Respiratory Regulation of Acid-Base Balance — MCQs
All of the following statements about acid-base disorders are true, EXCEPT:
A 75-year-old man develops acute confusion and drowsiness after a dental procedure. He has a history of severe chronic lung disease due to smoking. Earlier in the day, he had a tooth extraction and afterwards was given acetaminophen with codeine (Tylenol 3) for pain relief.For the above patient with new symptoms, select the most likely acid base disorder.
Central chemoreceptors are most sensitive to which of the following changes in blood?
The primary respiratory compensation for metabolic acidosis is?
The interpretation of the following ABG value is: pH = 7.5, pCO2 = 50 mm Hg, HCO3 = 30 mEq/L
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?
What is the primary mechanism for maintaining acid-base balance during prolonged vomiting?
In metabolic acidosis, what compensatory mechanism is activated first?
Carbonic anhydrase activity is found in all of the following except?
Which drug is given to prevent acute mountain sickness?
Respiratory Regulation of Acid-Base Balance Indian Medical PG Practice Questions and MCQs
Question 1: 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.
Question 2: A 75-year-old man develops acute confusion and drowsiness after a dental procedure. He has a history of severe chronic lung disease due to smoking. Earlier in the day, he had a tooth extraction and afterwards was given acetaminophen with codeine (Tylenol 3) for pain relief.For the above patient with new symptoms, select the most likely acid base disorder.
- A. respiratory acidosis (Correct Answer)
- B. metabolic alkalosis
- C. metabolic acidosis
- D. respiratory alkalosis
Explanation: ***Respiratory acidosis*** - The patient has **severe chronic lung disease** and was given **codeine**, an opioid, which can cause significant **respiratory depression**. - This combination leads to **hypoventilation**, resulting in **CO2 retention** and thus, respiratory acidosis, often manifesting as acute confusion and drowsiness. *Metabolic alkalosis* - This condition is characterized by an increase in **bicarbonate** levels, often due to vomiting, diuretic use, or hyperaldosteronism, none of which are suggested by the clinical scenario. - While patients with chronic lung disease can sometimes develop compensatory metabolic alkalosis, the acute presentation of confusion and drowsiness after opioid administration points away from this as the primary, acute problem. *Metabolic acidosis* - This typically results from an accumulation of **lactic acid** (e.g., shock), **ketoacids** (e.g., DKA), or renal failure, or loss of bicarbonate (e.g., severe diarrhea). - There is no clinical information provided that suggests these etiologies; rather, the acute symptoms point towards a respiratory cause. *Respiratory alkalosis* - This occurs due to **hyperventilation**, leading to excessive CO2 excretion and decreased PCO2. - The patient's acute confusion and drowsiness, combined with his history and recent medication, suggest **hypoventilation** rather than hyperventilation.
Question 3: Central chemoreceptors are most sensitive to which of the following changes in blood?
- A. PO2
- B. HCO3-
- C. pH
- D. PCO2 (Correct Answer)
Explanation: ***PCO2*** - Central chemoreceptors, located in the **medulla oblongata**, are exquisitely sensitive to changes in the **partial pressure of carbon dioxide (PCO2)** in the arterial blood. - An increase in blood PCO2 readily crosses the **blood-brain barrier** to the cerebrospinal fluid (CSF), where it is converted to carbonic acid and then to H+ and HCO3-. The resulting **drop in CSF pH** directly stimulates these chemoreceptors, leading to increased ventilation. *PO2* - While **peripheral chemoreceptors** (carotid and aortic bodies) are sensitive to changes in **PO2**, particularly when it drops significantly (below 60 mmHg), central chemoreceptors are not. - The primary role of central chemoreceptors is to monitor and respond to changes in CO2 and pH, rather than oxygen levels. *pH* - Central chemoreceptors are indirectly sensitive to **pH changes** in the cerebrospinal fluid (CSF), which result from blood PCO2 changes. - However, they are not directly or primarily sensitive to changes in **blood pH** because hydrogen ions do not readily cross the blood-brain barrier. *HCO3-* - Bicarbonate ions (**HCO3-**) are important in buffering pH, but central chemoreceptors do not directly sense bicarbonate levels. - Changes in HCO3- indirectly affect pH, and it is the resultant **H+ concentration** in the CSF, derived from CO2, that primarily stimulates central chemoreceptors.
Question 4: 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 5: 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 6: 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 7: What is the primary mechanism for maintaining acid-base balance during prolonged vomiting?
- A. Increased chloride reabsorption
- B. Increased potassium excretion
- C. Increased bicarbonate excretion (Correct Answer)
- D. Decreased hydrogen secretion
Explanation: ***Increased bicarbonate excretion*** - Prolonged vomiting leads to the loss of **gastric acid (HCl)**, causing **metabolic alkalosis**. The kidneys compensate by increasing the excretion of **bicarbonate (HCO3-)** to restore acid-base balance. - This renal compensation is the primary mechanism to eliminate the excess alkali from the body. *Increased chloride reabsorption* - In **metabolic alkalosis** due to vomiting, the body tends to reabsorb less chloride, not more, in an attempt to excrete bicarbonate. - **Chloride depletion** can actually hinder bicarbonate excretion by promoting sodium reabsorption with bicarbonate. *Increased potassium excretion* - **Hypokalemia** can occur with prolonged vomiting due to increased aldosterone activity and direct renal loss associated with metabolic alkalosis. - However, increased potassium excretion itself is not the primary mechanism for correcting the acid-base disorder; rather, it is a consequence or a contributing factor to the imbalance. *Decreased hydrogen secretion* - In response to alkalosis, the kidneys would typically decrease, not increase, **hydrogen ion (H+) secretion** in an effort to retain H+ and normalize pH. - Decreased H+ secretion is a compensatory mechanism, but the direct excretion of bicarbonate is more crucial for correcting the metabolic alkalosis.
Question 8: 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 9: Carbonic anhydrase activity is found in all of the following except?
- A. Brain
- B. Kidney
- C. RBC
- D. Plasma (Correct Answer)
Explanation: ***Plasma*** - **Carbonic anhydrase** is an intracellular enzyme that catalyzes the rapid interconversion of carbon dioxide and water to carbonic acid, **bicarbonate**, and protons. - It is notably **absent in plasma** in healthy individuals, as it is primarily found within cells where its function is crucial for pH regulation and CO2 transport. *Brain* - Carbonic anhydrase is found in various brain cells, including **neurons**, **oligodendrocytes**, and **astrocytes**. - It plays a vital role in pH regulation, fluid balance, and the production of cerebrospinal fluid (CSF) within the **central nervous system**. *Kidney* - The kidney is rich in carbonic anhydrase, particularly in the **proximal tubules** and collecting ducts. - It is critical for **bicarbonate reabsorption** and proton excretion, essential processes for maintaining acid-base balance. *RBC* - **Red blood cells (RBCs)** contain a high concentration of carbonic anhydrase (specifically CA-I and CA-II isoforms). - This enzyme facilitates the rapid conversion of CO2 to bicarbonate for transport to the lungs and the reverse reaction for **CO2 exhalation**.
Question 10: Which drug is given to prevent acute mountain sickness?
- A. Acetazolamide (Correct Answer)
- B. Dexamethasone
- C. Digoxin
- D. Diltiazem
Explanation: ***Acetazolamide*** - **Acetazolamide** is a **carbonic anhydrase inhibitor** that acidifies the blood and increases respiratory drive, thereby aiding acclimatization to high altitude. - It is the **drug of choice** for the prevention and treatment of acute mountain sickness (AMS) and high altitude cerebral edema (HACE). *Dexamethasone* - **Dexamethasone**, a corticosteroid, is primarily used for the **treatment** of severe AMS and HACE, rather than prevention. - It works by reducing swelling and inflammation in the brain but does not facilitate physiological acclimatization. *Digoxin* - **Digoxin** is a cardiac glycoside used to treat **heart failure** and **atrial fibrillation**. - It has no role in the prevention or treatment of acute mountain sickness. *Diltiazem* - **Diltiazem** is a **calcium channel blocker** used to treat hypertension, angina, and certain arrhythmias. - It is not indicated for the prevention or treatment of acute mountain sickness.
Want unlimited practice?
Get full access to all questions, explanations, and performance tracking.
Start For Free