Acid-Base Balance Practice Questions

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.

Q: All the following are the important intracellular buffers EXCEPT

Bicarbonates. ***Bicarbonates*** - Bicarbonate is the **most important extracellular buffer**, primarily working in the blood plasma and extracellular fluid. - While bicarbonate does exist intracellularly in small amounts, its **primary and most significant buffering role is in the extracellular compartment** (plasma and interstitial fluid). - It works in conjunction with carbonic acid (H₂CO₃) in the blood to maintain pH homeostasis. - **This is the correct answer** because the question asks for buffers that are NOT primarily intracellular. *Organic phosphate* - **Organic phosphates** such as 2,3-bisphosphoglycerate (2,3-BPG), ATP, ADP, and glucose-6-phosphate are abundant intracellularly, especially in red blood cells. - They act as effective **intracellular buffers** with their phosphate groups able to accept or donate protons. - Also play a crucial role in regulating oxygen affinity of hemoglobin. *Proteins* - **Intracellular proteins** (enzymes, structural proteins) contain numerous titratable acidic and basic groups, particularly the **imidazole group of histidine residues**. - They are the **most abundant and effective intracellular buffers** due to their high concentration within cells and amphoteric nature (can act as both acids and bases). - Protein buffering capacity is significant in all cells, not just RBCs. *Haemoglobin* - **Hemoglobin** is a major **intracellular buffer within red blood cells**, particularly important for buffering carbonic acid produced during CO₂ transport. - The **imidazole groups of histidine residues** (38 histidines per hemoglobin molecule) can readily bind or release hydrogen ions. - Accounts for approximately 70% of the buffering power of whole blood (intracellular contribution).

Q: The daily production of hydrogen ions from CO2 is primarily buffered by which of the following?

Red blood cell hemoglobin. ***Red blood cell hemoglobin*** - **Hemoglobin is the primary buffer** for the massive daily acid load from CO2 (approximately 12,500 mEq H+ per day). - CO2 diffuses into RBCs where **carbonic anhydrase** rapidly catalyzes: CO2 + H2O → H2CO3 → H+ + HCO3-. - **Deoxygenated hemoglobin** has a higher affinity for H+ than oxygenated hemoglobin (reduced hemoglobin is a weaker acid, thus better H+ acceptor). - This buffering is crucial for CO2 transport: **Hb + H+ → HHb**, preventing significant pH changes despite huge CO2 production. - The bicarbonate produced is then transported out via the **chloride shift** to maintain electrical neutrality. *Extracellular bicarbonate* - While the bicarbonate buffer system is quantitatively the largest extracellular buffer, it is **NOT the primary buffer for CO2-derived H+**. - The extracellular HCO3-/CO2 system primarily buffers **metabolic (non-volatile) acids** produced from dietary and metabolic sources (~50-100 mEq/day). - For CO2-derived acid, the buffering occurs **intracellularly in RBCs** via hemoglobin before bicarbonate enters the plasma. *Red blood cell bicarbonate* - Bicarbonate is produced within RBCs from the dissociation of carbonic acid, but it is **not the buffer itself**. - The bicarbonate is a **product** of the buffering reaction, not the buffering agent. - Most RBC-produced HCO3- is transported to plasma via the **anion exchanger (Band 3 protein)** in exchange for Cl-. *Plasma proteins* - Plasma proteins like **albumin** have buffering capacity due to ionizable groups (imidazole groups of histidine residues). - They contribute only about **1-5%** of total blood buffering capacity. - Far less important than hemoglobin for buffering the large CO2-derived acid load.

Q: In a patient with a plasma pH of 7.1 the HCO3 / H2CO3 ratio in plasma is:

10. ***Correct Answer: 10*** - The **Henderson-Hasselbalch equation** dictates that pH = pKa + log([HCO3-]/[H2CO3]). Given a normal pKa for carbonic acid of 6.1, a pH of 7.1 leads to 7.1 = 6.1 + log([HCO3-]/[H2CO3]), meaning log([HCO3-]/[H2CO3]) = 1, and thus [HCO3-]/[H2CO3] = 10^1 = **10**. - This ratio of 10 indicates **acidosis**, as the normal physiological ratio for a pH of 7.4 is 20:1. *Incorrect Option: 1* - A ratio of 1 ([HCO3-]/[H2CO3] = 1:1) would mean that log(1) = 0, which would result in a pH equal to the pKa, i.e., pH = 6.1. This is an **extremely acidic** condition incompatible with life. - This ratio would signify a severe and uncompensated metabolic and/or respiratory acidosis. *Incorrect Option: 20* - A ratio of 20 ([HCO3-]/[H2CO3] = 20:1) corresponds to a pH of **7.4**, which is the normal physiological pH. - Since the given plasma pH is 7.1, this ratio is incorrect, as a lower pH indicates a lower ratio. *Incorrect Option: 2* - A ratio of 2 ([HCO3-]/[H2CO3] = 2:1) would result in a pH calculation of pH = 6.1 + log(2) = 6.1 + 0.3 = 6.4. - This pH is also **too low** compared to the given pH of 7.1.

Question 1

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 2

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

Accepted Answer

Whole blood

Answer

Serum

Answer

RBC

Answer

Plasma

Question 3

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 4

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 5

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 6

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 7

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

Question 8

All the following are the important intracellular buffers EXCEPT

Accepted Answer

Bicarbonates

Answer

Organic phosphate

Answer

Proteins

Answer

Haemoglobin

Question 9

The daily production of hydrogen ions from CO2 is primarily buffered by which of the following?

Accepted Answer

Red blood cell hemoglobin

Answer

Red blood cell bicarbonate

Answer

Extracellular bicarbonate

Answer

Plasma proteins

Question 10

In a patient with a plasma pH of 7.1 the HCO3 / H2CO3 ratio in plasma is:

Accepted Answer

10

Answer

1

Answer

20

Answer

2

Acid-Base Balance — MCQs

Acid-Base Balance — MCQs

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