Hypoxia and Oxygen Transport — MCQs
Which factor predominantly influences the rightward shift of the oxygen dissociation curve?
Compensating mechanism involved in acclimatization to altitude is:
A healthy, 37-year-old woman loses her job at the auto factory. She picks up her three young children from school and is involved in a road traffic accident. Her 5-year-old son sustains a severe head injury. The woman was not hurt in the accident but is hyperventilating as she sits in the waiting room at the hospital. She complains of feeling faint and has blurred vision. Which of the following is decreased in this woman?
Which type of hypoxia is associated with reduced oxygen delivery to tissues despite normal arterial oxygen content?
Cells most sensitive to hypoxia are?
A patient with acute pulmonary embolism is found to have hypoxia. What is the most likely mechanism causing hypoxia in this condition?
CO poisoning causes which type of hypoxia?
During acclimatization to high altitude, all of the following take place except:
In which of the following conditions oxygen delivery is least to muscles?
Which is a feature of high-altitude pulmonary edema?
Hypoxia and Oxygen Transport 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: Compensating mechanism involved in acclimatization to altitude is:
- A. Respiratory depression
- B. Hypoventilation
- C. Hyperventilation (Correct Answer)
- D. Respiratory acidosis
Explanation: ***Hyperventilation*** - **Hyperventilation** is the primary immediate compensatory mechanism at altitude, increasing alveolar ventilation to improve **oxygen uptake** despite lower partial pressures of oxygen. - This response is mediated by the **carotid bodies**, which sense the reduced arterial PO2 and stimulate the respiratory center. *Respiratory depression* - **Respiratory depression** would worsen hypoxia at high altitude by further reducing **oxygen intake**. - This is not a compensatory, but rather a detrimental, response in this setting. *Hypoventilation* - **Hypoventilation** decreases the amount of air reaching the alveoli, exacerbating the **hypoxia** present at high altitudes. - This would further reduce the **partial pressure of oxygen** in the blood, which is counterproductive for acclimatization. *Respiratory acidosis* - **Respiratory acidosis** results from **hypoventilation** and CO2 retention. - Acclimatization leads to **respiratory alkalosis** due to increased CO2 excretion from hyperventilation, which is then partially compensated by renal mechanisms.
Question 3: A healthy, 37-year-old woman loses her job at the auto factory. She picks up her three young children from school and is involved in a road traffic accident. Her 5-year-old son sustains a severe head injury. The woman was not hurt in the accident but is hyperventilating as she sits in the waiting room at the hospital. She complains of feeling faint and has blurred vision. Which of the following is decreased in this woman?
- A. Arterial pH
- B. Cerebral blood flow (Correct Answer)
- C. Arterial oxygen content
- D. Arterial oxygen tension (PO2)
Explanation: ***Cerebral blood flow*** - **Hyperventilation** leads to a decrease in arterial **pCO2**, causing **vasoconstriction** of cerebral blood vessels. - Reduced cerebral blood flow results in symptoms like **dizziness**, **lightheadedness**, and **blurred vision** due to decreased oxygen delivery to the brain. *Arterial pH* - **Hyperventilation** causes a decrease in arterial pCO2, leading to **respiratory alkalosis** (increased arterial pH). - A decreased arterial pH would be characteristic of acidosis, which is the opposite of what occurs during hyperventilation. *Arterial oxygen content* - While hyperventilation increases the amount of oxygen in the blood, the **arterial oxygen content** (total oxygen bound to hemoglobin plus dissolved oxygen) is not significantly decreased in a healthy individual. - The primary effect of hyperventilation is on CO2 levels and pH, not a reduction in overall oxygen-carrying capacity. *Arterial oxygen tension (PO2)* - **Hyperventilation** actually leads to an **increase** in arterial PO2 due to increased alveolar ventilation. - A decreased arterial PO2 would indicate hypoxemia, which is not caused by hyperventilation and is generally associated with conditions causing impaired gas exchange.
Question 4: Which type of hypoxia is associated with reduced oxygen delivery to tissues despite normal arterial oxygen content?
- A. Stagnant hypoxia (related to blood flow issues) (Correct Answer)
- B. Ischemic hypoxia (inadequate blood supply)
- C. Anemic hypoxia (due to reduced hemoglobin functionality)
- D. Histotoxic hypoxia (cells cannot utilize oxygen due to toxins)
Explanation: ***Stagnant hypoxia (related to blood flow issues)*** - **Stagnant hypoxia**, also known as **circulatory hypoxia**, is characterized by **reduced oxygen delivery to tissues despite normal arterial oxygen content**. - The **oxygen content (CaO2)** in arterial blood is normal, but **oxygen delivery (DO2)** is reduced due to **decreased blood flow** or **reduced cardiac output**. - Conditions like **heart failure**, **shock**, or **localized vascular obstruction** reduce tissue perfusion, preventing adequate oxygen from reaching the tissues. - This perfectly matches the question stem: normal arterial O2 content but impaired delivery. *Anemic hypoxia (due to reduced hemoglobin functionality)* - **Anemic hypoxia** occurs when the **oxygen-carrying capacity of blood** is reduced due to decreased **functional hemoglobin**. - While **PaO2** (partial pressure) may be normal, the **arterial oxygen content (CaO2) is REDUCED** because CaO2 depends on hemoglobin concentration. - Conditions like **anemia** or **carbon monoxide poisoning** reduce the amount of oxygen that can be carried in blood. - This does NOT match the question stem because arterial oxygen content is reduced, not normal. *Histotoxic hypoxia (cells cannot utilize oxygen due to toxins)* - In **histotoxic hypoxia**, both **arterial oxygen content** and **oxygen delivery** are normal. - The problem is at the **cellular level** where cells cannot utilize oxygen despite adequate supply. - **Cyanide poisoning** inhibits **cytochrome oxidase** in mitochondria, preventing cellular respiration. - Does not match because delivery is normal, not reduced. *Ischemic hypoxia (inadequate blood supply)* - **Ischemic hypoxia** is a **localized form of stagnant hypoxia** caused by **regional blood flow obstruction**. - Examples include **arterial occlusion** in **stroke** or **myocardial infarction**. - While this involves reduced delivery, it's a subset of stagnant hypoxia and less precise than the primary answer.
Question 5: Cells most sensitive to hypoxia are?
- A. Myocardial cells
- B. Neurons (Correct Answer)
- C. Hepatocytes
- D. Renal tubular epithelial cells
Explanation: ***Neurons*** - Neurons have a very high metabolic rate and an **absolute requirement for oxygen** and glucose to maintain their complex electrochemical functions and ionic gradients. - Due to their lack of significant energy reserves and high metabolic demand, they can sustain **irreversible damage within minutes** (typically 3-5 minutes) of complete oxygen deprivation. *Myocardial cells* - While myocardial cells are highly susceptible to hypoxia and can undergo **ischemic necrosis** (e.g., in a myocardial infarction), they can often tolerate oxygen deprivation for somewhat longer periods than neurons due to some anaerobic metabolic capacity. - Significant damage to myocardial cells usually occurs after **20-30 minutes of severe ischemia**. *Hepatocytes* - Hepatocytes (liver cells) are relatively **resilient to hypoxia** compared to neurons, possessing significant metabolic flexibility and capacity for regeneration. - They can endure **longer periods of oxygen deprivation** before irreversible damage occurs, often hours. *Renal tubular epithelial cells* - Renal tubular epithelial cells are generally **sensitive to hypoxia**, especially those in the medulla, due to their high metabolic activity for reabsorption and secretion. - They are a common target for **acute tubular necrosis** in ischemic injury but generally have a **higher tolerance than neurons**, with damage becoming widespread after tens of minutes to an hour of severe ischemia.
Question 6: A patient with acute pulmonary embolism is found to have hypoxia. What is the most likely mechanism causing hypoxia in this condition?
- A. Hypoventilation
- B. Diffusion impairment
- C. Ventilation-perfusion mismatch (Correct Answer)
- D. Shunt
Explanation: ***Ventilation-perfusion mismatch*** - A pulmonary embolism blocks blood flow to a portion of the lung, creating areas that are **ventilated but not perfused** (increased dead space with high V/Q ratio). - Blood is redirected to the remaining perfused lung areas, which then become relatively **overperfused** (low V/Q ratio), impairing efficient oxygen uptake. - This V/Q mismatch—with both high V/Q (dead space) and low V/Q (relative shunt) areas—leads to **hypoxemia**, making it the **most common mechanism** of hypoxia in acute PE. *Hypoventilation* - This condition involves a generalized decrease in alveolar ventilation, leading to **hypercapnia** (increased CO2) and hypoxemia. - While PE can cause shortness of breath and tachypnea, the primary mechanism of hypoxia is not due to overall reduced ventilation, but rather disrupted matching of ventilation to perfusion. *Diffusion impairment* - Diffusion impairment occurs when the alveolar-capillary membrane is compromised, preventing proper oxygen transfer, as seen in conditions like **pulmonary fibrosis** or **interstitial lung disease**. - Pulmonary embolism primarily affects **blood flow distribution**, not the structural integrity or diffusion capacity of the alveolar-capillary membrane. *Shunt* - A true shunt occurs when deoxygenated blood bypasses ventilated alveoli entirely and enters systemic circulation, as seen in **intracardiac defects** or severe **ARDS**. - While massive PE can rarely lead to right-to-left shunting through a patent foramen ovale (due to increased right heart pressure), the **primary and most common mechanism** of hypoxia in typical acute PE is V/Q mismatch, not shunt.
Question 7: CO poisoning causes which type of hypoxia?
- A. Anemic hypoxia (Correct Answer)
- B. Hypoxic hypoxia
- C. Stagnant hypoxia
- D. Histotoxic hypoxia
Explanation: ***Anemic hypoxia*** - **Carbon monoxide (CO)** binds to **hemoglobin** with an affinity much higher than oxygen, forming **carboxyhemoglobin**. - This effectively reduces the **oxygen-carrying capacity of the blood**, mimicking a severe anemia, despite normal arterial PO2. *Hypoxic hypoxia* - Occurs when there is **insufficient oxygen delivery to the blood** due to low arterial PO2, as seen in high altitudes or respiratory diseases. - In CO poisoning, **arterial PO2 is typically normal**, distinguishing it from hypoxic hypoxia. *Stagnant hypoxia* - Results from **inadequate blood flow** to tissues, leading to reduced oxygen delivery, as observed in heart failure or shock. - CO poisoning primarily affects oxygen transport by hemoglobin, not the **rate of blood flow**. *Histotoxic hypoxia* - Characterized by the **inability of tissues to utilize oxygen** effectively, even when oxygen delivery is adequate, as seen in cyanide poisoning inhibiting cytochrome oxidase. - In CO poisoning, tissues can utilize oxygen; the problem is the **reduced availability of oxygen** from hemoglobin.
Question 8: During acclimatization to high altitude, all of the following take place except:
- A. Increase in minute ventilation
- B. Increase in the sensitivity of carotid body to hypoxia
- C. Shift in the oxygen dissociation curve to the left (Correct Answer)
- D. Increase in 2,3-BPG levels in red blood cells
Explanation: ***Shift in the oxygen dissociation curve to the left*** - Acclimatization to high altitude involves a **right shift** in the oxygen dissociation curve, not a left shift. This right shift, facilitated by an increase in 2,3-bisphosphoglycerate (2,3-BPG), allows for **improved oxygen unloading** to tissues at lower partial pressures of oxygen. - A left shift would mean that hemoglobin has a **higher affinity for oxygen**, hindering its release to the tissues, which would be detrimental in a low oxygen environment. - Note: Acute exposure causes a temporary left shift due to respiratory alkalosis, but chronic acclimatization produces a right shift. *Increase in minute ventilation* - A primary response to high altitude is an **increase in minute ventilation** (the total volume of air inhaled or exhaled per minute). - This increased breathing rate and depth helps to counteract the **lower partial pressure of oxygen** in the atmosphere, thereby maintaining alveolar oxygen levels. *Increase in the sensitivity of carotid body to hypoxia* - During acclimatization, the **carotid body's sensitivity to hypoxia** increases, leading to a stronger ventilatory response to low oxygen levels. - This enhanced sensitivity helps in maintaining adequate oxygenation by **stimulating increased breathing**. *Increase in 2,3-BPG levels in red blood cells* - Acclimatization leads to increased production of **2,3-bisphosphoglycerate (2,3-BPG)** in red blood cells. - This facilitates the **right shift of the oxygen dissociation curve**, promoting oxygen release to tissues at high altitude where oxygen partial pressure is low.
Question 9: In which of the following conditions oxygen delivery is least to muscles?
- A. Marathon runner at sea level
- B. Person with carbon monoxide poisoning (Correct Answer)
- C. Person inhaling 100 percent oxygen at the top of Mount Everest
- D. Person with anemia at sea level
Explanation: ***Person with carbon monoxide poisoning*** - **Carbon monoxide (CO)** binds to **hemoglobin** with an affinity 200-250 times greater than oxygen, forming **carboxyhemoglobin (COHb)**. This significantly reduces the **oxygen-carrying capacity** of the blood. - CO poisoning also shifts the **oxygen-hemoglobin dissociation curve** to the left, meaning that even the oxygen that *is* bound to hemoglobin is less readily released to the tissues, leading to severe **tissue hypoxia**. - **Dual mechanism** of impairment (reduced carrying capacity + impaired unloading) makes CO poisoning the most severe condition for oxygen delivery. *Marathon runner at sea level* - While a marathon runner at sea level experiences high oxygen demand, their **cardiovascular system** is highly adapted to deliver oxygen efficiently to the muscles. - The **partial pressure of oxygen** in the atmosphere is optimal, allowing for maximum oxygen saturation of hemoglobin and effective delivery. - Increased cardiac output and enhanced oxygen extraction compensate for high metabolic demands. *Person inhaling 100 percent oxygen at the top of Mount Everest* - Although the **atmospheric pressure** at the top of Mount Everest is very low, inhaling 100% oxygen significantly increases the **partial pressure of oxygen** in the inspired air. - This allows for a greater **driving pressure** for oxygen to enter the bloodstream and maintain higher oxygen saturation compared to breathing ambient air at altitude, mitigating the effects of hypoxia. - While not optimal, supplemental 100% O₂ can maintain adequate oxygen delivery despite low barometric pressure. *Person with anemia at sea level* - In anemia, there is a reduced **hemoglobin concentration**, which decreases the **oxygen-carrying capacity** of the blood. - However, unlike CO poisoning, the **oxygen-hemoglobin dissociation curve** remains normal, allowing for normal oxygen unloading to tissues. - Compensatory mechanisms include increased cardiac output and increased oxygen extraction, making it less severe than CO poisoning.
Question 10: Which is a feature of high-altitude pulmonary edema?
- A. Associated with low cardiac output
- B. Exercise has no effect
- C. Associated with pulmonary hypertension (Correct Answer)
- D. Occurs in both acclimatized and unacclimatized persons
Explanation: ***Associated with pulmonary hypertension*** - **High-altitude pulmonary edema (HAPE)** is characterized by **exaggerated hypoxic pulmonary vasoconstriction**, leading to significantly increased pulmonary artery pressures. - This **pulmonary hypertension** drives fluid extravasation into the alveolar spaces, causing non-cardiogenic pulmonary edema. *Associated with low cardiac output* - HAPE is typically associated with **normal or elevated cardiac output** in response to hypoxia, not low cardiac output. - Low cardiac output suggests conditions like cardiogenic shock or severe myocardial dysfunction, which are not primary features of HAPE. *Exercise has no effect* - **Physical exertion at altitude** is a significant risk factor and can worsen HAPE due to increased cardiac output and pulmonary blood flow, exacerbating pulmonary hypertension. - Rest and reduced activity are crucial components of preventing and treating HAPE, indicating that exercise does indeed have an effect. *Occurs in both acclimatized and unacclimatized persons* - HAPE primarily affects **unacclimatized individuals** or those who ascend rapidly to high altitudes. - While rare, it can occur in previously acclimatized individuals returning to altitude after a period at lower elevations or in those with predisposing factors, but it is predominantly a disease of the unacclimatized.
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