pH Regulation in Body Fluids — MCQs
Peripheral and central chemoreceptors may both contribute to the increased ventilation that occurs as a result of which of the following?
HCO3/H2CO3 is the best buffer because it is:
Hyperkalemia and metabolic acidosis are commonly associated with which type of renal tubular acidosis?
Hyperkalemia aciduria is seen in
The daily production of hydrogen ions from CO2 is primarily buffered by which of the following?
Cerebral blood flow is regulated by all, EXCEPT:
A male patient presents to the emergency department. The arterial blood gas report is as follows: pH, 7.2; pCO2, 81 mmHg; and HCO3, 40 meq/L. Which of the following is the most likely diagnosis?
A patient in renal failure exhibits metabolic acidosis. What compensatory mechanism is most likely activated?
In plasma, if pH is 5, what is the fraction of base to acid?
A patient with diabetes mellitus for the past 5 years presents with vomiting and abdominal pain. She is non-compliant with medication and appears dehydrated. Investigations revealed a blood sugar value of 500 mg/dl and the presence of ketone bodies. What is the next best step in management of this patient?
pH Regulation in Body Fluids Indian Medical PG Practice Questions and MCQs
Question 1: Peripheral and central chemoreceptors may both contribute to the increased ventilation that occurs as a result of which of the following?
- A. A decrease in arterial oxygen content
- B. A decrease in arterial blood pressure
- C. An increase in arterial carbon dioxide tension (Correct Answer)
- D. A decrease in arterial oxygen tension
Explanation: ***An increase in arterial carbon dioxide tension*** - An increase in **arterial PCO2** (hypercapnia) leads to a rapid decrease in the **pH of the cerebrospinal fluid (CSF)**, which strongly stimulates **central chemoreceptors** in the medulla. - While overwhelmingly driven by central chemoreceptors, a significant increase in **arterial PCO2** also causes a slight decrease in **arterial pH**, which can additionally stimulate **peripheral chemoreceptors** in the carotid and aortic bodies, leading to increased ventilation. *A decrease in arterial oxygen content* - A decrease in **arterial oxygen content** (e.g., due to anemia or carbon monoxide poisoning) without a significant drop in **arterial PO2** primarily affects oxygen delivery to tissues. - It does not directly stimulate peripheral chemoreceptors, which are sensitive to **PO2**, not content, nor does it affect central chemoreceptors directly to increase ventilation in this manner. *A decrease in arterial blood pressure* - A decrease in **arterial blood pressure** is sensed by **baroreceptors** and primarily triggers cardiovascular reflexes (e.g., increased heart rate and vasoconstriction) to restore blood pressure. - It does not directly stimulate peripheral or central chemoreceptors to significantly increase ventilation unless severe hypoperfusion leads to significant changes in arterial blood gases. *A decrease in arterial oxygen tension* - A decrease in **arterial oxygen tension (PO2)**, especially when it falls below approximately 60 mmHg, acts as a potent stimulus for **peripheral chemoreceptors**. - However, **central chemoreceptors** are primarily sensitive to **PCO2** and CSF pH, and a decrease in **arterial PO2** alone has little direct effect on their activity.
Question 2: HCO3/H2CO3 is the best buffer because it is:
- A. Its components can be increased or decreased in the body as needed (Correct Answer)
- B. Good acceptor and donor of H+ ions
- C. Combination of a weak acid and weak base
- D. pKa near physiological pH
Explanation: ***Its components can be increased or decreased in the body as needed*** - The **bicarbonate buffer system** is unique because its components, **bicarbonate (HCO3-)** and **carbon dioxide (CO2)**, are physiologically regulated by the kidneys and lungs, respectively. - This allows for dynamic adjustment of buffer concentrations to maintain **pH homeostasis**, making it highly effective even when its pKa is not perfectly matched to physiological pH. *Good acceptor and donor of H+ ions* - While bicarbonate acts as an **acceptor of H+ ions** and carbonic acid can donate H+ ions, this characteristic is true for all effective buffer systems. - This option does not highlight the unique advantage of the bicarbonate buffer over other physiological buffers. *Combination of a weak acid and weak base* - The bicarbonate buffer system indeed consists of **carbonic acid (H2CO3)**, a weak acid, and its conjugate base, **bicarbonate (HCO3-)**. - However, this is the definition of any buffer system and doesn't explain why it's the *best* physiological buffer compared to others. *pKa near physiological pH* - The **pKa of the bicarbonate buffer system is 6.1**, which is not exactly at the physiological pH of 7.4. - While buffers are generally most effective when their pKa is close to the pH they regulate, the **open nature and physiological regulation** of the bicarbonate system compensate for this difference.
Question 3: Hyperkalemia and metabolic acidosis are commonly associated with which type of renal tubular acidosis?
- A. Type II renal tubular acidosis
- B. Type I renal tubular acidosis
- C. Type IV renal tubular acidosis (Correct Answer)
- D. Type III renal tubular acidosis
Explanation: ***Type IV renal tubular acidosis*** - This type is characterized by **hypoaldosteronism** or **aldosterone resistance**, leading to impaired potassium excretion and bicarbonate reabsorption [2]. - The resulting **hyperkalemia** inhibits ammonium excretion, contributing to a **non-anion gap metabolic acidosis** [1]. *Type I renal tubular acidosis* - This is a **distal RTA** caused by a defect in acid secretion in the collecting duct, leading to an inability to acidify urine [1]. - It typically presents with **hypokalemia**, **nephrolithiasis** (kidney stones), and an alkaline urine pH. *Type II renal tubular acidosis* - This is a **proximal RTA** due to impaired bicarbonate reabsorption in the proximal tubule. - It is typically associated with **hypokalemia**, and the urine can be acidified when systemic acidosis is severe. *Type III renal tubular acidosis* - This is a rare, historically used term, sometimes referring to a combination of features from Type I and Type II RTA. - It is not routinely used in current classification systems and does not specifically or primarily feature hyperkalemia and metabolic acidosis as its defining characteristics.
Question 4: Hyperkalemia aciduria is seen in
- A. Type I Renal Tubular Acidosis
- B. Type IV Renal Tubular Acidosis (Correct Answer)
- C. Sigmoidocolostomy procedure
- D. Type II Renal Tubular Acidosis
Explanation: Type IV Renal Tubular Acidosis - This condition is characterized by **hyperkalemia** and **aciduria**, often due to a deficiency in aldosterone or a renal tubular insensitivity to aldosterone [1]. - The impaired aldosterone action leads to reduced potassium excretion and decreased ammonium production, both contributing to **hyperkalemia** and metabolic acidosis [1]. *Type I Renal Tubular Acidosis* - Type I RTA (distal RTA) is characterized by a defect in acid secretion in the distal tubule, leading to **hypokalemia** and metabolic acidosis with persistently high urine pH [2]. - Patients typically excrete an alkaline urine despite systemic acidosis, contrasting with the aciduria seen with hyperkalemia [2]. *Sigmoidocolostomy procedure* - A sigmoidocolostomy can lead to **hyperchloremic metabolic acidosis** due to the reabsorption of chloride and excretion of bicarbonate by the colonic mucosa. - However, it typically causes **hypokalemia** as potassium is secreted into the colonic lumen from the blood. *Type II Renal Tubular Acidosis* - Type II RTA (proximal RTA) involves a defect in bicarbonate reabsorption in the proximal tubule, resulting in **hypokalemia** and metabolic acidosis. - The kidney's ability to acidify urine is still largely intact in the distal nephron once the bicarbonate buffer system is overwhelmed.
Question 5: The daily production of hydrogen ions from CO2 is primarily buffered by which of the following?
- A. Red blood cell bicarbonate
- B. Extracellular bicarbonate
- C. Plasma proteins
- D. Red blood cell hemoglobin (Correct Answer)
Explanation: ***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.
Question 6: Cerebral blood flow is regulated by all, EXCEPT:
- A. Intracranial pressure
- B. Cerebral metabolic rate
- C. Potassium ions (Correct Answer)
- D. Arterial PCO2
Explanation: ***Potassium ions*** - While potassium ions play a crucial role in neuronal excitability and membrane potential, they are **not a primary direct regulator** of cerebral blood flow (CBF) in the same way as other factors listed. - Changes in extracellular potassium can affect vascular smooth muscle, but their direct impact on CBF auto-regulation is less pronounced compared to metabolic or pressure-related factors. *Intracranial pressure* - **Increased intracranial pressure (ICP)** can significantly decrease cerebral blood flow due to the **Monro-Kellie doctrine**, which states that an increase in ICP reduces the cerebral perfusion pressure (CPP). - A sustained and significant elevation in ICP can lead to **cerebral ischemia** as it opposes the arterial pressure driving blood into the brain. *Arterial PCO2* - **Arterial PCO2** is a potent regulator of cerebral blood flow, with **hypercapnia (high PCO2)** causing **vasodilation** and increased CBF. - Conversely, **hypocapnia (low PCO2)** leads to **vasoconstriction** and decreased CBF, which is a key mechanism in the management of cerebral edema. *Cerebral metabolic rate* - **Cerebral metabolic rate (CMR)** is directly coupled to cerebral blood flow, meaning that regions of the brain with higher metabolic activity receive increased blood flow. - This **neurovascular coupling** ensures adequate supply of oxygen and nutrients to meet the brain's metabolic demands.
Question 7: A male patient presents to the emergency department. The arterial blood gas report is as follows: pH, 7.2; pCO2, 81 mmHg; and HCO3, 40 meq/L. Which of the following is the most likely diagnosis?
- A. Respiratory alkalosis
- B. Metabolic acidosis
- C. Respiratory acidosis (Correct Answer)
- D. Metabolic alkalosis
Explanation: ***Respiratory acidosis*** - The **pH of 7.2** indicates **acidemia**, while the **elevated pCO2 (81 mmHg)** points to a primary respiratory problem [2]. - The elevated **HCO3 (40 meq/L)** suggests **renal compensation** attempting to buffer the increased carbonic acid [1]. *Respiratory alkalosis* - This condition presents with an **elevated pH (alkalemia)** and a **decreased pCO2**, which is opposite to the given ABG values [2]. - While there might be metabolic compensation with a decreased HCO3, the primary disturbance is an increase in respiratory rate leading to excessive CO2 exhalation. *Metabolic acidosis* - Metabolic acidosis is characterized by a **low pH** and a **low HCO3**, with a compensatory decrease in pCO2 [1]. - The given ABG shows a high HCO3, which rules out primary metabolic acidosis. *Metabolic alkalosis* - This condition would typically show an **elevated pH** and an **elevated HCO3**, with a compensatory increase in pCO2. - While both HCO3 and pCO2 are high in the given ABG, the low pH points to a primary acidosis, not alkalosis.
Question 8: A patient in renal failure exhibits metabolic acidosis. What compensatory mechanism is most likely activated?
- A. Hyperventilation (Correct Answer)
- B. Hypoventilation
- C. Increased renal HCO3- reabsorption
- D. Increased K+ excretion
Explanation: ***Hyperventilation*** - In metabolic acidosis, the body responds by increasing **respiratory rate and depth** to exhale more CO2, thereby reducing carbonic acid levels and raising pH. - This is a rapid compensatory mechanism to counteract the drop in blood pH caused by the accumulation of non-volatile acids or loss of bicarbonate. - In renal failure, this becomes the **primary compensatory mechanism** since renal compensation is impaired. *Hypoventilation* - **Hypoventilation** leads to CO2 retention, which would worsen metabolic acidosis by increasing carbonic acid and lowering pH further. - This mechanism is characteristic of primary respiratory acidosis, not a compensatory response to metabolic acidosis. *Increased renal HCO3- reabsorption* - While increased **renal bicarbonate reabsorption** and hydrogen ion excretion are fundamental renal compensatory mechanisms for metabolic acidosis, these are impaired in a patient with **renal failure**. - The kidneys are failing to perform this crucial function, which is the underlying cause of the metabolic acidosis in this scenario. - This is why respiratory compensation becomes the only available mechanism. *Increased K+ excretion* - **Increased K+ excretion** (or retention) is primarily a response to changes in potassium balance, though acid-base disturbances can influence it. - It is not a direct or primary compensatory mechanism for metabolic acidosis, although some renal tubular processes related to acid-base balance can affect potassium handling.
Question 9: In plasma, if pH is 5, what is the fraction of base to acid?
- A. 0.01
- B. 0.1 (Correct Answer)
- C. 1
- D. 10
Explanation: ***0.1*** - This question applies the **Henderson-Hasselbalch equation**: pH = pKa + log([base]/[acid]). For the **bicarbonate buffer system** (the primary plasma buffer), pKa ≈ 6.1. - Substituting the given values: $5 = 6.1 + \log([HCO_3^-] / [H_2CO_3])$ - Rearranging: $\log([HCO_3^-] / [H_2CO_3]) = 5 - 6.1 = -1.1$ - Therefore: $[HCO_3^-] / [H_2CO_3] = 10^{-1.1} ≈ 0.079$ - Among the given options, **0.079 is closest to 0.1**, making this the correct answer. - Note: pH 5 in plasma is physiologically impossible (incompatible with life), but this tests theoretical understanding of the buffer equation. *0.01* - This ratio would correspond to an even **more acidic** condition with $\log([base]/[acid]) = -2$. - Using Henderson-Hasselbalch: pH = 6.1 + (-2) = 4.1, which is lower than the given pH of 5. - The calculated ratio of 0.079 is much closer to 0.1 than to 0.01. *1* - A ratio of 1 means **equal concentrations** of base and acid, which occurs when pH = pKa. - This would give pH = 6.1, not the given pH of 5. - This represents a **neutral buffer condition**, not the acidic state described. *10* - This ratio indicates an **alkaline** solution with 10 times more base than acid. - Using Henderson-Hasselbalch: pH = 6.1 + log(10) = 6.1 + 1 = 7.1 (physiological alkalosis). - This contradicts the given acidic pH of 5.
Question 10: A patient with diabetes mellitus for the past 5 years presents with vomiting and abdominal pain. She is non-compliant with medication and appears dehydrated. Investigations revealed a blood sugar value of 500 mg/dl and the presence of ketone bodies. What is the next best step in management of this patient?
- A. Intravenous fluids
- B. Intravenous insulin
- C. Intravenous fluids with regular insulin (Correct Answer)
- D. Intravenous fluids with long-acting insulin
Explanation: Detailed management of diabetic ketoacidosis (DKA) requires both fluid resuscitation and insulin therapy. ***Intravenous fluids with regular insulin*** - The patient presents with classic signs of **diabetic ketoacidosis (DKA)**: hyperglycemia (blood sugar 500 mg/dl), ketone bodies, dehydration, and a history of diabetes non-compliance [1]. - Initial management for DKA involves aggressive **intravenous fluid resuscitation** to correct dehydration and then **intravenous regular insulin** to lower blood glucose and resolve ketosis [2]. *Intravenous fluids with long-acting insulin* - While fluids are essential, **long-acting insulin** is not appropriate for the acute management of DKA because its slow onset of action makes it inefficient for rapidly correcting hyperglycemia and ketosis. - **Regular insulin** is preferred as it has a quicker onset and shorter duration, allowing for more precise titration in an acute setting [2]. *Intravenous fluids* - Although crucial for correcting **dehydration** and improving renal perfusion, fluids alone will not address the underlying **insulin deficiency** and **ketosis** that define DKA. - Without insulin, the body will continue to produce ketones, exacerbating acidosis [3]. *Intravenous insulin* - Giving intravenous insulin without prior or concomitant **fluid resuscitation** can be dangerous, as it can worsen **hypovolemia** and potentially lead to circulatory collapse by shifting glucose and potassium into cells. - It is critical to first restore **circulating volume** before initiating insulin therapy [2].
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