Acid-Base Balance Practice Questions

Q: Vagal stimulation causes the following effects except?

Relaxation of bronchial musculature. **Explanation:** The Vagus nerve (CN X) is the primary mediator of the **parasympathetic nervous system** for the thorax and abdomen. Its effects are mediated via **muscarinic receptors** (primarily M2 and M3). **Why Option C is correct:** Vagal stimulation causes **bronchoconstriction** (contraction of bronchial smooth muscle) and increased mucus secretion via **M3 receptors**. Therefore, relaxation of bronchial musculature is the "except" option. Bronchodilation is actually a sympathetic response mediated by $\beta_2$-adrenergic receptors. **Analysis of incorrect options:** * **Option A (Increase in intestinal secretion):** Vagal activity stimulates gastric acid, pancreatic enzymes, and intestinal secretions to facilitate digestion (Rest and Digest). * **Option B (Constriction of intestinal musculature):** The vagus nerve increases gastrointestinal motility and tone by stimulating the smooth muscle of the gut wall while relaxing sphincters. * **Option C (Fall in blood pressure):** Vagal stimulation to the heart (M2 receptors at the SA and AV nodes) causes bradycardia and a decrease in cardiac output, which leads to a transient fall in blood pressure (Vasovagal response). **High-Yield NEET-PG Pearls:** * **Receptor Specificity:** Vagus $\rightarrow$ ACh $\rightarrow$ **M3** (Bronchoconstriction, Gut motility, Secretions) and **M2** (Decreased Heart Rate). * **Clinical Correlation:** Atropine (an anticholinergic) is used to treat symptomatic bradycardia because it blocks vagal inhibitory effects on the heart. * **Vagal Maneuvers:** Carotid sinus massage or the Valsalva maneuver increases vagal tone to terminate Supraventricular Tachycardia (SVT). * **Vagotomy:** Historically used to treat peptic ulcers to decrease vagally-mediated gastric acid secretion.

Q: What is the primary buffer system in human blood?

Bicarbonate (HCO3-). **Explanation:** The **Bicarbonate ($HCO_3^-$) buffer system** is the primary and most important buffer system in the **extracellular fluid (ECF)** and blood. Its dominance is due to two main factors: 1. **High Concentration:** It is present in high concentrations in the plasma. 2. **Open System:** Unlike other buffers, it is an "open system." The lungs can rapidly regulate $CO_2$ levels, and the kidneys can adjust $HCO_3^-$ levels, allowing for massive buffering capacity against metabolic acids. **Analysis of Options:** * **B. Chlorides:** Chloride ions are involved in maintaining electrical neutrality (e.g., Chloride shift) but do not have buffering capacity as they cannot accept or donate protons ($H^+$). * **C. Hemoglobin (Hb):** While Hemoglobin is the most important **intracellular** buffer within Red Blood Cells (RBCs) and contributes significantly to buffering $CO_2$ via the Bohr effect, it is not the *primary* system for the whole blood/ECF. * **D. Phosphates:** The phosphate buffer system is crucial in the **intracellular fluid (ICF)** and **renal tubules** (where its $pK_a$ of 6.8 is close to the fluid pH). However, its concentration in the plasma is too low to be the primary blood buffer. **High-Yield Clinical Pearls for NEET-PG:** * **Henderson-Hasselbalch Equation:** $pH = pK_a + \log([HCO_3^-] / [0.03 \times PCO_2])$. * **Ratio:** In a healthy state, the ratio of $HCO_3^-$ to dissolved $CO_2$ is **20:1**. * **Primary Intracellular Buffer:** Proteins (including Hemoglobin). * **Primary Urinary Buffer:** Phosphate (titratable acidity) and Ammonium ($NH_4^+$).

Q: What is the limiting pH in humans?

4.5. **Explanation:** The **limiting pH** refers to the maximum hydrogen ion ($H^+$) gradient that the renal tubular cells can maintain against the tubular fluid. In humans, the kidneys cannot acidify urine beyond a pH of **4.5**. **1. Why 4.5 is Correct:** The distal convoluted tubule and collecting ducts contain **Type A intercalated cells**, which utilize primary active transport ($H^+$-ATPase pumps) to secrete $H^+$ ions. Even with these powerful pumps, there is a limit to the concentration gradient they can overcome. When the urine pH reaches 4.5, the concentration of $H^+$ ions in the tubular lumen is approximately **1000 times higher** than in the blood (pH 7.4). At this point, the gradient becomes too steep, and further net secretion of $H^+$ ceases. **2. Analysis of Incorrect Options:** * **A (5.5):** This is often cited as the threshold for the bicarbonate buffer system in the urine to be completely depleted, but it is not the absolute physiological limit. * **B (5.0):** While this represents a very acidic urine sample, the kidneys possess the capacity to concentrate $H^+$ ions further. * **D (4.0):** This is beyond the physiological capability of human renal transport mechanisms. A pH of 4.0 would require a gradient that the $H^+$-ATPase pump cannot sustain. **3. NEET-PG Clinical Pearls & High-Yield Facts:** * **Urinary Buffers:** Because of the limiting pH, $H^+$ cannot be excreted as free ions. It must be buffered by **Phosphate** (Titratable acidity) and **Ammonia** ($NH_3 + H^+ \rightarrow NH_4^+$) to allow for more $H^+$ excretion without further lowering the pH. * **Distal Renal Tubular Acidosis (Type 1 RTA):** In this condition, the kidneys **cannot** reach the limiting pH (urine pH remains > 5.5) due to a defect in the $H^+$-ATPase pump in the distal tubule. * **Normal Urine pH:** Typically ranges from 4.5 to 8.0 depending on the systemic acid-base status.

Q: Which of the following is referred to as the alkali reserve of the body?

Bicarbonate. **Explanation:** The term **"Alkali Reserve"** refers to the concentration of **Bicarbonate ions ($HCO_3^-$)** in the blood. It represents the body's primary buffering capacity against non-volatile (fixed) acids like lactic acid or ketone bodies. **1. Why Bicarbonate is the Correct Answer:** In the Bicarbonate-Carbonic Acid buffer system (the most important extracellular buffer), the ratio of $HCO_3^-$ to $H_2CO_3$ is maintained at **20:1** at a physiological pH of 7.4. Because bicarbonate is present in such high concentrations and can be regulated by the kidneys, it acts as a "reserve" of base that can neutralize incoming hydrogen ions ($H^+$), preventing a drop in blood pH. **2. Why the Other Options are Incorrect:** * **Carbonic Acid ($H_2CO_3$):** This is the acidic component of the buffer system. It is formed when $CO_2$ dissolves in water. It represents the "acid" side of the balance, not the alkali reserve. * **Water ($H_2O$):** While water is the solvent for all biochemical reactions and participates in the formation of carbonic acid, it is neutral and does not function as a buffer or an alkali reserve. **High-Yield Clinical Pearls for NEET-PG:** * **Henderson-Hasselbalch Equation:** $pH = pKa + \log ([HCO_3^-] / [0.03 \times PCO_2])$. * **Normal Bicarbonate Levels:** 22–28 mEq/L. * **Metabolic Acidosis:** Characterized by a primary **decrease** in the alkali reserve ($HCO_3^-$). * **Anion Gap:** Useful in differentiating causes of metabolic acidosis; it is calculated as $Na^+ - (Cl^- + HCO_3^-)$. Normal range is 8–12 mEq/L.

Q: A 72-year-old male with diabetes mellitus presents with lethargy, disorientation, and Kussmaul respirations. Initial laboratory results show a high glucose level of 380 mg/dL and a venous blood pH of 7.3. Given normal bicarbonate levels of 22 to 28 mM and PCO2 values of 33 to 45 mmHg, which of the following additional test results would be consistent with the patient's pH and breathing pattern?

Bicarbonate of 15 mM and PCO2 of 30 mmHg. **Explanation:** The patient presents with classic signs of **Diabetic Ketoacidosis (DKA)**: hyperglycemia, lethargy, and **Kussmaul respirations** (deep, rapid breathing). In DKA, the accumulation of ketoacids leads to a **Metabolic Acidosis**, characterized by a low pH (<7.35) and low bicarbonate ($HCO_3^-$). 1. **Why Option B is correct:** The patient’s pH of 7.3 indicates acidosis. Kussmaul breathing is a physiological **respiratory compensation** where the lungs "blow off" $CO_2$ to raise the pH. Therefore, we expect a low $HCO_3^-$ (the primary metabolic cause) and a low $PCO_2$ (the respiratory compensation). Option B ($HCO_3^-$ 15 mM, $PCO_2$ 30 mmHg) reflects this compensated state. 2. **Why other options are incorrect:** * **Option A:** While it shows compensation, a $PCO_2$ of 10 mmHg is extreme and usually seen in much more severe acidosis (pH < 7.1). * **Option C:** Shows low $HCO_3^-$ but a normal $PCO_2$ (40 mmHg). This indicates a lack of respiratory compensation, which contradicts the clinical finding of Kussmaul respirations. * **Option D:** Shows a high $PCO_2$ (45 mmHg), which would indicate respiratory acidosis, further lowering the pH rather than compensating for it. **High-Yield NEET-PG Pearls:** * **Winters' Formula:** To check if respiratory compensation is adequate in metabolic acidosis: $Expected\ PCO_2 = (1.5 \times [HCO_3^-]) + 8 \pm 2$. * **Kussmaul Breathing:** A hallmark of metabolic acidosis (MUDPILES); it represents the body's attempt to induce a compensatory respiratory alkalosis. * **Anion Gap:** DKA always presents with a **High Anion Gap Metabolic Acidosis (HAGMA)** due to the presence of unmeasured anions (acetoacetate and beta-hydroxybutyrate).

Q: Given the lab values: pH 7.56, paCO2 20, HCO3 20. Interpret the acid-base status?

Respiratory alkalosis, partially compensated. ### Explanation To interpret any acid-base disorder, follow a systematic three-step approach: **1. Determine the Primary Disturbance (pH):** The normal pH range is 7.35–7.45. A pH of **7.56** indicates **Alkalemia**. **2. Identify the Cause (Respiratory vs. Metabolic):** * The **paCO2 is 20 mmHg** (Normal: 35–45). Low CO2 (hypocapnia) causes alkalosis. * The **HCO3 is 20 mEq/L** (Normal: 22–26). Low HCO3 causes acidosis. Since the low paCO2 matches the alkalemic pH, the primary diagnosis is **Respiratory Alkalosis**. **3. Determine Compensation:** In respiratory alkalosis, the kidneys compensate by excreting HCO3 to bring the pH back toward normal. * **Uncompensated:** HCO3 would be normal (24). * **Partially Compensated:** HCO3 has started to drop (as seen here, 20), but the **pH is still abnormal**. * **Fully Compensated:** HCO3 has dropped enough to return the pH to the 7.35–7.45 range. **Why Incorrect Options are Wrong:** * **Option A:** Incorrect because the HCO3 (20) is below the normal range, indicating the kidneys have already begun compensating. * **Option B:** Incorrect because the pH (7.56) is still outside the normal range. * **Option D:** Incorrect because a metabolic alkalosis would require a high pH and a high HCO3. ### High-Yield NEET-PG Pearls * **The "Rule of Thumb":** If the pH and the primary buffer (CO2 or HCO3) move in **opposite** directions, it is a respiratory disorder. If they move in the **same** direction, it is metabolic. * **Compensation Limits:** The body never "over-compensates." If the pH crosses the 7.40 midline, suspect a mixed acid-base disorder. * **Acute vs. Chronic:** In acute respiratory alkalosis, HCO3 drops by 2 mEq/L for every 10 mmHg drop in paCO2. In chronic cases, it drops by 4–5 mEq/L.

Q: What is the half-life of insulin?

4-6 minutes. **Explanation:** The correct answer is **B. 4-6 minutes.** **Underlying Medical Concept:** Insulin is a peptide hormone secreted by the beta cells of the pancreatic islets. Once it enters the systemic circulation, it is rapidly cleared from the plasma. The primary sites of insulin degradation are the **liver** (approx. 50-60%) and the **kidneys** (approx. 35-40%), mediated by the enzyme **insulin glutathione transhydrogenase** (insulinase). Because of this rapid degradation, insulin has a very short biological half-life, typically cited in standard physiology texts (like Guyton and Ganong) as being between **4 to 6 minutes**. This short half-life allows for precise, minute-to-minute regulation of blood glucose levels. **Analysis of Incorrect Options:** * **A. 1-2 minutes:** This is too short for insulin; however, it is more characteristic of the half-life of certain ultra-short-acting neurotransmitters or very unstable compounds. * **C. 10-12 minutes:** While some older studies suggested a slightly longer duration, 10-12 minutes exceeds the standard physiological range for endogenous insulin. * **D. 12-16 minutes:** This is significantly longer than the actual half-life. Such a duration would lead to delayed glucose responses and potential hypoglycemia during rapid physiological shifts. **High-Yield Clinical Pearls for NEET-PG:** * **C-Peptide:** Unlike insulin, C-peptide has a longer half-life (approx. **30 minutes**). This makes it a better clinical marker for endogenous insulin production. * **Exogenous Insulin:** While endogenous insulin lasts minutes, the "effective" half-life of injected insulin depends on the formulation (e.g., Lispro is rapid, Glargine is long-acting). * **Renal Failure:** In patients with chronic kidney disease (CKD), the half-life of insulin increases due to decreased clearance, often necessitating a reduction in insulin dosage to prevent hypoglycemia.

Q: Given a patient with pH = 7.27, HCO3 = 10 mEq/dl, and pCO2 = 23 mm Hg, what is the acid-base disorder?

Metabolic acidosis. To solve acid-base problems for NEET-PG, follow a systematic 3-step approach: **1. Analyze the pH:** The normal pH range is 7.35–7.45. A pH of **7.27** is less than 7.35, indicating **Acidosis**. This immediately eliminates options B and D. **2. Identify the Primary Cause:** * **Metabolic:** Look at Bicarbonate ($HCO_3^-$). Normal is 22–26 mEq/L. Here, $HCO_3^-$ is **10 mEq/L** (Low). Low bicarbonate causes acidosis. * **Respiratory:** Look at $pCO_2$. Normal is 35–45 mmHg. Here, $pCO_2$ is **23 mmHg** (Low). Low $pCO_2$ actually causes alkalosis. Since the low $HCO_3^-$ matches the acidic pH, the primary disorder is **Metabolic Acidosis**. **3. Evaluate Compensation:** The low $pCO_2$ (23 mmHg) represents the body’s respiratory attempt to compensate by "blowing off" acid to raise the pH back toward normal. **Why Incorrect Options are Wrong:** * **Metabolic Alkalosis:** Would present with a high pH (>7.45) and high $HCO_3^-$. * **Respiratory Acidosis:** Would present with a low pH ( 45 mmHg). * **Respiratory Alkalosis:** Would present with a high pH (>7.45) and low $pCO_2$. **High-Yield Clinical Pearls for NEET-PG:** * **Winters' Formula:** In metabolic acidosis, expected $pCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2$. Here: $(1.5 \times 10) + 8 = 23$. Since the measured $pCO_2$ matches, it is a **pure metabolic acidosis with appropriate compensation.** * **Anion Gap:** Always calculate the Anion Gap ($Na^+ - [Cl^- + HCO_3^-]$) in metabolic acidosis to narrow the differential (e.g., MUDPILES for High Anion Gap). * **ROME Mnemonic:** **R**espiratory **O**pposite (pH and $CO_2$ move in opposite directions), **M**etabolic **E**qual (pH and $HCO_3^-$ move in the same direction).

Question 1

Vagal stimulation causes the following effects except?

Accepted Answer

Relaxation of bronchial musculature

Answer

Increase in intestinal secretion

Answer

Constriction of intestinal musculature

Answer

Fall in the blood pressure

Question 2

What is the primary buffer system in human blood?

Accepted Answer

Bicarbonate (HCO3-)

Answer

Chlorides

Answer

Hemoglobin (Hb)

Answer

Phosphates

Question 3

What is the limiting pH in humans?

Accepted Answer

4.5

Answer

5.5

Answer

5.0

Answer

4.0

Question 4

Which of the following is referred to as the alkali reserve of the body?

Accepted Answer

Bicarbonate

Answer

Carbonic acid

Answer

Water

Answer

All of the above

Question 5

A 72-year-old male with diabetes mellitus presents with lethargy, disorientation, and Kussmaul respirations. Initial laboratory results show a high glucose level of 380 mg/dL and a venous blood pH of 7.3. Given normal bicarbonate levels of 22 to 28 mM and PCO2 values of 33 to 45 mmHg, which of the following additional test results would be consistent with the patient's pH and breathing pattern?

Accepted Answer

Bicarbonate of 15 mM and PCO2 of 30 mmHg

Answer

Bicarbonate of 5 mM and PCO2 of 10 mmHg

Answer

Bicarbonate of 15 mM and PCO2 of 40 mmHg

Answer

Bicarbonate of 20 mM and PCO2 of 45 mmHg

Question 6

What is the sympathetic postganglionic neurotransmitter in sweat glands?

Accepted Answer

Acetylcholine

Answer

Epinephrine

Answer

Norepinephrine

Answer

Serotonin

Question 7

Given the lab values: pH 7.56, paCO2 20, HCO3 20. Interpret the acid-base status?

Accepted Answer

Respiratory alkalosis, partially compensated

Answer

Respiratory alkalosis, uncompensated

Answer

Respiratory alkalosis, compensated

Answer

Metabolic alkalosis, partially compensated

Question 8

What is the half-life of insulin?

Accepted Answer

4-6 minutes

Answer

1-2 minutes

Answer

0-12 minutes

Answer

12-16 minutes

Question 9

Increased anion gap acidosis is seen in which of the following conditions?

Accepted Answer

Diabetic ketoacidosis (DKA), Alcoholic ketoacidosis (AKA), Renal tubular acidosis (RTA), Alcoholic fatty liver disease (AFLD)

Answer

Renal tubular acidosis (RTA), Alcoholic fatty liver disease (AFLD), Organic aciduria

Answer

Alcoholic fatty liver disease (AFLD), Organic aciduria, Diarrhoea

Answer

Diabetic ketoacidosis (DKA), Alcoholic fatty liver disease (AFLD), Diarrhoea

Question 10

Given a patient with pH = 7.27, HCO3 = 10 mEq/dl, and pCO2 = 23 mm Hg, what is the acid-base disorder?

Accepted Answer

Metabolic acidosis

Answer

Metabolic alkalosis

Answer

Respiratory acidosis

Answer

Respiratory alkalosis

Acid-Base Balance — MCQs

Acid-Base Balance — MCQs

On this page

Practice by Chapter

Want unlimited practice?