Bicarbonate Buffer System — MCQs
What is the primary enzyme responsible for the conversion of carbon dioxide to bicarbonate in erythrocytes?
The most important physiological anion other than chloride is:
Most important buffer system in human blood:
Assertion: In a patient with chronic kidney disease (CKD) and metabolic acidosis, sodium bicarbonate should be initiated to correct acidosis. Reason: Sodium bicarbonate therapy reduces the progression of kidney disease by decreasing tubular injury and slowing fibrosis.
The primary respiratory compensation for metabolic acidosis is?
A hyperventilating patient has the following ABG values: pH=7.53, pCO2=20 mmHg, HCO3= 26 mEq/L. What is the most likely diagnosis?
The transport of CO2 in the blood is primarily influenced by which of the following factors?
HCO3/H2CO3 is the best buffer because it is:
A patient is going skiing high in the Rockies and is given acetazolamide to protect against altitude sickness. Unfortunately, the patient is also a type 1 diabetic. He is admitted to the hospital in a worsening ketoacidosis. In which of the following cells has acetazolamide inhibited a reaction that has led to the severity of the metabolic acidosis?
The pH of body fluids is stabilized by buffer systems. Which of the following compounds is the most effective buffer at physiologic pH?
Bicarbonate Buffer System Indian Medical PG Practice Questions and MCQs
Question 1: What is the primary enzyme responsible for the conversion of carbon dioxide to bicarbonate in erythrocytes?
- A. The high solubility of CO2 in water
- B. The role of hemoglobin in CO2 transport
- C. The conversion of carbon dioxide to carbonic acid
- D. The action of carbonic anhydrase in erythrocytes (Correct Answer)
Explanation: ***The action of carbonic anhydrase in erythrocytes*** - **Carbonic anhydrase** is an enzyme found in high concentrations within **red blood cells (erythrocytes)**, catalyzing the rapid interconversion of carbon dioxide and water to **carbonic acid**. - This enzyme is crucial for the efficient transport of carbon dioxide from the tissues to the lungs, as carbonic acid quickly dissociates into **bicarbonate ions**, which are easily transported in the plasma. *The high solubility of CO2 in water* - While **CO2** does have some solubility in water, this process is too slow on its own to account for the rapid and efficient transport of the large amounts of metabolic CO2 produced by the body. - The direct dissolution of CO2 in plasma accounts for only a small fraction of its total transport. *The role of hemoglobin in CO2 transport* - **Hemoglobin** does play a role in CO2 transport by forming **carbaminohemoglobin**, binding to the amino groups on the globin chains. - However, this mechanism represents only about 20-30% of CO2 transport and does not involve the conversion to **bicarbonate**. *The conversion of carbon dioxide to carbonic acid* - The conversion of CO2 to **carbonic acid (H2CO3)** is indeed an intermediate step in bicarbonate formation. - However, this reaction is very slow in the absence of an enzyme and does not address the primary catalyst responsible for this rapid conversion.
Question 2: The most important physiological anion other than chloride is:
- A. Bicarbonate (Correct Answer)
- B. Phosphate
- C. Sulphate
- D. Nitrate
Explanation: ***Bicarbonate*** - **Bicarbonate (HCO3-)** is the second most abundant physiological anion after chloride and plays a crucial role in maintaining **acid-base balance** as part of the bicarbonate buffer system. - It is vital for transporting **carbon dioxide** from tissues to the lungs to be exhaled. *Phosphate* - While an important physiological anion, **phosphate** is primarily involved in **energy metabolism** (ATP, ADP), bone mineralization, and intracellular buffering, making its extracellular concentration far lower than bicarbonate. - Its role as an extracellular buffer is less significant than bicarbonate's due to its lower concentration and pKa in physiological conditions. *Sulphate* - **Sulphate (SO4^2-)** is present in the body but in much lower concentrations than chloride or bicarbonate. - Its primary roles are in metabolism and detoxification, not as a major component of electrolyte balance or acid-base regulation. *Nitrate* - **Nitrate (NO3-)** is generally found in very low, non-physiologically significant concentrations in the body under normal circumstances. - It is not considered a major physiological anion and does not play a direct role in maintaining electrolyte balance or acid-base homeostasis.
Question 3: Most important buffer system in human blood:
- A. Hemoglobin
- B. Chloride ions
- C. Bicarbonates (Correct Answer)
- D. Phosphate buffer system
Explanation: ***Bicarbonates*** - The **bicarbonate buffer system** is the most significant extracellular buffer in human blood due to its high concentration and the ability of its components (CO2 and HCO3-) to be regulated by the lungs and kidneys, respectively. - It rapidly equilibrates with dissolved CO2, making it highly effective at buffering both acid and base imbalances to maintain **blood pH**. *Hemoglobin* - **Hemoglobin** is an important intracellular buffer within red blood cells, primarily buffering carbonic acid formed from CO2 transport. - While powerful within the red blood cell, it is not the primary buffer system in the overall plasma (extracellular fluid). *Chloride ions* - **Chloride ions** are crucial for maintaining electroneutrality in red blood cell buffering processes (e.g., the **chloride shift**), but they do not directly act as a buffer in the traditional sense of accepting or donating protons. - Their primary role in pH balance is indirect, supporting the function of other buffer systems. *Phosphate buffer system* - The **phosphate buffer system** is important, particularly in intracellular fluid and renal tubules, due to its pKa being close to physiological pH. - However, its concentration in the extracellular fluid (blood plasma) is relatively low compared to bicarbonate, making it less significant for overall blood buffering.
Question 4: Assertion: In a patient with chronic kidney disease (CKD) and metabolic acidosis, sodium bicarbonate should be initiated to correct acidosis. Reason: Sodium bicarbonate therapy reduces the progression of kidney disease by decreasing tubular injury and slowing fibrosis.
- A. Assertion is false, but Reason is true
- B. Both Assertion and Reason are true, and Reason is the correct explanation of Assertion
- C. Assertion is true, but Reason is false
- D. Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion (Correct Answer)
Explanation: The **Assertion** is true: **KDIGO guidelines** recommend sodium bicarbonate therapy for CKD patients when serum bicarbonate falls below **22 mEq/L** to correct metabolic acidosis [2]. - The **Reason** is also true: studies demonstrate that bicarbonate therapy has **nephroprotective effects**, reducing CKD progression through decreased **tubular injury** and **interstitial fibrosis**. However, this describes a secondary benefit rather than the primary indication for initiating therapy. *Both Assertion and Reason are true, and Reason is the correct explanation of Assertion* - While both statements are medically accurate, the Reason does not explain the primary indication for bicarbonate initiation in CKD patients. - The main purpose is **acid-base correction** and prevention of acidosis complications like **bone disease**, **muscle wasting**, and **cardiovascular effects**, not primarily nephroprotection [1], [2]. *Assertion is false, but Reason is true* - The Assertion is medically correct: sodium bicarbonate is **standard therapy** for metabolic acidosis in CKD according to nephrology guidelines. - CKD patients develop acidosis due to impaired **renal acid excretion** and reduced **bicarbonate regeneration**, making correction clinically necessary [2]. *Assertion is true, but Reason is false* - The Reason is actually supported by **clinical evidence**: randomized controlled trials show bicarbonate therapy slows CKD progression. - Mechanisms include reduced **complement activation**, decreased **endothelin production**, and preservation of **residual kidney function**.
Question 5: The primary respiratory compensation for metabolic acidosis is?
- A. HCO3 loss
- B. Cl- loss
- C. Hyperventilation (Correct Answer)
- D. Ammonia excretion in kidney
Explanation: ***Hyperventilation*** - In **metabolic acidosis**, the body attempts to raise the pH by decreasing the **partial pressure of carbon dioxide (PCO2)**. - **Hyperventilation** increases the excretion of CO2, a volatile acid, which directly reduces the amount of carbonic acid in the blood and helps to buffer the excess acid. *HCO3 loss* - **Bicarbonate (HCO3-) loss** is a cause or consequence of metabolic acidosis, not a compensatory mechanism. - The kidneys generally try to *retain* or regenerate bicarbonate during acidosis, rather than losing it. *Cl- loss* - **Chloride ion (Cl-) loss** is not a primary respiratory compensatory mechanism for metabolic acidosis. - While shifts in chloride can occur in acid-base imbalances, they are typically related to renal handling or fluid shifts, not direct respiratory compensation. *Ammonia excretion in kidney* - **Ammonia excretion** by the kidneys is a renal (kidney) compensatory mechanism, not a respiratory one. - The kidneys excrete ammonia to excrete hydrogen ions (H+), thereby regenerating bicarbonate and helping to correct the acidosis over a longer period.
Question 6: A hyperventilating patient has the following ABG values: pH=7.53, pCO2=20 mmHg, HCO3= 26 mEq/L. What is the most likely diagnosis?
- A. Metabolic alkalosis
- B. Metabolic acidosis
- C. Respiratory alkalosis (Correct Answer)
- D. Respiratory acidosis
Explanation: ***Respiratory alkalosis*** - The pH of 7.53 indicates **alkalemia**, and the low pCO2 (20 mmHg) is the primary driver, signifying **respiratory alkalosis** - A hyperventilating patient exhales more CO2, leading to a decrease in its partial pressure in the blood and a subsequent rise in pH - The HCO3 is within normal range (26 mEq/L), indicating **uncompensated respiratory alkalosis** *Metabolic alkalosis* - This would be characterized by a high pH and an elevated **HCO3**, but the HCO3 is within the normal range (26 mEq/L) - While it causes alkalemia, the primary disturbance here is respiratory, not metabolic *Metabolic acidosis* - This would present with a **low pH** and a low **HCO3**, which is contrary to the given ABG values - The patient's pH is elevated, indicating an alkalotic state, not acidotic *Respiratory acidosis* - This would be defined by a **low pH** and an elevated **pCO2**, which is the exact opposite of the provided ABG results - The patient's high pH and low pCO2 rule out respiratory acidosis
Question 7: The transport of CO2 in the blood is primarily influenced by which of the following factors?
- A. Binding to hemoglobin as carbaminohemoglobin
- B. Conversion to bicarbonate ions by carbonic anhydrase (Correct Answer)
- C. Transport as carbonic acid in red blood cells
- D. Direct dissolution in blood plasma
Explanation: ***Conversion to bicarbonate ions by carbonic anhydrase*** - This is the **primary mechanism** for CO2 transport, accounting for approximately **70%** of total CO2 transport in blood. - Inside red blood cells, CO2 combines with water to form carbonic acid (H2CO3), catalyzed by the enzyme **carbonic anhydrase**. - Carbonic acid **immediately dissociates** into hydrogen ions (H+) and **bicarbonate ions (HCO3-)**. - Bicarbonate ions then diffuse into plasma in exchange for chloride ions (chloride shift), making this the most quantitatively significant transport mechanism. - **Carbonic anhydrase** is the key enzyme that influences this process by accelerating the reaction by approximately **5000-fold**. *Binding to hemoglobin as carbaminohemoglobin* - Approximately **20-23%** of CO2 is transported by directly binding to amino groups on hemoglobin to form **carbaminohemoglobin**. - This is significant but less than bicarbonate transport. - Deoxygenated hemoglobin binds CO2 more readily than oxygenated hemoglobin (Haldane effect). *Transport as carbonic acid in red blood cells* - This is **not correct** because carbonic acid (H2CO3) is only a **transient intermediate** that exists momentarily. - It immediately dissociates into H+ and HCO3-, so CO2 is not actually transported "as carbonic acid" but rather as **bicarbonate ions**. - The carbonic acid step is part of the mechanism, but bicarbonate is the actual transport form. *Direct dissolution in blood plasma* - Only about **7-10%** of CO2 is transported dissolved directly in plasma. - CO2 has limited solubility in plasma, making this the least significant mechanism. - This dissolved CO2 contributes to the partial pressure of CO2 (PCO2) in blood.
Question 8: HCO3/H2CO3 is the best buffer because it is:
- A. Its components can be increased or decreased in the body as needed (Correct Answer)
- B. Good acceptor and donor of H+ ions
- C. Combination of a weak acid and weak base
- D. pKa near physiological pH
Explanation: ***Its components can be increased or decreased in the body as needed*** - The **bicarbonate buffer system** is unique because its components, **bicarbonate (HCO3-)** and **carbon dioxide (CO2)**, are physiologically regulated by the kidneys and lungs, respectively. - This allows for dynamic adjustment of buffer concentrations to maintain **pH homeostasis**, making it highly effective even when its pKa is not perfectly matched to physiological pH. *Good acceptor and donor of H+ ions* - While bicarbonate acts as an **acceptor of H+ ions** and carbonic acid can donate H+ ions, this characteristic is true for all effective buffer systems. - This option does not highlight the unique advantage of the bicarbonate buffer over other physiological buffers. *Combination of a weak acid and weak base* - The bicarbonate buffer system indeed consists of **carbonic acid (H2CO3)**, a weak acid, and its conjugate base, **bicarbonate (HCO3-)**. - However, this is the definition of any buffer system and doesn't explain why it's the *best* physiological buffer compared to others. *pKa near physiological pH* - The **pKa of the bicarbonate buffer system is 6.1**, which is not exactly at the physiological pH of 7.4. - While buffers are generally most effective when their pKa is close to the pH they regulate, the **open nature and physiological regulation** of the bicarbonate system compensate for this difference.
Question 9: A patient is going skiing high in the Rockies and is given acetazolamide to protect against altitude sickness. Unfortunately, the patient is also a type 1 diabetic. He is admitted to the hospital in a worsening ketoacidosis. In which of the following cells has acetazolamide inhibited a reaction that has led to the severity of the metabolic acidosis?
- A. Cells in the lens of the eye
- B. Liver cells
- C. Immune system cells
- D. Renal tubular cells (Correct Answer)
Explanation: ***Renal tubular cells*** - Acetazolamide is a **carbonic anhydrase inhibitor**, primarily acting in the **proximal renal tubular cells** to block the enzyme carbonic anhydrase. - This inhibition prevents **bicarbonate (HCO₃⁻) reabsorption** in the proximal tubule, causing bicarbonate wasting in urine and resulting in **metabolic acidosis** (specifically type 2 renal tubular acidosis). - In this patient already suffering from **diabetic ketoacidosis** (DKA), which is itself a metabolic acidosis with low bicarbonate, the additional bicarbonate loss from acetazolamide **worsens the severity** of the acidosis. - This represents a clinically important drug-disease interaction. *Cells in the lens of the eye* - While carbonic anhydrase is present in the eye and acetazolamide can reduce **intraocular pressure** (used therapeutically for glaucoma), this mechanism is unrelated to systemic metabolic acidosis. - Inhibition here affects aqueous humor production but does not directly or significantly contribute to **acid-base balance** in the blood. *Liver cells* - The liver is crucial for metabolism and ammonia detoxification, but acetazolamide's primary action on acid-base balance is not directly through **hepatic carbonic anhydrase**. - Liver dysfunction can impact acid-base balance, but the liver is not the direct target or primary cause of acetazolamide-induced acidosis. *Immune system cells* - Carbonic anhydrase activity in **immune cells** is involved in processes like **pH regulation within phagosomes** and T-cell activation. - However, modulation of immune cell function by acetazolamide does not significantly contribute to its effect on systemic **metabolic acidosis**.
Question 10: The pH of body fluids is stabilized by buffer systems. Which of the following compounds is the most effective buffer at physiologic pH?
- A. NH4OH, pKa = 9.24
- B. Na2HPO4, pKa = 12.32
- C. NaH2PO4, pKa = 7.21 (Correct Answer)
- D. CH3CO2H, pKa = 4.75
Explanation: ***NaH2PO4, pKa = 7.21*** - A buffer's maximum effectiveness is typically within 1 pH unit of its **pKa value**. - With a **pKa of 7.21**, the H2PO4⁻/HPO4²⁻ buffer system (phosphate buffer) is optimally positioned to buffer fluctuations around the physiologic pH of **7.35-7.45**. - This makes the phosphate buffer system highly effective in intracellular and urinary pH regulation. *NH4OH, pKa = 9.24* - This compound is a **weak base** with a pKa of 9.24, meaning it would be effective at a pH much higher than the physiologic range. - Its buffering capacity would be minimal at **pH 7.4**, as the system would be predominantly in one form, reducing its ability to resist pH changes. *Na2HPO4, pKa = 12.32* - This represents the **second dissociation** of phosphoric acid (HPO4²⁻ ⇌ PO4³⁻ + H⁺) with a very high **pKa of 12.32**. - This dissociation occurs at extremely alkaline pH levels, far above the physiological range. - At physiologic pH, this equilibrium would be almost entirely shifted to HPO4²⁻, providing no buffering capacity. *CH3CO2H, pKa = 4.75* - **Acetic acid** has a pKa of 4.75, making it an effective buffer in the acidic range (around pH 3.75-5.75). - It would be almost entirely dissociated at **physiologic pH**, offering very little buffering capacity against pH changes in body fluids.
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