Compensatory Mechanisms — MCQs
In a patient with a pH of 7.2 and a bicarbonate level of 15 mEq/L, what does this imply about the acid-base status?
The interpretation of the following ABG value is: pH = 7.5, pCO2 = 50 mm Hg, HCO3 = 30 mEq/L
A patient in renal failure exhibits metabolic acidosis. What compensatory mechanism is most likely activated?
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?
In metabolic acidosis, what compensatory mechanism is activated first?
Which of the following medications is known to cause increased renin levels with prolonged use?
What is the primary cardiovascular compensatory mechanism in acute hemorrhage?
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 lab reports of a patient given below: pH = 7.2, HCO3 = 10 mEq/L, PCO2 = 30 mmHg. This exemplifies which of the following disorders?
All of the following statements about acid-base disorders are true, EXCEPT:
Compensatory Mechanisms Indian Medical PG Practice Questions and MCQs
Question 1: In a patient with a pH of 7.2 and a bicarbonate level of 15 mEq/L, what does this imply about the acid-base status?
- A. Normal acid-base status
- B. Respiratory acidosis with partial metabolic compensation
- C. Respiratory acidosis
- D. Metabolic acidosis (Correct Answer)
Explanation: ***Metabolic acidosis*** - A **low pH (7.2)** indicates **acidosis**, and a **low bicarbonate level (15 mEq/L)** is the primary disturbance suggesting a metabolic origin. - This combination points to an excess of acid or a loss of bicarbonate, leading to **metabolic acidosis**. *Normal acid-base status* - A **normal pH** would typically range from 7.35 to 7.45, and **bicarbonate** levels would be between 22-26 mEq/L. - The given values (pH 7.2, HCO3 15 mEq/L) are significantly outside of these normal ranges. *Respiratory acidosis with partial metabolic compensation* - Respiratory acidosis is characterized by a **low pH** and an **elevated pCO2**, not primarily a low bicarbonate. - Metabolic compensation would involve an increase in bicarbonate to buffer the acidosis, but the primary disturbance here is low bicarbonate, indicating a metabolic rather than respiratory etiology. *Respiratory acidosis* - Respiratory acidosis is primarily caused by **hypoventilation**, leading to an **increase in pCO2** and a decrease in pH. - In this case, the primary abnormality is the **low bicarbonate**, not an elevated pCO2, which characterizes metabolic acidosis.
Question 2: 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 3: 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 4: 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 5: In metabolic acidosis, what compensatory mechanism is activated first?
- A. Decreased CO2 excretion
- B. Increased respiratory rate (Correct Answer)
- C. Increased renal HCO3- excretion
- D. Increased renal H+ secretion
Explanation: ***Increased respiratory rate*** - In metabolic acidosis, the body attempts to **decrease PCO2** through increasing ventilation, thus reducing the **acid load** by expelling more CO2. - This **respiratory compensation** is rapid and begins within minutes to hours of the onset of acidosis. *Decreased CO2 excretion* - This option is incorrect because the body's compensatory mechanism for acidosis involves **increasing CO2 excretion** through hyperventilation, not decreasing it. - Decreased CO2 excretion would lead to **respiratory acidosis**, further worsening the metabolic acidosis. *Increased renal HCO3- excretion* - In metabolic acidosis, the kidneys aim to **conserve bicarbonate**, not excrete it, to buffer the excess acid. - Increased renal HCO3- excretion would exacerbate acidosis by reducing the body's primary **buffer system**. *Increased renal H+ secretion* - This is a renal compensatory mechanism that occurs in metabolic acidosis, but it is **slower to activate** (hours to days) compared to respiratory compensation. - While important for long-term acid-base balance, it is **not the first mechanism** to be activated.
Question 6: Which of the following medications is known to cause increased renin levels with prolonged use?
- A. Clonidine
- B. Enalapril (Correct Answer)
- C. Methyldopa
- D. Propranolol
Explanation: ***Enalapril*** - **Enalapril** is an **ACE inhibitor** which blocks the conversion of angiotensin I to angiotensin II, leading to decreased levels of angiotensin II [1]. - Reduced angiotensin II levels remove the **negative feedback** on renin release from the juxtaglomerular cells, thus increasing renin secretion [1], [2]. *Clonidine* - Clonidine is a **central alpha-2 adrenergic agonist** that reduces sympathetic outflow from the central nervous system. - This reduction in sympathetic activity leads to a **decrease in renin release**, as sympathetic stimulation normally promotes renin secretion [3]. *Methyldopa* - Methyldopa is a **central alpha-2 adrenergic agonist** that works similarly to clonidine by reducing sympathetic tone. - It consequently causes a **decrease in plasma renin activity** due to reduced sympathetic stimulation of the juxtaglomerular apparatus [3]. *Propranolol* - Propranolol is a **non-selective beta-blocker** that blocks beta-1 receptors in the juxtaglomerular cells of the kidney. - This blockade **inhibits the release of renin** stimulated by sympathetic activity, leading to reduced renin levels [3].
Question 7: What is the primary cardiovascular compensatory mechanism in acute hemorrhage?
- A. Decreased myocardial contractility
- B. Increased respiratory rate
- C. Decreased heart rate
- D. Increased heart rate (Correct Answer)
Explanation: ***Increased heart rate*** - In acute hemorrhage, the body senses a decrease in **blood volume** and **blood pressure**, triggering the **baroreceptor reflex**. - This reflex leads to increased sympathetic nervous system activity, causing an immediate compensatory **increase in heart rate** to maintain **cardiac output** and tissue perfusion. *Decreased myocardial contractility* - A decrease in myocardial contractility would worsen the situation in hemorrhage by further reducing **cardiac output** and is not a primary compensatory mechanism. - While prolonged severe hemorrhage can lead to myocardial depression due to ischemia, it is a pathological consequence, not a compensatory response. *Decreased heart rate* - A decrease in heart rate would reduce **cardiac output** and further compromise blood flow to vital organs during hemorrhage, which is precisely the opposite of what the body needs. - This response is usually seen with vagal stimulation, not in response to hypovolemic shock. *Increased respiratory rate* - An **increased respiratory rate** is a compensatory mechanism for conditions like **metabolic acidosis** (which can occur in severe shock due to lactic acid accumulation) or to improve oxygenation, but it is not the primary cardiovascular compensatory mechanism for maintaining blood pressure and cardiac output in acute hemorrhage. - While it often accompanies hemorrhage, it acts to regulate oxygen and CO2 levels, not directly blood volume or pressure.
Question 8: 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 9: 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 10: All of the following statements about acid-base disorders are true, EXCEPT:
- A. Metabolic acidosis is compensated by increasing Pco2 (Correct Answer)
- B. Buffering may be intra & extra cellular
- C. pH determined by Pco2 and HCO3
- D. Respiratory acidosis is compensated by HCO3
Explanation: ***Metabolic acidosis is compensated by increasing Pco2*** - In **metabolic acidosis**, the primary problem is a decrease in **bicarbonate (HCO3-)**. - The compensatory response is **respiratory**, involving an increase in **respiratory rate** and depth to **decrease Pco2**, thereby *raising* the pH back towards normal. Increasing Pco2 would worsen the acidosis. *Buffering may be intra & extra cellular* - **Buffering systems** operate both **intracellularly** (e.g., proteins, phosphates) and **extracellularly** (e.g., bicarbonate-carbonic acid system, hemoglobin). - This dual buffering ensures a rapid and widespread response to changes in acid-base balance throughout the body. *pH determined by Pco2 and HCO3* - According to the **Henderson-Hasselbalch equation**, pH is directly proportional to the ratio of **bicarbonate (HCO3-)** to **Pco2**. - This means that changes in either Pco2 (respiratory component) or HCO3- (metabolic component) will directly influence the overall pH of the blood. *Respiratory acidosis is compensated by HCO3* - In **respiratory acidosis**, the primary problem is an increase in **Pco2** due to hypoventilation. - The compensatory response is **renal**, involving increased reabsorption of **bicarbonate (HCO3-)** and increased excretion of H+ ions to buffer the excess acid.
Want unlimited practice?
Get full access to all questions, explanations, and performance tracking.
Start For Free