Acid-Base Balance Practice Questions

Q: The most important physiological anion other than chloride is:

Bicarbonate. ***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.

Q: Hydrogen ions are directly eliminated from the body primarily by:

Kidney. ***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.

Q: Bulimia nervosa is associated with

alkalosis. ***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

Q: Uncompensated metabolic acidosis shows

Decreased pH with Decreased HCO3-. ***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.

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

Whole blood. ***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.

Q: 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?

Respiratory acidosis +Metabolic acidosis. ***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.

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

Metabolic acidosis is compensated by increasing Pco2. ***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.

Q: What is the normal pH of the blood?

7.35–7.45. ***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.

Q: 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

Combined metabolic and respiratory acidosis. ***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.

Question 1

The most important physiological anion other than chloride is:

Accepted Answer

Bicarbonate

Answer

Phosphate

Answer

Sulphate

Answer

Nitrate

Question 2

Hydrogen ions are directly eliminated from the body primarily by:

Accepted Answer

Kidney

Answer

Lungs

Answer

Liver

Answer

Stomach

Question 3

Bulimia nervosa is associated with

Accepted Answer

alkalosis

Answer

acidosis

Answer

obesity

Answer

decreased sexual drive

Question 4

Uncompensated metabolic acidosis shows

Accepted Answer

Decreased pH with Decreased HCO3-

Answer

Increased pH with increased HCO3-

Answer

Decreased pH with increased HCO3-

Answer

Increased pH with decreased HCO3-

Question 5

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

Accepted Answer

Whole blood

Answer

Serum

Answer

RBC

Answer

Plasma

Question 6

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?

Accepted Answer

Respiratory acidosis +Metabolic acidosis

Answer

Respiratory alkalosis + Metabolic acidosis

Answer

Respiratory alkalosis + Metabolic alkalosis

Answer

Respiratory acidosis + Metabolic alkalosis

Question 7

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

Accepted Answer

Metabolic acidosis is compensated by increasing Pco2

Answer

Buffering may be intra & extra cellular

Answer

pH determined by Pco2 and HCO3

Answer

Respiratory acidosis is compensated by HCO3

Question 8

What is the normal pH of the blood?

Accepted Answer

7.35–7.45

Answer

7.45–7.55

Answer

7.30–7.40

Answer

7.20–7.30

Question 9

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

Accepted Answer

Decreased K+ excretion

Answer

Increased Na+ reabsorption

Answer

Increased Na+ excretion

Answer

Increased K+ excretion

Question 10

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

Accepted Answer

Combined metabolic and respiratory acidosis

Answer

Metabolic alkalosis with respiratory compensation

Answer

Combined metabolic and respiratory alkalosis

Answer

Respiratory acidosis with renal compensation

Acid-Base Balance — MCQs

Acid-Base Balance — MCQs

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