Acid-Base Balance — MCQs

Acid-Base Balance Indian Medical PG Practice Questions and MCQs

Question 141: All the following are the important intracellular buffers EXCEPT

A. Organic phosphate
B. Proteins
C. Haemoglobin
D. Bicarbonates (Correct Answer)

Explanation: ***Bicarbonates*** - Bicarbonate is the **most important extracellular buffer**, primarily working in the blood plasma and extracellular fluid. - While bicarbonate does exist intracellularly in small amounts, its **primary and most significant buffering role is in the extracellular compartment** (plasma and interstitial fluid). - It works in conjunction with carbonic acid (H₂CO₃) in the blood to maintain pH homeostasis. - **This is the correct answer** because the question asks for buffers that are NOT primarily intracellular. *Organic phosphate* - **Organic phosphates** such as 2,3-bisphosphoglycerate (2,3-BPG), ATP, ADP, and glucose-6-phosphate are abundant intracellularly, especially in red blood cells. - They act as effective **intracellular buffers** with their phosphate groups able to accept or donate protons. - Also play a crucial role in regulating oxygen affinity of hemoglobin. *Proteins* - **Intracellular proteins** (enzymes, structural proteins) contain numerous titratable acidic and basic groups, particularly the **imidazole group of histidine residues**. - They are the **most abundant and effective intracellular buffers** due to their high concentration within cells and amphoteric nature (can act as both acids and bases). - Protein buffering capacity is significant in all cells, not just RBCs. *Haemoglobin* - **Hemoglobin** is a major **intracellular buffer within red blood cells**, particularly important for buffering carbonic acid produced during CO₂ transport. - The **imidazole groups of histidine residues** (38 histidines per hemoglobin molecule) can readily bind or release hydrogen ions. - Accounts for approximately 70% of the buffering power of whole blood (intracellular contribution).

Question 142: 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 143: 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 144: A person with type 1 diabetes ran out of her prescription insulin and has not been able to inject insulin for the past 3 days. The patient is hyperventilating to compensate for her metabolic acidosis. Which of the following reactions explains this respiratory compensation for metabolic acidosis?

A. H2O ⇌ H+ + OH-
B. H+ + NH3 ⇌ NH4+
C. CH3CHOHCH2COOH ⇌ CH3CHOHCH2COO- + H+
D. CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3- (Correct Answer)

Explanation: ***CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3-*** - This reaction represents the **bicarbonate buffer system**, which is central to maintaining **pH balance** in the body. - In response to **metabolic acidosis**, the body hyperventilates to **decrease CO2** levels, shifting the equilibrium to the left and reducing H+ which compensates for the increased acidity. *H2O ⇌ H+ + OH-* - This reaction describes the **autoionization of water**, which is fundamental but does not directly explain the body's respiratory compensation mechanism for metabolic acidosis. - While it shows the presence of H+ ions, it doesn't illustrate how the respiratory system manipulates CO2 to influence pH. *H+ + NH3 ⇌ NH4+* - This reaction represents the **ammonia buffer system** primarily active in the **kidneys** for acid excretion. - It plays a role in renal compensation for pH imbalances, but it is not the mechanism for respiratory compensation. *CH3CHOHCH2COOH ⇌ CH3CHOHCH2COO- + H+* - This represents the **dissociation of beta-hydroxybutyric acid**, a **ketone body** produced in diabetic ketoacidosis (DKA). - While DKA is the cause of the metabolic acidosis in this patient, this specific reaction describes the *production* of H+ ions, not the *respiratory compensatory mechanism* to address it.

Question 145: 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 146: The lab reports of a patient given below: pH = 7.2, HCO3 = 10 mEq/L, PCO2 = 30 mmHg. This exemplifies which of the following disorders?

A. Metabolic alkalosis
B. Respiratory acidosis
C. Metabolic acidosis (Correct Answer)
D. Respiratory alkalosis

Explanation: ***Metabolic acidosis*** - The pH of 7.2 is acidic, and the **bicarbonate (HCO3) of 10 mEq/L** is significantly low (normal: 22-28 mEq/L), indicating a primary metabolic disturbance causing acidosis. - The **PCO2 of 30 mmHg** is also low (normal: 35-45 mmHg), which represents **partial respiratory compensation** through hyperventilation to blow off CO2 and raise pH. - This is a classic example of **metabolic acidosis with respiratory compensation**. *Metabolic alkalosis* - This condition would be characterized by a **high pH** and a **high bicarbonate (HCO3)** level, which is the opposite of the given values. - The body would attempt to compensate by increasing PCO2 through hypoventilation. *Respiratory acidosis* - This would present with a **low pH** and a **high PCO2** (>45 mmHg), indicating a primary respiratory problem leading to CO2 retention and acid accumulation. - Metabolic compensation would show elevated HCO3, not the low HCO3 (10 mEq/L) seen here. *Respiratory alkalosis* - This condition is characterized by a **high pH** (>7.45) and a **low PCO2**, due to excessive ventilation causing CO2 elimination. - While PCO2 is low in the given scenario, the pH is acidic (7.2), not alkalotic, ruling out this diagnosis.

Question 147: 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 148: A patient in renal failure exhibits metabolic acidosis. What compensatory mechanism is most likely activated?

A. Hyperventilation (Correct Answer)
B. Hypoventilation
C. Increased renal HCO3- reabsorption
D. Increased K+ excretion

Explanation: ***Hyperventilation*** - In metabolic acidosis, the body responds by increasing **respiratory rate and depth** to exhale more CO2, thereby reducing carbonic acid levels and raising pH. - This is a rapid compensatory mechanism to counteract the drop in blood pH caused by the accumulation of non-volatile acids or loss of bicarbonate. - In renal failure, this becomes the **primary compensatory mechanism** since renal compensation is impaired. *Hypoventilation* - **Hypoventilation** leads to CO2 retention, which would worsen metabolic acidosis by increasing carbonic acid and lowering pH further. - This mechanism is characteristic of primary respiratory acidosis, not a compensatory response to metabolic acidosis. *Increased renal HCO3- reabsorption* - While increased **renal bicarbonate reabsorption** and hydrogen ion excretion are fundamental renal compensatory mechanisms for metabolic acidosis, these are impaired in a patient with **renal failure**. - The kidneys are failing to perform this crucial function, which is the underlying cause of the metabolic acidosis in this scenario. - This is why respiratory compensation becomes the only available mechanism. *Increased K+ excretion* - **Increased K+ excretion** (or retention) is primarily a response to changes in potassium balance, though acid-base disturbances can influence it. - It is not a direct or primary compensatory mechanism for metabolic acidosis, although some renal tubular processes related to acid-base balance can affect potassium handling.

Question 149: The interpretation of the following ABG value is: pH = 7.5, pCO2 = 50 mm Hg, HCO3 = 30 mEq/L

A. Respiratory acidosis
B. Metabolic acidosis
C. Metabolic alkalosis (Correct Answer)
D. Normal acid-base balance

Explanation: ***Metabolic alkalosis (partially compensated)*** - The **pH of 7.5** indicates **alkalosis**, and the elevated **bicarbonate (HCO3) of 30 mEq/L** is the primary driver of this high pH. - The elevated **pCO2 of 50 mm Hg** represents **partial respiratory compensation**, where the body retains CO2 to lower the pH toward normal. - Since the pH remains elevated (not normalized to 7.35-7.45), this is **partially compensated** rather than fully compensated. *Respiratory acidosis* - This would be characterized by a **low pH** and an **elevated pCO2**, which is not seen here as the pH is high. - Although pCO2 is elevated, the **high pH** and **high bicarbonate** rule out primary respiratory acidosis. *Metabolic acidosis* - This would present with a **low pH** and a **low bicarbonate** concentration. - The given values show a **high pH** and **high bicarbonate**, which is the opposite of metabolic acidosis. *Normal acid-base balance* - A normal acid-base balance would have a **pH between 7.35-7.45**, a **pCO2 between 35-45 mm Hg**, and an **HCO3 between 22-26 mEq/L**. - All three values are outside of their normal ranges, indicating an acid-base disturbance.

Question 150: In a case of uncontrolled diabetes mellitus, when ketoacidosis develops, evaluate the primary compensatory mechanism that the body employs to correct the resulting metabolic acidosis.

A. Increased renal reabsorption of bicarbonate
B. Hyperventilation to blow off CO2 (Correct Answer)
C. Increased production of lactate
D. Increased ammonia excretion

Explanation: ***Hyperventilation to blow off CO2*** - In **diabetic ketoacidosis (DKA)**, the accumulation of **ketone bodies** leads to a significant drop in blood pH, causing **metabolic acidosis**. - The body's primary immediate compensatory mechanism is to increase the respiratory rate and depth (**Kussmaul respirations**) to **exhale more CO2**, thereby reducing carbonic acid and increasing blood pH. *Increased renal reabsorption of bicarbonate* - While the kidneys do try to **conserve bicarbonate**, this is a slower, renal compensatory mechanism that is not the primary immediate response to acute metabolic acidosis. - The renal response involves **bicarbonate reabsorption** and **acid excretion**, but its onset and maximal effect are delayed compared to respiratory compensation. *Increased production of lactate* - **Lactate production** is an endogenous source of acid if it accumulates (e.g., in lactic acidosis), and would worsen rather than correct metabolic acidosis. - It is not a compensatory mechanism but rather a potential cause or consequence of abnormal metabolism. *Increased ammonia excretion* - The kidneys **increase ammonia excretion** as part of their long-term acid-base regulation to generate new bicarbonate. - This is a slow, renal mechanism that contributes to **acid excretion** but is not the immediate and primary compensatory response to acute metabolic acidosis in DKA.

Practice by Chapter

Acid-Base Chemistry

Practice Questions

Respiratory Regulation of Acid-Base Balance

Practice Questions

Renal Regulation of Acid-Base Balance

Practice Questions

Bicarbonate Buffer System

Practice Questions

Non-Bicarbonate Buffer Systems

Practice Questions

Respiratory Acidosis and Alkalosis

Practice Questions

Metabolic Acidosis and Alkalosis

Practice Questions

Mixed Acid-Base Disorders

Practice Questions

Compensatory Mechanisms

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Clinical Assessment of Acid-Base Status

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