Oxygen Transport and Oxygen-Hemoglobin Dissociation Curve — MCQs
Which factor predominantly influences the rightward shift of the oxygen dissociation curve?
The oxygen-hemoglobin dissociation curve is sigmoid because
Which poison shows cherry red discoloration of blood but normal PaO2 on blood gas analysis?
Which of the following laboratory findings is most consistent with a diagnosis of carbon monoxide poisoning?
How many Fe²⁺ atoms are present in one molecule of hemoglobin (Hb)?
The daily production of hydrogen ions from CO2 is primarily buffered by which of the following?
A pregnant woman is able to transfer oxygen to her fetus because fetal hemoglobin has a greater affinity for oxygen than does adult hemoglobin. Why is the affinity of fetal hemoglobin for oxygen higher?
Which porphyrin forms the organic component of heme?
Why is blood stored in citrate-phosphate-dextrose considered more beneficial for hypoxic patients compared to blood stored in acidic-citrate-dextrose?
Which factor has the most significant influence on the oxygen dissociation curve?
Oxygen Transport and Oxygen-Hemoglobin Dissociation Curve Indian Medical PG Practice Questions and MCQs
Question 1: Which factor predominantly influences the rightward shift of the oxygen dissociation curve?
- A. pH (Bohr effect)
- B. 2,3-Bisphosphoglycerate (2,3-BPG) (Correct Answer)
- C. Temperature increase
- D. Carbon monoxide levels
Explanation: ***2,3-Bisphosphoglycerate (2,3-BPG)*** - **2,3-BPG** is an organic phosphate found in **red blood cells** that serves as the **predominant regulator** of oxygen-hemoglobin affinity under physiological conditions. - An increase in **2,3-BPG** levels binds to the **beta chains of deoxyhemoglobin**, stabilizing the T (tense) state and reducing hemoglobin's affinity for oxygen, thereby shifting the curve to the right and facilitating **oxygen release** to tissues. - **2,3-BPG** is especially important in **chronic adaptations** to hypoxia (high altitude, chronic lung disease, anemia) and is the **primary mechanism** for sustained alterations in oxygen delivery. - Normal RBC 2,3-BPG concentration is approximately equal to hemoglobin concentration, making it a **quantitatively significant** regulatory factor. *pH (Bohr effect)* - A decrease in blood **pH** (increased acidity) due to higher **CO2** and **H+** concentrations also shifts the oxygen dissociation curve to the right via the **Bohr effect**. - While physiologically important for **acute regulation** in metabolically active tissues, the Bohr effect operates in conjunction with other factors rather than as the predominant standalone regulator. - The effect is mediated by **protonation of histidine residues** on hemoglobin, causing conformational changes that reduce oxygen affinity. *Temperature increase* - An increase in **temperature** reduces hemoglobin's affinity for oxygen, shifting the oxygen dissociation curve to the right. - This effect is vital for **oxygen delivery** to actively metabolizing tissues (which generate heat), but is generally a **secondary factor** compared to 2,3-BPG in terms of overall regulation. - The temperature effect is more situational, occurring primarily in tissues with elevated metabolic activity. *Carbon monoxide levels* - **Carbon monoxide (CO)** causes a **leftward shift** of the oxygen dissociation curve, not a rightward shift. - CO binds to hemoglobin with 200-250 times greater affinity than oxygen, forming **carboxyhemoglobin** (COHb). - This not only reduces oxygen-carrying capacity but also **increases hemoglobin's affinity** for the remaining oxygen, making it harder to release oxygen to tissues. - CO poisoning is therefore dangerous both because it displaces oxygen and because it impairs oxygen delivery through leftward shift.
Question 2: The oxygen-hemoglobin dissociation curve is sigmoid because
- A. Binding of one oxygen molecule decreases the affinity of binding other O2 molecules
- B. Oxygen affinity of Hemoglobin decreases when the pH of blood falls
- C. Binding of oxygen to Hemoglobin reduces the affinity of Hb for CO2
- D. Binding of one oxygen molecule increases the affinity of binding other O2 molecules (Correct Answer)
Explanation: ***Binding of one oxygen molecule increases the affinity of binding other O2 molecules*** - The **sigmoid shape** of the oxygen-hemoglobin dissociation curve reflects the cooperative binding of oxygen. When one oxygen molecule binds to a heme unit in hemoglobin, it causes a conformational change that increases the affinity of the remaining heme units for oxygen. - This **cooperative binding** means that at low partial pressures of oxygen, very little oxygen binds to hemoglobin. However, once a few oxygen molecules bind, subsequent binding occurs much more readily and steeply, leading to the characteristic 'S' shape. *Binding of one oxygen molecule decreases the affinity of binding other O2 molecules* - This statement is incorrect as it describes **negative cooperativity**, which is the opposite of what occurs with oxygen and hemoglobin. - Decreased affinity after initial binding would lead to a **hyperbolic (rectangular)** curve rather than a sigmoid one, similar to myoglobin's oxygen binding curve. *Oxygen affinity of Hemoglobin decreases when the pH of blood falls* - This describes the **Bohr effect**, where a decrease in pH (acidosis) or an increase in CO2 shifts the curve to the right, indicating reduced oxygen affinity and enhanced oxygen release to tissues. - While this is an important physiological phenomenon, it explains the **shift** of the curve rather than its inherent **sigmoid shape**. *Binding of oxygen to Hemoglobin reduces the affinity of Hb for CO2* - This phenomenon is known as the **Haldane effect**, where oxygen binding promotes the release of CO2 from hemoglobin in the lungs. - The Haldane effect is another crucial aspect of hemoglobin function but does not explain the **sigmoid shape** of the oxygen-hemoglobin dissociation curve itself.
Question 3: Which poison shows cherry red discoloration of blood but normal PaO2 on blood gas analysis?
- A. Cyanide
- B. Hydrogen sulfide
- C. Carbon monoxide (Correct Answer)
- D. Nitrites
Explanation: ***Carbon monoxide*** - **Carbon monoxide (CO)** binds to **hemoglobin** with a much higher affinity than oxygen, forming **carboxyhemoglobin**. This complex is bright red, causing the characteristic **cherry-red discoloration of blood** and skin. - Despite the impaired oxygen delivery, the partial pressure of dissolved oxygen in the blood (**PaO2**) remains normal because CO poisoning affects oxygen binding to hemoglobin rather than the amount of oxygen dissolved in plasma. *Cyanide* - **Cyanide** inhibits **cytochrome c oxidase**, impairing cellular oxygen utilization and leading to **lactic acidosis** and cellular hypoxia. - While it can cause cellular hypoxia, it does not typically produce cherry-red discoloration and usually results in an **arteriovenous oxygen difference** that is small as tissues cannot extract oxygen from the blood effectively. *Hydrogen sulfide* - **Hydrogen sulfide (H2S)** also inhibits **cytochrome c oxidase**, leading to cellular hypoxia similar to cyanide. - Although it can cause a "rotten egg" smell and rapid collapse, it does not typically produce the characteristic **cherry-red discoloration** of blood. *Nitrites* - **Nitrites** (and other oxidizing agents) cause **methemoglobinemia**, where the iron in hemoglobin is oxidized from the ferrous (Fe2+) to the ferric (Fe3+) state, which cannot bind oxygen. - This condition causes the blood to appear **chocolate brown** or **bluish-gray**, not cherry-red, and can lead to a **functional anemia** despite normal PaO2.
Question 4: Which of the following laboratory findings is most consistent with a diagnosis of carbon monoxide poisoning?
- A. Increased PaCO2 and decreased pH
- B. Decreased PaO2 with normal oxygen saturation
- C. Normal PaO2 with decreased oxygen saturation (Correct Answer)
- D. Decreased PaCO2 with normal PaO2
Explanation: ***Normal PaO2 with decreased oxygen saturation*** - Carbon monoxide (CO) binds to hemoglobin with an affinity 200-250 times greater than oxygen, forming **carboxyhemoglobin (COHb)** [2]. This reduces the **oxygen-carrying capacity** of the blood and shifts the oxygen dissociation curve to the left, but it does **not affect the partial pressure of oxygen (PaO2)** dissolved in the plasma [1]. - The pulse oximeter, which typically measures oxygen saturation, will show a falsely high reading because it cannot differentiate between oxyhemoglobin and carboxyhemoglobin, but actual **oxygen saturation is decreased**. *Increased PaCO2 and decreased pH* - This pattern suggests **respiratory acidosis**, which is not a direct or primary finding of carbon monoxide poisoning. - While severe CO poisoning can lead to lactic acidosis, an increase in PaCO2 points to impaired ventilation, not specifically CO toxicity [3]. *Decreased PaO2 with normal oxygen saturation* - A decreased PaO2 with normal oxygen saturation is a contradictory finding and not physiologically consistent, as oxygen saturation is directly dependent on PaO2. - This pattern would indicate a measurement error or a highly unusual physiological state, neither of which is characteristic of CO poisoning. *Decreased PaCO2 with normal PaO2* - This suggests **respiratory alkalosis**, often due to hyperventilation. - While patients with CO poisoning may hyperventilate due to hypoxia, this ABG pattern is not the defining laboratory finding for CO poisoning, and **PaO2 would remain normal** until very late stages.
Question 5: How many Fe²⁺ atoms are present in one molecule of hemoglobin (Hb)?
- A. One Fe²⁺ atom
- B. Two Fe²⁺ atoms
- C. Four Fe²⁺ atoms (Correct Answer)
- D. Eight Fe²⁺ atoms
Explanation: ***Four Fe²⁺ atoms*** - A single molecule of **hemoglobin** is composed of **four globin chains**, each containing one **heme group**. - Each **heme group** in hemoglobin contains one central **ferrous iron (Fe²⁺) atom**, allowing for the binding of one oxygen molecule per heme group. *One Fe²⁺ atom* - This is incorrect because hemoglobin is a **tetramer**, meaning it has multiple subunits. - Only one heme group (and thus one Fe²⁺ atom) is present in **myoglobin**, which is a single polypeptide chain, not hemoglobin. *Two Fe²⁺ atoms* - This is incorrect as it does not account for the **tetrameric structure** of adult hemoglobin. - While some developmental forms of hemoglobin could be considered to have two alpha and two beta chains, each still has its own heme group. *Eight Fe²⁺ atoms* - This is incorrect as it would imply two Fe²⁺ atoms per heme group or multiple heme groups per globin chain. - The 1:1 ratio of heme group to Fe²⁺ atom and globin chain to heme group is fundamental to hemoglobin structure.
Question 6: The daily production of hydrogen ions from CO2 is primarily buffered by which of the following?
- A. Red blood cell bicarbonate
- B. Extracellular bicarbonate
- C. Plasma proteins
- D. Red blood cell hemoglobin (Correct Answer)
Explanation: ***Red blood cell hemoglobin*** - **Hemoglobin is the primary buffer** for the massive daily acid load from CO2 (approximately 12,500 mEq H+ per day). - CO2 diffuses into RBCs where **carbonic anhydrase** rapidly catalyzes: CO2 + H2O → H2CO3 → H+ + HCO3-. - **Deoxygenated hemoglobin** has a higher affinity for H+ than oxygenated hemoglobin (reduced hemoglobin is a weaker acid, thus better H+ acceptor). - This buffering is crucial for CO2 transport: **Hb + H+ → HHb**, preventing significant pH changes despite huge CO2 production. - The bicarbonate produced is then transported out via the **chloride shift** to maintain electrical neutrality. *Extracellular bicarbonate* - While the bicarbonate buffer system is quantitatively the largest extracellular buffer, it is **NOT the primary buffer for CO2-derived H+**. - The extracellular HCO3-/CO2 system primarily buffers **metabolic (non-volatile) acids** produced from dietary and metabolic sources (~50-100 mEq/day). - For CO2-derived acid, the buffering occurs **intracellularly in RBCs** via hemoglobin before bicarbonate enters the plasma. *Red blood cell bicarbonate* - Bicarbonate is produced within RBCs from the dissociation of carbonic acid, but it is **not the buffer itself**. - The bicarbonate is a **product** of the buffering reaction, not the buffering agent. - Most RBC-produced HCO3- is transported to plasma via the **anion exchanger (Band 3 protein)** in exchange for Cl-. *Plasma proteins* - Plasma proteins like **albumin** have buffering capacity due to ionizable groups (imidazole groups of histidine residues). - They contribute only about **1-5%** of total blood buffering capacity. - Far less important than hemoglobin for buffering the large CO2-derived acid load.
Question 7: A pregnant woman is able to transfer oxygen to her fetus because fetal hemoglobin has a greater affinity for oxygen than does adult hemoglobin. Why is the affinity of fetal hemoglobin for oxygen higher?
- A. There is less 2,3-BPG in the fetal circulation as compared to maternal circulation
- B. Fetal hemoglobin binds 2,3-BPG with fewer ionic bonds than the adult form. (Correct Answer)
- C. The tense form of hemoglobin is more prevalent in the circulation of the fetus
- D. The oxygen-binding curve of fetal hemoglobin is shifted to the right.
Explanation: ***Fetal hemoglobin binds 2,3-BPG with fewer ionic bonds than the adult form.*** * **Fetal hemoglobin (HbF)**, composed of two alpha and two gamma subunits, interacts less effectively with **2,3-bisphosphoglycerate (2,3-BPG)** due to a difference in its gamma subunits compared to the beta subunits of **adult hemoglobin (HbA)**. * The reduced binding of 2,3-BPG to HbF stabilizes its **R (relaxed) state**, which has a higher oxygen affinity, facilitating oxygen transfer from the mother to the fetus. *There is less 2,3-BPG in the fetal circulation as compared to maternal circulation* * While 2,3-BPG plays a crucial role in regulating oxygen affinity, the primary reason for **fetal hemoglobin's higher oxygen affinity** is its inherent structural difference that leads to weaker binding of 2,3-BPG, not necessarily the concentration of 2,3-BPG in the fetal circulation. * The **concentration of 2,3-BPG is typically similar or even slightly higher in fetal blood** to enhance oxygen unloading at the tissues, but its effect on HbF is diminished. *The tense form of hemoglobin is more prevalent in the circulation of the fetus* * The **tense form (T-state)** of hemoglobin has a **lower affinity for oxygen**, and its prevalence would lead to reduced oxygen binding, which is contrary to the physiological need of the fetus to extract oxygen from the maternal blood. * **Fetal hemoglobin's higher oxygen affinity** means it spends more time in the **relaxed form (R-state)**, which is responsible for tighter oxygen binding. *The oxygen-binding curve of fetal hemoglobin is shifted to the right.* * An **oxygen-binding curve shifted to the right** indicates a **decreased affinity for oxygen** and would facilitate oxygen unloading, not oxygen loading. * For fetal hemoglobin to effectively extract oxygen from maternal blood, its **oxygen-binding curve must be shifted to the left**, signifying a higher oxygen affinity.
Question 8: Which porphyrin forms the organic component of heme?
- A. Uroporphyrin
- B. Coproporphyrin
- C. Deuteroporphyrin
- D. Protoporphyrin IX (Correct Answer)
Explanation: ***Protoporphyrin IX*** - **Heme** is formed by the insertion of an **iron atom (Fe2+)** into the center of **protoporphyrin IX**. - **Protoporphyrin IX** is the immediate precursor to heme in the **heme synthesis pathway**. *Uroporphyrin* - **Uroporphyrin** is an earlier precursor in the **heme synthesis pathway** and is much more hydrophilic than protoporphyrin. - It accumulates in diseases like **congenital erythropoietic porphyria (CEP)**, leading to photosensitivity. *Coproporphyrin* - **Coproporphyrin** is an intermediate in the **heme synthesis pathway**, formed after uroporphyrinogen. - It is also more water-soluble than protoporphyrin and its accumulation can be seen in various porphyrias. *Deuteroporphyrin* - **Deuteroporphyrin** is a synthetic porphyrin or a less common natural porphyrin that is not directly involved as the organic component of heme in mammals. - While it is structurally similar to protoporphyrin, it does not serve as the direct precursor for heme formation in the human body.
Question 9: Why is blood stored in citrate-phosphate-dextrose considered more beneficial for hypoxic patients compared to blood stored in acidic-citrate-dextrose?
- A. The fall in 2,3-DPG is less. (Correct Answer)
- B. It has a higher pH level than acidic-citrate-dextrose.
- C. It is more effective in oxygen delivery.
- D. It has a longer shelf life than acidic-citrate-dextrose.
Explanation: ***The fall in 2,3-DPG is less.*** * **Citrate-phosphate-dextrose (CPD)** better preserves levels of **2,3-bisphosphoglycerate (2,3-DPG)** in stored red blood cells. * Higher 2,3-DPG levels are crucial for **oxygen unloading** from hemoglobin in tissues, which is particularly beneficial for hypoxic patients who need efficient oxygen delivery. *It has a higher pH level than acidic-citrate-dextrose.* * While CPD does maintain a **less acidic pH** than acid-citrate-dextrose (ACD), which is generally favorable for red blood cell viability, the most direct benefit for hypoxic patients relates to 2,3-DPG. * The slightly higher pH indirectly contributes to better 2,3-DPG preservation but isn't the primary reason for improved oxygen delivery. *It is more effective in oxygen delivery.* * While the *consequence* of using CPD is **more effective oxygen delivery** due to better 2,3-DPG preservation, this option describes the outcome rather than the underlying mechanism compared to the more specific answer regarding 2,3-DPG. * The increased efficacy in oxygen delivery is directly attributable to the preserved 2,3-DPG levels. *It has a longer shelf life than acidic-citrate-dextrose.* * The storage solutions primarily impact red blood cell viability and function, but the **shelf life** (typically 21-35 days depending on the anticoagulant/preservative) is generally determined by other factors, including the additive solutions used with the anticoagulant. * While CPD improves red blood cell quality, the primary advantage for hypoxic patients specifically lies in oxygen affinity rather than overall storage duration.
Question 10: Which factor has the most significant influence on the oxygen dissociation curve?
- A. 2,3-BPG (Correct Answer)
- B. pH
- C. Temperature
- D. All of these
Explanation: ***2,3-BPG*** - **2,3-bisphosphoglycerate (2,3-BPG)** is a metabolic intermediate produced specifically in red blood cells that serves as the primary physiological regulator of hemoglobin's oxygen affinity. - It binds to the central cavity of deoxygenated hemoglobin, stabilizing the tense (T) state and significantly decreasing oxygen affinity, shifting the curve to the right. - Its concentration increases in chronic hypoxic conditions (high altitude, anemia, chronic lung disease), providing sustained adaptation for oxygen delivery to tissues. - **2,3-BPG levels can increase by 50% or more** during chronic hypoxia, representing the most significant **long-term physiological mechanism** for modulating the oxygen dissociation curve. *pH* - A decrease in **pH** (Bohr effect) shifts the oxygen dissociation curve to the right by stabilizing the T state of hemoglobin. - This is primarily an **acute response** to metabolic conditions rather than a sustained regulatory mechanism. - While clinically important, pH changes are typically secondary to metabolic states rather than a primary regulatory mechanism. *Temperature* - An increase in **temperature** causes a rightward shift of the oxygen dissociation curve, promoting oxygen release from hemoglobin. - Temperature effects are generally **passive responses** to environmental or metabolic conditions rather than active regulatory mechanisms. - The magnitude of temperature-induced shifts is typically smaller than those produced by 2,3-BPG in physiological conditions. *All of these* - While pH, temperature, and 2,3-BPG all influence the oxygen dissociation curve, the question asks for the factor with the **most significant influence**. - **2,3-BPG** is unique as the only factor that represents an **active, sustained, physiological regulatory mechanism** specifically evolved for oxygen delivery modulation. - pH and temperature effects are important but represent **passive responses** to metabolic conditions rather than primary regulatory control mechanisms.
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