Acid-Base Chemistry and Buffers — MCQs
In a patient with a plasma pH of 7.1 the HCO3 / H2CO3 ratio in plasma is:
A patient with pH of 7, pCO2 of 30 mmHg and Bicarbonate levels of 10 meq. What is the acid base abnormality?
Which of the following is considered the most important intracellular buffer in human physiology?
HCO3/H2CO3 is the best buffer because it is:
The daily production of hydrogen ions from CO2 is primarily buffered by which of the following?
The primary respiratory compensation for metabolic acidosis is?
In plasma, if pH is 5, what is the fraction of base to acid?
Which one of the following biochemical abnormalities can be produced by repeated vomiting?
Increased serum calcium is seen in all conditions except:
Which is the most effective buffer system in the blood that is controlled by respiration?
Acid-Base Chemistry and Buffers Indian Medical PG Practice Questions and MCQs
Question 1: In a patient with a plasma pH of 7.1 the HCO3 / H2CO3 ratio in plasma is:
- A. 1
- B. 20
- C. 2
- D. 10 (Correct Answer)
Explanation: ***Correct Answer: 10*** - The **Henderson-Hasselbalch equation** dictates that pH = pKa + log([HCO3-]/[H2CO3]). Given a normal pKa for carbonic acid of 6.1, a pH of 7.1 leads to 7.1 = 6.1 + log([HCO3-]/[H2CO3]), meaning log([HCO3-]/[H2CO3]) = 1, and thus [HCO3-]/[H2CO3] = 10^1 = **10**. - This ratio of 10 indicates **acidosis**, as the normal physiological ratio for a pH of 7.4 is 20:1. *Incorrect Option: 1* - A ratio of 1 ([HCO3-]/[H2CO3] = 1:1) would mean that log(1) = 0, which would result in a pH equal to the pKa, i.e., pH = 6.1. This is an **extremely acidic** condition incompatible with life. - This ratio would signify a severe and uncompensated metabolic and/or respiratory acidosis. *Incorrect Option: 20* - A ratio of 20 ([HCO3-]/[H2CO3] = 20:1) corresponds to a pH of **7.4**, which is the normal physiological pH. - Since the given plasma pH is 7.1, this ratio is incorrect, as a lower pH indicates a lower ratio. *Incorrect Option: 2* - A ratio of 2 ([HCO3-]/[H2CO3] = 2:1) would result in a pH calculation of pH = 6.1 + log(2) = 6.1 + 0.3 = 6.4. - This pH is also **too low** compared to the given pH of 7.1.
Question 2: A patient with pH of 7, pCO2 of 30 mmHg and Bicarbonate levels of 10 meq. What is the acid base abnormality?
- A. Respiratory alkalosis
- B. Metabolic alkalosis
- C. Respiratory Acidosis
- D. Metabolic Acidosis (Correct Answer)
Explanation: ***Metabolic Acidosis*** - The pH is 7, which is severely **acidotic** (normal range 7.35-7.45). This indicates an acid-base disorder where the body is too acidic. - The **bicarbonate level is 10 mEq/L** (normal range 22-26 mEq/L), which is significantly low, directly contributing to the acidosis and pointing towards a metabolic origin. *Respiratory alkalosis* - This condition involves an **elevated pH** (alkalosis) due to a primary decrease in pCO2. - In this case, the pH is acidic, not alkaline. *Metabolic alkalosis* - This condition involves an **elevated pH** (alkalosis) due to a primary increase in bicarbonate levels. - Here, the pH is acidic and bicarbonate is low, directly contradicting metabolic alkalosis. *Respiratory Acidosis* - This condition involves a **decreased pH** (acidosis) due to a primary increase in pCO2. - Although the pH is acidotic, the pCO2 is 30 mmHg (normal range 35-45 mmHg), which is low, indicating a respiratory compensation rather than the primary cause.
Question 3: Which of the following is considered the most important intracellular buffer in human physiology?
- A. Albumin protein
- B. Ammonia buffer
- C. Bicarbonate buffer
- D. Phosphate buffer (Correct Answer)
Explanation: ***Phosphate buffer*** - The **phosphate buffer system (H₂PO₄⁻/HPO₄²⁻)** is the most important intracellular buffer due to relatively high concentrations of inorganic phosphates within cells - The pKa₂ of approximately **6.8 is close to intracellular pH** (~7.0-7.2), providing optimal buffering capacity - Plays a crucial role in buffering acids and bases generated by metabolic processes within cells and is also important in renal tubular buffering *Albumin protein* - **Proteins**, including albumin, are important **extracellular buffers** in plasma due to their abundant ionizable amino acid residues - While proteins do contribute to intracellular buffering (especially hemoglobin in RBCs), the **phosphate system is more significant** for general intracellular pH regulation *Ammonia buffer* - The **ammonia buffer system (NH₃/NH₄⁺)** is primarily a **renal buffer system** that plays a crucial role in acid excretion via urine - It is not considered the primary intracellular buffer for metabolic acid-base balance within cells *Bicarbonate buffer* - The **bicarbonate buffer system (HCO₃⁻/H₂CO₃)** is the **most important extracellular buffer system**, critical for maintaining blood pH - Although present intracellularly, its buffering capacity is less prominent than phosphate within cells due to lower intracellular bicarbonate concentration and its pKa of 6.1 being further from intracellular pH
Question 4: 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 5: 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 6: 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 7: In plasma, if pH is 5, what is the fraction of base to acid?
- A. 0.01
- B. 0.1 (Correct Answer)
- C. 1
- D. 10
Explanation: ***0.1*** - This question applies the **Henderson-Hasselbalch equation**: pH = pKa + log([base]/[acid]). For the **bicarbonate buffer system** (the primary plasma buffer), pKa ≈ 6.1. - Substituting the given values: $5 = 6.1 + \log([HCO_3^-] / [H_2CO_3])$ - Rearranging: $\log([HCO_3^-] / [H_2CO_3]) = 5 - 6.1 = -1.1$ - Therefore: $[HCO_3^-] / [H_2CO_3] = 10^{-1.1} ≈ 0.079$ - Among the given options, **0.079 is closest to 0.1**, making this the correct answer. - Note: pH 5 in plasma is physiologically impossible (incompatible with life), but this tests theoretical understanding of the buffer equation. *0.01* - This ratio would correspond to an even **more acidic** condition with $\log([base]/[acid]) = -2$. - Using Henderson-Hasselbalch: pH = 6.1 + (-2) = 4.1, which is lower than the given pH of 5. - The calculated ratio of 0.079 is much closer to 0.1 than to 0.01. *1* - A ratio of 1 means **equal concentrations** of base and acid, which occurs when pH = pKa. - This would give pH = 6.1, not the given pH of 5. - This represents a **neutral buffer condition**, not the acidic state described. *10* - This ratio indicates an **alkaline** solution with 10 times more base than acid. - Using Henderson-Hasselbalch: pH = 6.1 + log(10) = 6.1 + 1 = 7.1 (physiological alkalosis). - This contradicts the given acidic pH of 5.
Question 8: Which one of the following biochemical abnormalities can be produced by repeated vomiting?
- A. Metabolic acidosis
- B. Metabolic alkalosis (Correct Answer)
- C. Ketosis
- D. Uraemia
Explanation: ***Metabolic alkalosis*** - Repeated vomiting leads to the loss of **hydrochloric acid (HCl)** from the stomach, causing **hypochloremic metabolic alkalosis** with an increase in serum **bicarbonate (HCO3-)** and a rise in blood pH. - The loss of H+ and Cl- ions results in **compensatory hypokalemia** as the kidneys exchange K+ for H+ to maintain electroneutrality. - **Volume depletion** from vomiting triggers aldosterone secretion, which further promotes K+ loss and H+ excretion, perpetuating the alkalosis (contraction alkalosis). - This is one of the most common causes of metabolic alkalosis in clinical practice. *Metabolic acidosis* - This condition is characterized by a decrease in **serum pH** and **bicarbonate levels**, typically due to excess acid production or bicarbonate loss from diarrhea or renal tubular acidosis. - Vomiting does not directly cause metabolic acidosis; rather, it leads to the opposite effect by removing acidic gastric contents. *Ketosis* - **Ketosis** occurs when the body metabolizes fat for energy, producing **ketone bodies**, common in conditions like uncontrolled diabetes or prolonged starvation. - While severe, prolonged vomiting with reduced oral intake can indirectly lead to starvation ketosis, the primary and most characteristic biochemical abnormality of repeated vomiting is metabolic alkalosis, not ketosis. *Uraemia* - **Uraemia** is a syndrome caused by the accumulation of **nitrogenous waste products** (urea, creatinine) in the blood, primarily due to kidney failure. - Vomiting may be a *symptom* of uraemia, but it does not *cause* uraemia. Kidney function is the primary determinant of urea levels.
Question 9: Increased serum calcium is seen in all conditions except:
- A. Myxedema (Correct Answer)
- B. Multiple myeloma
- C. Sarcoidosis
- D. Primary hyperparathyroidism
Explanation: ### Explanation **Correct Answer: A. Myxedema** **1. Why Myxedema is the correct answer:** Myxedema refers to severe **hypothyroidism**. In this condition, serum calcium levels are typically **normal or slightly decreased**, but never increased. Thyroid hormones normally stimulate bone resorption; therefore, in a hypothyroid state, there is a decrease in bone turnover. In contrast, it is *Hyperthyroidism* that is occasionally associated with mild hypercalcemia due to increased osteoclastic activity. **2. Analysis of Incorrect Options (Causes of Hypercalcemia):** * **Multiple Myeloma:** This is a plasma cell dyscrasia where malignant cells produce "Osteoclast Activating Factors" (like IL-6 and TNF-beta). This leads to extensive bone resorption (punched-out lesions) and significant hypercalcemia. * **Sarcoidosis:** This granulomatous disease involves macrophages that express the enzyme **1-alpha-hydroxylase**. This enzyme converts Vitamin D to its active form (1,25-dihydroxyvitamin D), leading to increased intestinal calcium absorption and hypercalcemia. * **Primary Hyperparathyroidism:** Usually caused by a parathyroid adenoma, it results in excessive secretion of Parathyroid Hormone (PTH). PTH increases bone resorption, renal calcium reabsorption, and intestinal absorption (via Vitamin D activation), making it the most common cause of hypercalcemia in outpatient settings. **3. NEET-PG High-Yield Pearls:** * **Most common cause of hypercalcemia (Outpatient):** Primary Hyperparathyroidism. * **Most common cause of hypercalcemia (Inpatient/Hospitalized):** Malignancy. * **Milk-Alkali Syndrome:** A classic triad of hypercalcemia, metabolic alkalosis, and renal failure due to excessive ingestion of calcium carbonate. * **ECG Finding:** Hypercalcemia causes a **shortened QT interval**, whereas hypocalcemia causes a prolonged QT interval.
Question 10: Which is the most effective buffer system in the blood that is controlled by respiration?
- A. Bicarbonates (Correct Answer)
- B. Hemoglobin
- C. Proteins
- D. Phosphates
Explanation: **Explanation:** The **Bicarbonate buffer system ($HCO_3^- / CO_2$)** is the most important and effective extracellular buffer in the blood. Its effectiveness stems from being an **"open system."** Unlike other buffers, its components are independently regulated by two major organs: the **lungs** (controlling $CO_2$ via respiration) and the **kidneys** (controlling $HCO_3^-$ excretion and reabsorption). According to the Henderson-Hasselbalch equation, the pH of blood is determined by the ratio of bicarbonate to dissolved $CO_2$. By increasing or decreasing the rate of respiration (ventilation), the body can rapidly adjust $pCO_2$ levels to maintain this ratio, making it the primary respiratory-controlled buffer. **Analysis of Incorrect Options:** * **Hemoglobin (B):** While hemoglobin is a powerful buffer (due to histidine residues) and is the most important buffer **inside erythrocytes**, it is not primarily controlled by respiration; it depends on the oxygenation state (Bohr effect). * **Proteins (C):** Plasma proteins (like albumin) act as buffers in the blood, but their concentration remains relatively static and is not acutely regulated by respiratory changes. * **Phosphates (D):** The phosphate buffer system is crucial **intracellularly** and in the **renal tubules** (where its pKa of 6.8 is close to tubular pH). However, its concentration in the plasma is too low to be the most effective blood buffer. **High-Yield Clinical Pearls for NEET-PG:** * **Normal $HCO_3^- : CO_2$ ratio:** 20:1 (maintains physiological pH of 7.4). * **First line of defense:** Chemical buffers (seconds). * **Second line of defense:** Respiratory system (minutes). * **Third line of defense:** Renal system (hours to days). * **Isohydric Principle:** All buffer systems in the body are in equilibrium with each other; a change in one affects all others.
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