal hemoglobin?

Left-shifted curve and increased oxygen binding

No shift in the curve and increased oxygen binding

Left-shifted curve and decreased oxygen binding

Right-shifted curve and decreased oxygen binding

Right-shifted curve and increased oxygen binding

A 24-year-old professional athlete is advised to train in the mountains to enhance his performance. After 5 months of training at an altitude of 1.5 km (5,000 feet), he is able to increase his running pace while competing at sea-level venues. Which of the following changes would produce the same effect on the oxygen-hemoglobin dissociation curve as this athlete's training did?

Decreased 2,3-bisphosphoglycerate

Increased carbon monoxide inhalation

Increased partial pressure of oxygen

Oxygen-hemoglobin dissociation curve for USMLE Step 2 CK: High-Yield Physiology Lessons

O₂-Hgb Curve - The S-Shaped Star

Oxygen-hemoglobin dissociation curve

Sigmoid (S-shape) curve due to cooperative binding: O₂ binding to one heme group ↑ affinity of others.
P₅₀: O₂ partial pressure where hemoglobin is 50% saturated; normally ~26 mmHg.

Right Shift (↓ Affinity)	Left Shift (↑ Affinity)
↑ CO₂, ↑ Acid (↓ pH)	↓ CO₂, ↓ Acid (↑ pH)
↑ 2,3-DPG, ↑ Temp	↓ 2,3-DPG, ↓ Temp
↑ Exercise	Fetal Hb (HbF), CO, MetHb

⭐ Bohr Effect: In peripheral tissues, ↑ H⁺ and ↑ CO₂ promote O₂ release from hemoglobin, causing a rightward shift of the curve.

Curve Shifts - Righty Tighty, Lefty Loosey

The curve's position reflects hemoglobin's O₂ affinity. A right shift promotes O₂ unloading in tissues (lower affinity), while a left shift enhances O₂ binding in the lungs (higher affinity).

Oxygen-hemoglobin dissociation curve shifts

Feature	Right Shift (↓ Affinity)	Left Shift (↑ Affinity)
P₅₀	↑ P₅₀	↓ P₅₀
Causes	↑ CO₂, ↑ Acid/H⁺ (↓pH), ↑ 2,3-DPG, ↑ Temperature, Exercise	↓ CO₂, ↓ Acid/H⁺ (↑pH), ↓ 2,3-DPG, ↓ Temperature, Fetal Hemoglobin (HbF), CO, Methemoglobinemia
Mnemonic	📌 CADET, face Right! (CO₂, Acid, 2,3-DPG, Exercise, Temperature)

Clinical Curves - When Hemoglobin Misbehaves

Curve Shifts: Changes in hemoglobin's affinity for oxygen ($O_2$).
- Left Shift: ↑ Affinity (Loves $O_2$). Less $O_2$ delivered to tissues.
- Right Shift: ↓ Affinity (Releases $O_2$). More $O_2$ delivered to tissues.

Oxygen-hemoglobin dissociation curve with shifts and factors

Left Shift (↑ Affinity)	Factor	Right Shift (↓ Affinity)
↓ $H^+$ (↑ pH)	Acidity	↑ $H^+$ (↓ pH)
↓ Temp	Temp.	↑ Temp
↓ 2,3-BPG	2,3-BPG	↑ 2,3-BPG
↑ CO	CO
↑ Met-Hb	Methemoglobin
↑ HbF	Fetal Hb

⭐ In CO poisoning, $PaO_2$ is normal, but $SaO_2$ is low. CO causes a left shift, trapping $O_2$ on hemoglobin.

The sigmoidal shape of the curve is due to cooperative binding.

A right shift indicates decreased O₂ affinity, facilitating O₂ unloading to tissues.

Factors causing a right shift (CADET): ↑ CO₂, Acid (↓pH), 2,3-DPG, Exercise, Temperature.

A left shift signifies increased O₂ affinity, promoting O₂ loading in the lungs.

Key causes of a left shift include fetal hemoglobin (HbF) and CO poisoning.

P50 is the PO₂ where hemoglobin is 50% saturated; an ↑ P50 reflects a right shift.

Continue reading on Oncourse

CONTINUE READING — FREE

or get the app

App Store|Google Play