Acid-Base Balance — MCQs

A 28 year old woman was admitted electively to a HDU following C-section. A diagnosis of 'fatty liver of pregnancy' had been made preoperatively. She was commenced on a continuous morphine infusion at 5 mg/hr and received oxygen by mask. This was continued overnight and she was noted to be quite drowsy the next day. Arterial blood gases were- pH 7.16, pCO2- 61.9 mmHg, pO2- 115 mmHg and HCO3- 21.2 mmol/l?

Q137

All of the following statements about acid-base disorders are true, EXCEPT:

Q138

What is the normal pH of the blood?

Q139

In metabolic acidosis, which of the following changes are seen?

Q140

Use the following laboratory values to find the best option that describes the acid-base disorder: Plasma pH = 7.12, Plasma PCO2 = 60 mm Hg, Plasma HCO3- = 19 mEq/L

Acid-Base Balance Indian Medical PG Practice Questions and MCQs

Question 131: The most important physiological anion other than chloride is:

A. Bicarbonate (Correct Answer)
B. Phosphate
C. Sulphate
D. Nitrate

Explanation: ***Bicarbonate*** - **Bicarbonate (HCO3-)** is the second most abundant physiological anion after chloride and plays a crucial role in maintaining **acid-base balance** as part of the bicarbonate buffer system. - It is vital for transporting **carbon dioxide** from tissues to the lungs to be exhaled. *Phosphate* - While an important physiological anion, **phosphate** is primarily involved in **energy metabolism** (ATP, ADP), bone mineralization, and intracellular buffering, making its extracellular concentration far lower than bicarbonate. - Its role as an extracellular buffer is less significant than bicarbonate's due to its lower concentration and pKa in physiological conditions. *Sulphate* - **Sulphate (SO4^2-)** is present in the body but in much lower concentrations than chloride or bicarbonate. - Its primary roles are in metabolism and detoxification, not as a major component of electrolyte balance or acid-base regulation. *Nitrate* - **Nitrate (NO3-)** is generally found in very low, non-physiologically significant concentrations in the body under normal circumstances. - It is not considered a major physiological anion and does not play a direct role in maintaining electrolyte balance or acid-base homeostasis.

Question 132: Hydrogen ions are directly eliminated from the body primarily by:

A. Lungs
B. Kidney (Correct Answer)
C. Liver
D. Stomach

Explanation: ***Kidney*** - The kidneys play a crucial role in **long-term acid-base balance** by excreting excess hydrogen ions (H+) and reabsorbing bicarbonate. - This process involves the secretion of H+ into the renal tubules, primarily by **proximal tubule cells** and **intercalated cells** of the collecting ducts. *Lungs* - The lungs eliminate carbon dioxide (CO2), which is in equilibrium with carbonic acid (H2CO3) and hydrogen ions in the blood. This provides **short-term acid-base regulation**. - While essential for pH balance, the lungs primarily control volatile acids, not directly eliminating hydrogen ions as a waste product in the same way the kidneys do. *Liver* - The liver is involved in various metabolic processes, including the metabolism of proteins and some organic acids, but it does **not directly eliminate hydrogen ions** as a primary function of acid-base regulation. - Its role in acid-base balance is indirect, such as producing urea from ammonia, which helps remove nitrogenous waste products. *Stomach* - The stomach secretes **hydrochloric acid (HCl)**, contributing to an acidic environment for digestion, but it does **not eliminate hydrogen ions** from the body to maintain systemic acid-base balance. - The secreted hydrogen ions are primarily involved in the digestive process within the gastrointestinal tract.

Question 133: Bulimia nervosa is associated with

A. acidosis
B. obesity
C. alkalosis (Correct Answer)
D. decreased sexual drive

Explanation: ***Alkalosis*** - **Recurrent self-induced vomiting** in bulimia nervosa causes loss of gastric HCl (hydrochloric acid) - This results in **metabolic alkalosis** with hypochloremia and hypokalemia - Elevated blood pH with compensatory hypoventilation is characteristic - **Directly related to the pathophysiology** of purging behaviors *Acidosis* - Would require excessive acid load or bicarbonate loss - Not typical in bulimia where gastric acid is lost through vomiting - Opposite acid-base disturbance from what occurs *Obesity* - Bulimics are typically **normal weight or slightly overweight** - Binge-purge cycle prevents substantial weight gain - Distinguishes bulimia from binge eating disorder *Decreased sexual drive* - Not a primary physiological association of bulimia nervosa - Sexual function is variable and depends on multiple psychological factors - Amenorrhea is less common than in anorexia nervosa

Question 134: Uncompensated metabolic acidosis shows

A. Increased pH with increased HCO3-
B. Decreased pH with increased HCO3-
C. Increased pH with decreased HCO3-
D. Decreased pH with Decreased HCO3- (Correct Answer)

Explanation: ***Decreased pH with Decreased HCO3-*** - In **metabolic acidosis**, the primary disturbance is a **decrease in bicarbonate (HCO3-)**, which leads directly to a **decrease in pH**. - Since it is **uncompensated**, there is no significant change in the **PaCO2** to counteract the pH drop, maintaining the low pH. - This is the classic finding in uncompensated metabolic acidosis. *Increased pH with increased HCO3-* - This profile describes **metabolic alkalosis**, where a primary increase in **HCO3-** drives the **pH up**. - This is the opposite of acidosis, where pH is low. *Decreased pH with increased HCO3-* - A **decreased pH** with **increased HCO3-** typically represents **compensated respiratory acidosis**, where the primary problem is **increased PaCO2** (causing low pH), and the kidneys have retained HCO3- as compensation. - This does not represent uncompensated metabolic acidosis, where HCO3- would be decreased, not increased. *Increased pH with decreased HCO3-* - An **increased pH** with **decreased HCO3-** would represent **compensated respiratory alkalosis**, where the primary disturbance is a decrease in **PaCO2** (causing pH to rise), and the kidneys have decreased HCO3- as compensation. - This is unrelated to metabolic acidosis.

Question 135: For clinical assessment of blood pH, the measurement is typically done in:

A. Serum
B. Whole blood (Correct Answer)
C. RBC
D. Plasma

Explanation: ***Whole blood*** - Blood pH for clinical assessment is measured using **arterial blood gas (ABG)** or venous blood gas (VBG) analysis, which uses **whole blood samples**. - Modern blood gas analyzers measure pH directly in whole blood using pH-sensitive electrodes that detect hydrogen ion concentration in the sample. - Using whole blood is essential because it: - Maintains the natural **equilibrium between plasma and red blood cells** - Prevents CO2 loss that would occur with sample processing - Provides immediate, accurate representation of **in vivo acid-base status** - While the pH value reflects primarily the **plasma/extracellular compartment**, the measurement technique requires whole blood. *Plasma* - Although plasma pH is the physiologically relevant parameter (representing extracellular fluid pH), **plasma is not used for the actual measurement** in clinical practice. - Separating plasma would be impractical, time-consuming, and would alter the acid-base status due to CO2 equilibration and temperature changes. - Blood gas analyzers are designed to analyze whole blood, not separated plasma. *Serum* - Serum is plasma from which clotting factors have been removed through the clotting process. - The clotting process involves metabolic activity, release of substances from cells and platelets, and time delay, all of which would significantly **alter the pH** from its true in vivo value. - Serum is **never used for pH measurement** in clinical acid-base assessment. *RBC* - Red blood cells have an intracellular pH (~7.2) that is lower than plasma pH (~7.4). - Direct measurement of RBC pH alone would not reflect the clinically relevant **extracellular/plasma pH**. - RBCs are included in whole blood samples, but the measurement represents the equilibrated system, not isolated RBC pH.

Question 136: A 28 year old woman was admitted electively to a HDU following C-section. A diagnosis of 'fatty liver of pregnancy' had been made preoperatively. She was commenced on a continuous morphine infusion at 5 mg/hr and received oxygen by mask. This was continued overnight and she was noted to be quite drowsy the next day. Arterial blood gases were- pH 7.16, pCO2- 61.9 mmHg, pO2- 115 mmHg and HCO3- 21.2 mmol/l?

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

Explanation: ***Respiratory acidosis + Metabolic acidosis*** - The **low pH (7.16)** indicates severe acidosis, and the **elevated pCO2 (61.9 mmHg)** confirms primary respiratory acidosis, likely due to **morphine-induced respiratory depression**. - The **bicarbonate level (21.2 mmol/l) is below normal (22-26 mmol/l)**, indicating **inadequate metabolic compensation** and concurrent metabolic acidosis. - In pure acute respiratory acidosis, the expected HCO3- should be ~26 mmol/l; the actual value of 21.2 mmol/l confirms a **superimposed metabolic acidosis**, likely related to hepatic dysfunction from fatty liver of pregnancy. *Respiratory alkalosis + Metabolic acidosis* - This option is incorrect because the **pCO2 is elevated (61.9 mmHg)**, indicating respiratory acidosis, not alkalosis. - **Morphine infusion** and patient drowsiness clearly point to respiratory depression, not hyperventilation. *Respiratory alkalosis + Metabolic alkalosis* - This option is incorrect because the **pH is profoundly acidic (7.16)**, contradicting both respiratory and metabolic alkalosis. - The clinical picture of **morphine-induced respiratory depression** is inconsistent with alkalosis. *Respiratory acidosis + Metabolic alkalosis* - This option correctly identifies **respiratory acidosis (high pCO2)** but incorrectly identifies the metabolic component. - The **bicarbonate level (21.2 mmol/l) is below normal**, not elevated, ruling out metabolic alkalosis and confirming concurrent metabolic acidosis instead.

Question 137: All of the following statements about acid-base disorders are true, EXCEPT:

A. Metabolic acidosis is compensated by increasing Pco2 (Correct Answer)
B. Buffering may be intra & extra cellular
C. pH determined by Pco2 and HCO3
D. Respiratory acidosis is compensated by HCO3

Explanation: ***Metabolic acidosis is compensated by increasing Pco2*** - In **metabolic acidosis**, the primary problem is a decrease in **bicarbonate (HCO3-)**. - The compensatory response is **respiratory**, involving an increase in **respiratory rate** and depth to **decrease Pco2**, thereby *raising* the pH back towards normal. Increasing Pco2 would worsen the acidosis. *Buffering may be intra & extra cellular* - **Buffering systems** operate both **intracellularly** (e.g., proteins, phosphates) and **extracellularly** (e.g., bicarbonate-carbonic acid system, hemoglobin). - This dual buffering ensures a rapid and widespread response to changes in acid-base balance throughout the body. *pH determined by Pco2 and HCO3* - According to the **Henderson-Hasselbalch equation**, pH is directly proportional to the ratio of **bicarbonate (HCO3-)** to **Pco2**. - This means that changes in either Pco2 (respiratory component) or HCO3- (metabolic component) will directly influence the overall pH of the blood. *Respiratory acidosis is compensated by HCO3* - In **respiratory acidosis**, the primary problem is an increase in **Pco2** due to hypoventilation. - The compensatory response is **renal**, involving increased reabsorption of **bicarbonate (HCO3-)** and increased excretion of H+ ions to buffer the excess acid.

Question 138: What is the normal pH of the blood?

A. 7.45–7.55
B. 7.30–7.40
C. 7.20–7.30
D. 7.35–7.45 (Correct Answer)

Explanation: ***7.35–7.45*** - The human body maintains a very **narrow pH range** to ensure optimal functioning of physiological processes and enzyme activity. - A pH within this range is crucial for **acid-base homeostasis**, which is tightly regulated by buffer systems, the respiratory system, and the renal system. *7.45–7.55* - A blood pH above **7.45 is considered alkalosis**, indicating an excess of base or a deficit of acid. - Such a high pH can lead to various medical complications, including **neurological dysfunction** and **cardiac arrhythmias**. *7.30–7.40* - While part of this range (7.35-7.40) is normal, a pH below **7.35 is considered acidosis**, indicating an excess of acid or a deficit of base. - Sustained acidosis can impair cellular function and lead to **organ damage**. *7.20–7.30* - This range represents **moderate to severe acidosis**, which requires immediate medical intervention. - A pH this low can significantly depress the central nervous system, leading to **coma and death** if not corrected.

Question 139: In metabolic acidosis, which of the following changes are seen?

A. Increased Na+ reabsorption
B. Decreased K+ excretion (Correct Answer)
C. Increased Na+ excretion
D. Increased K+ excretion

Explanation: ***Decreased K+ excretion*** - This answer requires important context: the renal K+ handling in metabolic acidosis is **complex and varies by acidosis type**. - In metabolic acidosis, H+ moves into cells and K+ shifts out, causing **hyperkalemia** (this transcellular shift is consistent across all types). - In **non-anion gap (hyperchloremic) acidosis** and some chronic forms, renal K+ excretion may be reduced as the kidney prioritizes H+ secretion over K+ secretion in the distal nephron. - However, in **high anion gap acidosis** (diabetic ketoacidosis, lactic acidosis), K+ excretion typically increases due to enhanced distal delivery of Na+ with organic anions. - This option is the "best" answer in the classical teaching context of renal tubular acidosis, though it is not universally applicable to all metabolic acidoses. *Increased Na+ reabsorption* - While the kidney does increase HCO3- reabsorption (often coupled with Na+) as a compensatory mechanism in metabolic acidosis, this is **not the most characteristic or direct change**. - Na+ handling is primarily driven by volume status and aldosterone rather than acid-base status directly. - This is not the defining renal response to metabolic acidosis. *Increased Na+ excretion* - Increased Na+ excretion would lead to volume depletion and is not a characteristic compensatory response to metabolic acidosis. - The kidney generally conserves Na+ to maintain extracellular volume, especially during acid-base disturbances. - This does not represent a typical adaptation to acidosis. *Increased K+ excretion* - While this actually occurs in **high anion gap metabolic acidoses** (most common clinically), the classical teaching emphasizes that acidosis causes **decreased K+ secretion** in the distal tubule. - The hyperkalemia seen in metabolic acidosis is primarily due to the **transcellular K+ shift** (out of cells), not necessarily reduced excretion in all cases. - In the context of traditional teaching about renal tubular acidosis and the expected answer for this question type, this is considered incorrect.

Question 140: Use the following laboratory values to find the best option that describes the acid-base disorder: Plasma pH = 7.12, Plasma PCO2 = 60 mm Hg, Plasma HCO3- = 19 mEq/L

A. Combined metabolic and respiratory acidosis (Correct Answer)
B. Metabolic alkalosis with respiratory compensation
C. Combined metabolic and respiratory alkalosis
D. Respiratory acidosis with renal compensation

Explanation: ***Combined metabolic and respiratory acidosis*** - The **pH of 7.12** indicates profound **acidemia**, meaning the blood is more acidic than normal. - The **PCO2 of 60 mm Hg** (normal 35-45 mm Hg) indicates **respiratory acidosis** as the elevated CO2 drives the pH down; the **HCO3- of 19 mEq/L** (normal 22-26 mEq/L) indicates **metabolic acidosis** as the decreased bicarbonate also drives the pH down, making both components contribute to the acidemia. *Metabolic alkalosis with respiratory compensation* - This would present with an **elevated pH** (alkalemia) and an **elevated HCO3-**, compensated by an elevated PCO2. - The given values show a **low pH** and a **low HCO3-**, which contradicts metabolic alkalosis. *Combined metabolic and respiratory alkalosis* - This would involve an **elevated pH** with both a **low PCO2** (respiratory alkalosis) and an **elevated HCO3-** (metabolic alkalosis). - The patient's pH is very low, unequivocally ruling out any form of alkalosis. *Respiratory acidosis with renal compensation* - While respiratory acidosis is present due to the high PCO2, the **low bicarbonate (19 mEq/L)** indicates a **metabolic acidosis** rather than renal compensation. - In compensated respiratory acidosis, the kidneys would retain bicarbonate, leading to an **elevated HCO3-**, which is not seen here.

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

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