To keep blood pH at 7.4, what should be the HCO3- : H2CO3 ratio?
Q162
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?
Q163
The primary respiratory compensation for metabolic acidosis is?
Q164
Which of the following conditions does not cause metabolic acidosis?
Acid-Base Balance Indian Medical PG Practice Questions and MCQs
Question 161: To keep blood pH at 7.4, what should be the HCO3- : H2CO3 ratio?
A. 20:1 (Correct Answer)
B. 30:1
C. 15:1
D. 1:1
Explanation: ***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**.
Question 162: 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 163: 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 164: Which of the following conditions does not cause metabolic acidosis?
A. Pyloric stenosis (Correct Answer)
B. Biliary fistula
C. Ureterosigmoidostomy
D. Renal failure
Explanation: ***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**.
