Acid-Base Balance Practice Questions

Q: 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?

CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3-. ***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.

Q: 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?

Metabolic acidosis. ***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.

Q: A patient with pH of 7, pCO2 of 30 mmHg and Bicarbonate levels of 10 meq. What is the acid base abnormality?

Metabolic Acidosis. ***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.

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

Hyperventilation. ***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.

Q: A patient presents with severe vomiting and confusion. ABG shows pH 7.55, HCO3 30 mEq/L, and pCO2 48 mmHg. What is the diagnosis?

Metabolic alkalosis. ### ***Metabolic alkalosis*** - The **pH of 7.55** indicates **alkalemia (alkalosis)**, as it is significantly above the normal range of 7.35-7.45 [2]. - The **elevated HCO3 (30 mEq/L)** is the primary driver of the alkalemia, pointing to a **metabolic origin**. The pCO2 of 48 mmHg indicates a compensatory respiratory acidosis [1]. ### *Metabolic acidosis* - This would be characterized by a **low pH** (acidemia) and a **low HCO3** [3]. - The patient's pH is high, and HCO3 is also high, directly contradicting metabolic acidosis. ### *Respiratory acidosis* - This would present with a **low pH** due to **elevated pCO2** [2]. - While the pCO2 is elevated, the pH is high, and the primary disturbance is the high HCO3, not the pCO2. ### *Mixed acidosis* - This indicates the presence of **both metabolic and respiratory acidosis**, resulting in a very low pH, which is not seen here. - The patient's pH is elevated, indicating an alkalosis, not an acidosis [3].

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

Metabolic alkalosis. ***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.

Q: 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.

Hyperventilation to blow off CO2. ***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.

Q: A 30-year-old male with metabolic alkalosis shows decreased respiratory rate. Which compensatory mechanism explains this finding?

Decreased CO2 excretion. ***Decreased CO2 excretion*** - In **metabolic alkalosis**, the body tries to compensate by **retaining CO2** to lower the pH, making it more acidic and thus countering the severe alkalosis. - This **CO2 retention** is achieved through **hypoventilation**, which is deliberately slowing down breathing to reduce the amount of CO2 expelled from the lungs. *Increased CO2 excretion* - **Increased CO2 excretion** would lead to a further decrease in CO2 levels, which would worsen the **alkalosis** rather than compensate for it. - This mechanism is typically seen in **metabolic acidosis**, where the body tries to blow off CO2 to raise the pH. *Increased bicarbonate reabsorption* - **Increased bicarbonate reabsorption** by the kidneys would further **increase the bicarbonate levels** in the blood, thereby exacerbating the **metabolic alkalosis**. - Renal compensation for metabolic alkalosis typically involves increased bicarbonate excretion, not reabsorption. *Decreased bicarbonate reabsorption* - While ultimately part of renal compensation for metabolic alkalosis, **decreased bicarbonate reabsorption** would involve the kidneys, not immediate respiratory mechanisms. - The question specifically asks about a mechanism leading to **hypoventilation**, which is a **respiratory compensatory mechanism**.

Q: Which of the following is a likely cause of respiratory acidosis?

Ventilatory failure. ***Ventilatory failure*** - **Ventilatory failure** leads to inadequate CO2 excretion from the lungs, causing CO2 to accumulate in the blood. - The accumulation of CO2 (a byproduct of metabolism) results in an increase in **PCO2** and a decrease in **pH**, which is the definition of **respiratory acidosis**. *Hyperventilation* - **Hyperventilation** involves increased removal of CO2 from the body through rapid and deep breathing. - This leads to a decrease in **PCO2** and an increase in **pH**, which causes **respiratory alkalosis**, not acidosis. *Excessive bicarbonate loss* - **Excessive bicarbonate loss**, often seen in conditions like severe diarrhea or renal tubular acidosis, directly reduces the blood's buffer capacity. - This results in a decrease in **HCO3-** and a decrease in **pH**, leading to **metabolic acidosis**. *Loss of gastric acid* - **Loss of gastric acid**, typically due to persistent vomiting or nasogastric suction, removes hydrogen ions from the body. - This leads to an increase in **HCO3-** and an increase in **pH**, causing **metabolic alkalosis**.

Question 1

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?

Accepted Answer

CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3-

Answer

H2O ⇌ H+ + OH-

Answer

H+ + NH3 ⇌ NH4+

Answer

CH3CHOHCH2COOH ⇌ CH3CHOHCH2COO- + H+

Question 2

HCO3/H2CO3 is the best buffer because it is:

Accepted Answer

Its components can be increased or decreased in the body as needed

Answer

Good acceptor and donor of H+ ions

Answer

Combination of a weak acid and weak base

Answer

pKa near physiological pH

Question 3

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?

Accepted Answer

Metabolic acidosis

Answer

Metabolic alkalosis

Answer

Respiratory acidosis

Answer

Respiratory alkalosis

Question 4

A patient with pH of 7, pCO2 of 30 mmHg and Bicarbonate levels of 10 meq. What is the acid base abnormality?

Accepted Answer

Metabolic Acidosis

Answer

Respiratory alkalosis

Answer

Metabolic alkalosis

Answer

Respiratory Acidosis

Question 5

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

Accepted Answer

Hyperventilation

Answer

Hypoventilation

Answer

Increased renal HCO3- reabsorption

Answer

Increased K+ excretion

Question 6

A patient presents with severe vomiting and confusion. ABG shows pH 7.55, HCO3 30 mEq/L, and pCO2 48 mmHg. What is the diagnosis?

Accepted Answer

Metabolic alkalosis

Answer

Metabolic acidosis

Answer

Respiratory acidosis

Answer

Mixed acidosis

Question 7

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

Accepted Answer

Metabolic alkalosis

Answer

Respiratory acidosis

Answer

Metabolic acidosis

Answer

Normal acid-base balance

Question 8

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.

Accepted Answer

Hyperventilation to blow off CO2

Answer

Increased renal reabsorption of bicarbonate

Answer

Increased production of lactate

Answer

Increased ammonia excretion

Question 9

A 30-year-old male with metabolic alkalosis shows decreased respiratory rate. Which compensatory mechanism explains this finding?

Accepted Answer

Decreased CO2 excretion

Answer

Increased CO2 excretion

Answer

Increased bicarbonate reabsorption

Answer

Decreased bicarbonate reabsorption

Question 10

Which of the following is a likely cause of respiratory acidosis?

Accepted Answer

Ventilatory failure

Answer

Hyperventilation

Answer

Excessive bicarbonate loss

Answer

Loss of gastric acid

Acid-Base Balance — MCQs

Acid-Base Balance — MCQs

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