Acid-Base Balance Practice Questions

Q: To keep blood pH at 7.4, what should be the HCO3- : H2CO3 ratio?

20:1. ***20:1*** - The **Henderson-Hasselbalch equation** (pH = pKa + log [HCO3-]/[H2CO3]) is used to determine this ratio. - With a normal blood pH of 7.4 and the pKa of the bicarbonate buffering system being 6.1, a ratio of **20:1** (log 20 ≈ 1.3) yields the correct pH (6.1 + 1.3 = 7.4). *30:1* - A **30:1 ratio** of HCO3-:H2CO3 would result in a higher pH, indicating **alkalosis**, as it shifts the equilibrium towards a more alkaline state. - Log(30) is approximately 1.48, which would result in a pH of 6.1 + 1.48 = 7.58, which is **too alkaline**. *15:1* - A **15:1 ratio** would lead to a lower pH, suggesting **acidosis**, because there isn't enough bicarbonate relative to carbonic acid to buffer the blood properly. - Log(15) is approximately 1.18, resulting in a pH of 6.1 + 1.18 = 7.28, which is **too acidic**. *1:1* - A **1:1 ratio** between bicarbonate and carbonic acid would result in a pH equal to the **pKa (6.1)**, which is significantly acidic and incompatible with life. - This extreme imbalance would indicate severe **metabolic and respiratory acidosis**.

Q: 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?

Metabolic acidosis. ***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.

Q: The primary respiratory compensation for metabolic acidosis is?

Hyperventilation. ***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.

Q: Which of the following conditions does not cause metabolic acidosis?

Pyloric stenosis. ***Pyloric stenosis*** - **Pyloric stenosis** causes persistent vomiting of gastric contents, leading to the loss of hydrochloric acid. - The loss of acid results in **metabolic alkalosis**, characterized by an elevated pH due to increased bicarbonate levels. *Renal failure* - In **renal failure**, the kidneys are unable to excrete hydrogen ions and reabsorb bicarbonate effectively. - This leads to the accumulation of acids in the body and a decrease in bicarbonate, causing **metabolic acidosis**. *Biliary fistula* - A **biliary fistula** results in the loss of bicarbonate-rich bile from the body. - The loss of bicarbonate leads to a decrease in the body's buffer capacity, resulting in **metabolic acidosis**. *Ureterosigmoidostomy* - In a **ureterosigmoidostomy**, urine is diverted into the sigmoid colon, allowing prolonged contact between urine and colonic mucosa. - The colon reabsorbs chloride and excretes bicarbonate in exchange, leading to **hyperchloremic metabolic acidosis**.

Question 1

To keep blood pH at 7.4, what should be the HCO3- : H2CO3 ratio?

Accepted Answer

20:1

Answer

30:1

Answer

15:1

Answer

1:1

Question 2

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?

Accepted Answer

Metabolic acidosis

Answer

Normal acid-base status

Answer

Respiratory acidosis with partial metabolic compensation

Answer

Respiratory acidosis

Question 3

The primary respiratory compensation for metabolic acidosis is?

Accepted Answer

Hyperventilation

Answer

HCO3 loss

Answer

Cl- loss

Answer

Ammonia excretion in kidney

Question 4

Which of the following conditions does not cause metabolic acidosis?

Accepted Answer

Pyloric stenosis

Answer

Biliary fistula

Answer

Ureterosigmoidostomy

Answer

Renal failure

Acid-Base Balance — MCQs

Acid-Base Balance — MCQs

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