Acid-Base Balance — MCQs

Acid-Base Balance Indian Medical PG Practice Questions and MCQs

Question 151: A 30-year-old male with metabolic alkalosis shows decreased respiratory rate. Which compensatory mechanism explains this finding?

A. Increased CO2 excretion
B. Decreased CO2 excretion (Correct Answer)
C. Increased bicarbonate reabsorption
D. Decreased bicarbonate reabsorption

Explanation: ***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**.

Question 152: A patient presents with hyperventilation and reports feeling light-headed. Which of the following physiological changes is most likely responsible for these symptoms?

A. Increased blood CO2 levels
B. Decreased blood O2 levels
C. Decreased blood CO2 levels (Correct Answer)
D. Increased blood lactate levels

Explanation: ***Decreased blood CO2 levels*** - **Hyperventilation** leads to excessive exhalation of carbon dioxide, causing a decrease in blood CO2 levels, a condition known as **hypocapnia**. - This reduction in CO2 results in **cerebral vasoconstriction**, decreasing blood flow to the brain and causing symptoms like light-headedness and dizziness. *Increased blood CO2 levels* - An increase in blood CO2 (hypercapnia) would typically lead to increased respiratory drive (to blow off CO2) and symptoms like headache, confusion, and lethargy, not light-headedness from hyperventilation. - This condition is usually seen in hypoventilation or respiratory failure, the opposite of the described scenario. *Decreased blood O2 levels* - While low oxygen (hypoxia) can cause light-headedness, hyperventilation typically **increases** blood oxygen saturation due to more efficient gas exchange, unless there is an underlying lung pathology. - The primary physiological change directly caused by hyperventilation is related to CO2, not O2. *Increased blood lactate levels* - Elevated lactate, or **lactic acidosis**, is associated with metabolic conditions, strenuous exercise, or shock, and while it can cause symptoms like nausea and weakness, it is not a direct consequence of hyperventilation. - Hyperventilation itself does not primarily drive significant increases in blood lactate in this context.

Question 153: Which of the following is a likely cause of respiratory acidosis?

A. Hyperventilation
B. Ventilatory failure (Correct Answer)
C. Excessive bicarbonate loss
D. Loss of gastric acid

Explanation: ***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 154: A 40-year-old male presents with hyperventilation and dizziness. Which acid-base disturbance is he most likely experiencing?

A. Metabolic acidosis
B. Respiratory alkalosis (Correct Answer)
C. Metabolic alkalosis
D. Respiratory acidosis

Explanation: ***Respiratory alkalosis (hyperventilation)*** - **Hyperventilation** directly leads to an excessive exhalation of **carbon dioxide (CO2)**, causing a decrease in blood pCO2, which is the primary characteristic of respiratory alkalosis. - The drop in CO2 reduces the amount of **carbonic acid** in the blood, leading to an increase in pH (alkalosis) and symptoms like **dizziness and lightheadedness**. *Metabolic acidosis (compensatory hyperventilation possible)* - While metabolic acidosis can lead to **compensatory hyperventilation** to blow off CO2, the primary cause here is the hyperventilation itself, not a metabolic derangement. - In metabolic acidosis, there would be a **low bicarbonate level** and often a primary metabolic cause like DKA or lactic acidosis, which are not suggested by the isolated hyperventilation. *Metabolic alkalosis (typically associated with hypoventilation)* - **Metabolic alkalosis** is characterized by an elevated bicarbonate level, often due to vomiting or diuretic use. - It would typically lead to **compensatory hypoventilation** to retain CO2 and lower pH, which is the opposite of the patient's presentation. *Respiratory acidosis (hypoventilation)* - **Respiratory acidosis** results from **hypoventilation**, leading to CO2 retention and an increase in blood pCO2. - The patient is actively hyperventilating, which would decrease CO2, making respiratory acidosis an incorrect diagnosis.

Question 155: Which buffer system is most effective in regulating the pH of blood during exercise?

A. Phosphate buffer
B. Protein buffer
C. Carbonic acid-bicarbonate buffer
D. Hemoglobin buffer (Correct Answer)

Explanation: ***Hemoglobin buffer*** - During exercise, hemoglobin is the **most quantitatively significant buffer system** in blood, accounting for approximately **60-70% of total buffering capacity** for H+ ions generated during physical activity. - Exercise produces increased **lactic acid** and **CO2** from working muscles, which enter red blood cells where hemoglobin immediately buffers the excess H+ ions through its abundant imidazole groups (histidine residues). - Deoxygenated hemoglobin (reduced Hb) is a **better buffer than oxygenated hemoglobin**, making it particularly effective during exercise when O2 is being released to tissues and CO2/H+ are being picked up. - The high concentration of hemoglobin in blood (**150 g/L**) provides enormous buffering capacity that exceeds other buffer systems during acute metabolic challenges. *Carbonic acid-bicarbonate buffer* - While this is the **major extracellular buffer system** and crucial for overall acid-base homeostasis, it accounts for only about **10% of buffering** during acute exercise. - The bicarbonate system is more important for **long-term regulation** through respiratory compensation (adjusting CO2) and renal compensation (adjusting HCO3- reabsorption). - During exercise, the rapid production of H+ requires immediate intracellular buffering, which bicarbonate provides less effectively than hemoglobin. *Phosphate buffer* - The phosphate buffer system is important primarily **intracellularly** and in **renal tubules** for urinary acid excretion. - Its concentration in extracellular fluid is **significantly lower** than bicarbonate or hemoglobin, limiting its role in blood pH regulation during exercise. *Protein buffer* - Plasma proteins (mainly **albumin**) do contribute to buffering through ionizable side chains, particularly histidine residues. - However, their total buffering capacity in blood during exercise is **much less** than hemoglobin due to lower concentration and fewer buffering sites per molecule.

Question 156: A patient with severe burns is at risk for metabolic acidosis. Which buffer system is primarily involved in compensating for this condition?

A. Bicarbonate buffer system (Correct Answer)
B. Phosphate buffer system
C. Protein buffer system
D. Ammonia buffer system

Explanation: ***Bicarbonate buffer system*** - The **bicarbonate buffer system (HCO3-/H2CO3)** is the **most important extracellular buffer** and the **primary system for compensating metabolic acidosis** from severe burns. - It has the **highest buffering capacity in plasma** (~53% of total blood buffering) and works rapidly to neutralize excess H+ ions produced in metabolic acidosis. - In metabolic acidosis, excess acid (H+) reacts with bicarbonate (HCO3-) to form carbonic acid (H2CO3), which dissociates into **CO2 and H2O**. The CO2 is then **exhaled by the lungs**, providing respiratory compensation. - This system's effectiveness comes from being an **open buffer system** connected to ventilation, allowing elimination of CO2. *Phosphate buffer system* - The **phosphate buffer system (HPO4²-/H2PO4-)** is important for **intracellular buffering** and in **renal tubular fluid**, where phosphate concentrations are higher. - However, its **low concentration in plasma** (only ~5% of blood buffering capacity) makes it ineffective as the primary buffer for systemic metabolic acidosis. - It plays a greater role in chronic pH regulation through the kidneys, not acute compensation. *Protein buffer system* - The **protein buffer system** includes **hemoglobin** (within RBCs) and **plasma proteins** (albumin), which buffer via their ionizable amino acid side chains (especially histidine residues). - Proteins contribute ~35% of blood buffering, but this is mainly **intracellular (hemoglobin)** rather than in extracellular fluid. - While important, proteins cannot be eliminated from the body like CO2, making them less effective for compensating metabolic disturbances compared to the bicarbonate system. *Ammonia buffer system* - The **ammonia buffer system (NH3/NH4+)** operates primarily in the **renal tubules**, where ammonia accepts H+ ions to form ammonium (NH4+), which is then excreted in urine. - This is a **renal compensation mechanism** for chronic acid-base disorders, taking **days to fully activate**, not an immediate blood buffer. - It is not involved in the acute, primary buffering of metabolic acidosis in plasma.

Question 157: Isocapnic buffering is?

A. None of the options
B. Increased pCO2 with increased CO2
C. Increased pCO2 with decreased CO2
D. Normal pCO2 with increased CO2 (Correct Answer)

Explanation: ***Normal pCO2 with increased CO2*** - Isocapnic buffering refers to the process where the body buffers an **increase in lactic acid** or other metabolic acids without a significant change (maintaining it within a normal range) in **arterial partial pressure of carbon dioxide (pCO2)**. - This is achieved by an increase in **ventilation** stimulated by the acid, which expels more CO2 to compensate for the additional CO2 produced from the buffering reaction, thereby keeping pCO2 stable. *Increased pCO2 with increased CO2* - This scenario would indicate **hypoventilation** or a failure of the respiratory compensation mechanism to maintain pCO2 within normal limits during an increased metabolic CO2 load. - **Increased pCO2** would signify a state of **respiratory acidosis** or inadequate respiratory compensation, not isocapnic buffering. *Increased pCO2 with decreased CO2* - This statement is inherently contradictory; it is not possible to have an **increased pCO2** simultaneously with **decreased CO2** in the context of buffering. - **pCO2** is a measure of the partial pressure of carbon dioxide, directly related to the amount of CO2 present and dissolved in the blood. *None of the options* - This option is incorrect because "Normal pCO2 with increased CO2" accurately describes the physiological phenomenon of **isocapnic buffering**.

Question 158: Which of the following is considered the most important intracellular buffer in human physiology?

A. Albumin protein
B. Ammonia buffer
C. Bicarbonate buffer
D. Phosphate buffer (Correct Answer)

Explanation: ***Phosphate buffer*** - The **phosphate buffer system (H₂PO₄⁻/HPO₄²⁻)** is the most important intracellular buffer due to relatively high concentrations of inorganic phosphates within cells - The pKa₂ of approximately **6.8 is close to intracellular pH** (~7.0-7.2), providing optimal buffering capacity - Plays a crucial role in buffering acids and bases generated by metabolic processes within cells and is also important in renal tubular buffering *Albumin protein* - **Proteins**, including albumin, are important **extracellular buffers** in plasma due to their abundant ionizable amino acid residues - While proteins do contribute to intracellular buffering (especially hemoglobin in RBCs), the **phosphate system is more significant** for general intracellular pH regulation *Ammonia buffer* - The **ammonia buffer system (NH₃/NH₄⁺)** is primarily a **renal buffer system** that plays a crucial role in acid excretion via urine - It is not considered the primary intracellular buffer for metabolic acid-base balance within cells *Bicarbonate buffer* - The **bicarbonate buffer system (HCO₃⁻/H₂CO₃)** is the **most important extracellular buffer system**, critical for maintaining blood pH - Although present intracellularly, its buffering capacity is less prominent than phosphate within cells due to lower intracellular bicarbonate concentration and its pKa of 6.1 being further from intracellular pH

Question 159: What is the most important extracellular buffer?

A. Bicarbonates (Correct Answer)
B. Phosphate buffer
C. Plasma protein buffer
D. Ammonium buffer

Explanation: ***Bicarbonates*** - The **bicarbonate buffer system** is the most important extracellular buffer because its components (carbonic acid and bicarbonate) are present in high concentrations and their levels can be regulated by both the lungs (CO2 excretion) and the kidneys (bicarbonate reabsorption/secretion). - Its pKa (6.1) makes it an effective buffer against metabolically produced acids, which frequently challenge blood pH. *Phosphate buffer* - The **phosphate buffer system** is more important as an intracellular buffer and in renal tubular fluid due to its higher concentration in these compartments. - Its concentration in the extracellular fluid is relatively low compared to bicarbonate, limiting its capacity as the primary extracellular buffer. *Plasma protein buffer* - **Plasma proteins**, particularly albumin, have numerous ionizable groups and contribute to buffering in the extracellular fluid. - However, their overall buffering capacity is less significant than that of the bicarbonate system due to lower concentration compared to bicarbonate and less dynamic regulation. *Ammonium buffer* - The **ammonium buffer system** (ammonia/ammonium) is primarily important for acid-base regulation by the kidneys, where it plays a critical role in excreting excess acid, particularly in chronic acidosis. - It is not a major extracellular fluid buffer in the systemic circulation under normal physiological conditions.

Question 160: Tetany is seen in

A. Respiratory alkalosis (Correct Answer)
B. Respiratory acidosis
C. Metabolic acidosis
D. Hyperkalemia

Explanation: ***Respiratory alkalosis*** - **Respiratory alkalosis** is caused by **hyperventilation**, which leads to a decrease in arterial partial pressure of carbon dioxide (**PaCO2**). - This decrease in PaCO2 causes an increase in pH (alkalemia) and a shift in the albumin-bound calcium equilibrium, reducing the amount of **ionized calcium** in the blood, leading to symptoms of **hypocalcemia** such as tetany. *Respiratory acidosis* - **Respiratory acidosis** is characterized by an increase in PaCO2 and a decrease in pH due to inadequate ventilation, which would not typically cause tetany. - In fact, the acidosis would tend to increase **ionized calcium** levels, thereby counteracting any tendency towards symptoms of hypocalcemia. *Metabolic acidosis* - **Metabolic acidosis** involves a decrease in bicarbonate concentration and pH, often due to conditions like diabetic ketoacidosis or lactic acidosis. - Similar to respiratory acidosis, the acidic environment of **metabolic acidosis** tends to increase **ionized calcium** levels, making tetany unlikely. *Hyperkalemia* - **Hyperkalemia** refers to elevated potassium levels in the blood, which primarily affects cardiac and neuromuscular function. - While it can cause muscle weakness and cardiac arrhythmias, it does not directly lead to **tetany**, which is a sign of **hypocalcemia**.

Practice by Chapter

Acid-Base Chemistry

Practice Questions

Respiratory Regulation of Acid-Base Balance

Practice Questions

Renal Regulation of Acid-Base Balance

Practice Questions

Bicarbonate Buffer System

Practice Questions

Non-Bicarbonate Buffer Systems

Practice Questions

Respiratory Acidosis and Alkalosis

Practice Questions

Metabolic Acidosis and Alkalosis

Practice Questions

Mixed Acid-Base Disorders

Practice Questions

Compensatory Mechanisms

Practice Questions

Clinical Assessment of Acid-Base Status

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free