Acid-Base and Electrolyte Balance Practice Questions

Q: The normal value of anion gap is:

8-16 mEq/l. ***8-16 mEq/l*** - The **anion gap** is a calculated value representing the difference between the primary measured cation (sodium) and the primary measured anions (chloride and bicarbonate) in serum. - The standard formula is: **Anion Gap = Na+ - (Cl- + HCO3-)**. - A normal anion gap typically falls within the range of **8 to 16 mEq/L**, indicating a balance between unmeasured anions and cations. *20-25 mEq/l* - An anion gap in this range would be significantly **elevated**, suggesting the presence of unmeasured anions in the blood. - This typically indicates a **high anion gap metabolic acidosis**, which can be caused by conditions like lactic acidosis or ketoacidosis. *75 mEq/l* - An anion gap of 75 mEq/L is an **extremely high** value and would be indicative of a severe life-threatening metabolic acidosis. - Such a high value would suggest a profound imbalance, likely due to a massive accumulation of **unmeasured acids**. *50 mEq/l* - An anion gap of 50 mEq/L is also significantly **elevated** and suggests a severe high anion gap metabolic acidosis. - This value is well above the normal range and would prompt immediate investigation for underlying causes of **acid accumulation**.

Q: The pH of body fluids is stabilized by buffer systems. Which of the following compounds is the most effective buffer at physiologic pH?

NaH2PO4, pKa = 7.21. ***NaH2PO4, pKa = 7.21*** - A buffer's maximum effectiveness is typically within 1 pH unit of its **pKa value**. - With a **pKa of 7.21**, the H2PO4⁻/HPO4²⁻ buffer system (phosphate buffer) is optimally positioned to buffer fluctuations around the physiologic pH of **7.35-7.45**. - This makes the phosphate buffer system highly effective in intracellular and urinary pH regulation. *NH4OH, pKa = 9.24* - This compound is a **weak base** with a pKa of 9.24, meaning it would be effective at a pH much higher than the physiologic range. - Its buffering capacity would be minimal at **pH 7.4**, as the system would be predominantly in one form, reducing its ability to resist pH changes. *Na2HPO4, pKa = 12.32* - This represents the **second dissociation** of phosphoric acid (HPO4²⁻ ⇌ PO4³⁻ + H⁺) with a very high **pKa of 12.32**. - This dissociation occurs at extremely alkaline pH levels, far above the physiological range. - At physiologic pH, this equilibrium would be almost entirely shifted to HPO4²⁻, providing no buffering capacity. *CH3CO2H, pKa = 4.75* - **Acetic acid** has a pKa of 4.75, making it an effective buffer in the acidic range (around pH 3.75-5.75). - It would be almost entirely dissociated at **physiologic pH**, offering very little buffering capacity against pH changes in body fluids.

Q: In plasma, if pH is 5, what is the fraction of base to acid?

0.1. ***0.1*** - This question applies the **Henderson-Hasselbalch equation**: pH = pKa + log([base]/[acid]). For the **bicarbonate buffer system** (the primary plasma buffer), pKa ≈ 6.1. - Substituting the given values: $5 = 6.1 + \log([HCO_3^-] / [H_2CO_3])$ - Rearranging: $\log([HCO_3^-] / [H_2CO_3]) = 5 - 6.1 = -1.1$ - Therefore: $[HCO_3^-] / [H_2CO_3] = 10^{-1.1} ≈ 0.079$ - Among the given options, **0.079 is closest to 0.1**, making this the correct answer. - Note: pH 5 in plasma is physiologically impossible (incompatible with life), but this tests theoretical understanding of the buffer equation. *0.01* - This ratio would correspond to an even **more acidic** condition with $\log([base]/[acid]) = -2$. - Using Henderson-Hasselbalch: pH = 6.1 + (-2) = 4.1, which is lower than the given pH of 5. - The calculated ratio of 0.079 is much closer to 0.1 than to 0.01. *1* - A ratio of 1 means **equal concentrations** of base and acid, which occurs when pH = pKa. - This would give pH = 6.1, not the given pH of 5. - This represents a **neutral buffer condition**, not the acidic state described. *10* - This ratio indicates an **alkaline** solution with 10 times more base than acid. - Using Henderson-Hasselbalch: pH = 6.1 + log(10) = 6.1 + 1 = 7.1 (physiological alkalosis). - This contradicts the given acidic pH of 5.

Q: Most important buffer system in human blood:

Bicarbonates. ***Bicarbonates*** - The **bicarbonate buffer system** is the most significant extracellular buffer in human blood due to its high concentration and the ability of its components (CO2 and HCO3-) to be regulated by the lungs and kidneys, respectively. - It rapidly equilibrates with dissolved CO2, making it highly effective at buffering both acid and base imbalances to maintain **blood pH**. *Hemoglobin* - **Hemoglobin** is an important intracellular buffer within red blood cells, primarily buffering carbonic acid formed from CO2 transport. - While powerful within the red blood cell, it is not the primary buffer system in the overall plasma (extracellular fluid). *Chloride ions* - **Chloride ions** are crucial for maintaining electroneutrality in red blood cell buffering processes (e.g., the **chloride shift**), but they do not directly act as a buffer in the traditional sense of accepting or donating protons. - Their primary role in pH balance is indirect, supporting the function of other buffer systems. *Phosphate buffer system* - The **phosphate buffer system** is important, particularly in intracellular fluid and renal tubules, due to its pKa being close to physiological pH. - However, its concentration in the extracellular fluid (blood plasma) is relatively low compared to bicarbonate, making it less significant for overall blood buffering.

Question 1

The normal value of anion gap is:

Accepted Answer

8-16 mEq/l

Answer

20-25 mEq/l

Answer

75 mEq/l

Answer

50 mEq/l

Question 2

The pH of body fluids is stabilized by buffer systems. Which of the following compounds is the most effective buffer at physiologic pH?

Accepted Answer

NaH2PO4, pKa = 7.21

Answer

NH4OH, pKa = 9.24

Answer

Na2HPO4, pKa = 12.32

Answer

CH3CO2H, pKa = 4.75

Question 3

In plasma, if pH is 5, what is the fraction of base to acid?

Accepted Answer

0.1

Answer

0.01

Answer

1

Answer

10

Question 4

Most important buffer system in human blood:

Accepted Answer

Bicarbonates

Answer

Hemoglobin

Answer

Chloride ions

Answer

Phosphate buffer system

Question 5

Which one of the following has the maximum ionization potential?

Accepted Answer

Helium ion

Answer

Hydrogen ion

Answer

Neutron

Answer

Helium atom

Acid-Base and Electrolyte Balance — MCQs

Acid-Base and Electrolyte Balance — MCQs

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