Acid-Base Balance

Acid-Base Balance Indian Medical PG 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

Acid-Base Balance 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.

Acid-Base Balance Indian Medical PG Question 2: All of the following substances have decreased concentration on the luminal side of the proximal convoluted tubule except:

• A. Glucose
• B. Amino acids
• C. Bicarbonate
• D. Chloride (Correct Answer)

Acid-Base Balance Explanation: ***Chloride*** - As **water and other solutes** are reabsorbed from the proximal tubule, the concentration of **chloride** actually increases in the remaining luminal fluid due to continued water reabsorption. - This increased luminal **chloride concentration** then drives passive reabsorption of chloride later in the tubule. *Glucose* - **Glucose** is almost completely reabsorbed from the tubular lumen by **secondary active transport** in the early part of the proximal tubule. - Therefore, its concentration in the remaining luminal fluid rapidly decreases. *Amino acids* - Similar to glucose, **amino acids** are extensively reabsorbed by **secondary active transport** mechanisms in the proximal tubule. - Consequently, their luminal concentration significantly decreases. *Bicarbonate* - Most **bicarbonate** is reabsorbed in the proximal tubule through a process involving **carbonic anhydrase**, converting it to CO2 and water, which then diffuse into the cell. - This efficient reabsorption results in a substantial decrease in luminal bicarbonate concentration.

Acid-Base Balance Indian Medical PG Question 3: A 55 year old male presents with tachypnea and mental confusion. Blood glucose 350 mg/dl, pH = 7.0. What is the most likely acid base disorder?

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

Acid-Base Balance Explanation: Metabolic acidosis - A **pH of 7.0** indicates significant acidemia, and **hyperglycemia (350 mg/dL)** in conjunction with clinical symptoms (tachypnea, mental confusion) strongly suggests **diabetic ketoacidosis (DKA)**, a common cause of high anion gap metabolic acidosis [1]. - Tachypnea is often a **compensatory mechanism** (Kussmaul breathing) to blow off carbon dioxide and raise pH in metabolic acidosis [1], [2]. Metabolic alkalosis - This would present with an **elevated pH (alkalemia)**, which is opposite to the patient's measured pH of 7.0 [2]. - It is typically caused by conditions like severe vomiting or diuretic use, which are not suggested by the clinical presentation [3]. Respiratory alkalosis - This condition involves a **high pH** and a **low PCO2**, often due to hyperventilation [2]. - While the patient is tachypneic, the profound acidemia (pH 7.0) contradicts a primary respiratory alkalosis. Respiratory acidosis - While leading to a low pH, respiratory acidosis is characterized by **elevated PCO2** due to hypoventilation. - The patient's **tachypnea** indicates hyperventilation, which would tend to lower PCO2, making primary respiratory acidosis unlikely.

Acid-Base Balance Indian Medical PG 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

Acid-Base Balance 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.

Acid-Base Balance Indian Medical PG Question 5: Renal tubular acidosis with ABG value pH = 7.24 PO2=80; PaCO2= 36 Na = 131; HCO3 = 14 Cl= 90; BE = -13 Glucose = 135 the above ABG picture suggests –

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

Acid-Base Balance Explanation: The ABG shows a pH of 7.24, indicating **acidemia** [1]. The HCO3 is 14 mEq/L, which is significantly **low**, and the base excess (BE) is -13 [1]. The PaCO2 of 36 mmHg is within the normal range, indicating no significant primary respiratory derangement [2]. The overall picture is consistent with an uncompensated or partially compensated **metabolic acidosis** [1][2]. ***Metabolic acidosis*** - The **low pH (acidemia)**, **low bicarbonate (HCO3)**, and **negative base excess (BE)** are direct indicators of metabolic acidosis [1]. - The **PaCO2 within normal limits** or slightly decreased suggests either no respiratory compensation or insufficient compensation for the metabolic derangement [1][2]. *Respiratory acidosis* - This would present with a **low pH** and an **elevated PaCO2** as the primary defect, which is not seen here (PaCO2 is normal) [1]. - Bicarbonate would typically be normal or elevated if compensated, not significantly decreased. *Respiratory alkalosis* - This would be characterized by an **elevated pH** and a **low PaCO2**, which is the opposite of the findings in this ABG [1]. - HCO3 would be normal or low if compensated. *Metabolic alkalosis* - This would present with an **elevated pH** and an **elevated HCO3**, which contradicts the given ABG values (low pH and low HCO3) [2].

Acid-Base Balance Indian Medical PG Question 6: 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)

Acid-Base Balance 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.

Acid-Base Balance Indian Medical PG Question 7: Central chemoreceptors are most sensitive to which of the following changes in blood?

• A. PO2
• B. HCO3-
• C. pH
• D. PCO2 (Correct Answer)

Acid-Base Balance Explanation: ***PCO2*** - Central chemoreceptors, located in the **medulla oblongata**, are exquisitely sensitive to changes in the **partial pressure of carbon dioxide (PCO2)** in the arterial blood. - An increase in blood PCO2 readily crosses the **blood-brain barrier** to the cerebrospinal fluid (CSF), where it is converted to carbonic acid and then to H+ and HCO3-. The resulting **drop in CSF pH** directly stimulates these chemoreceptors, leading to increased ventilation. *PO2* - While **peripheral chemoreceptors** (carotid and aortic bodies) are sensitive to changes in **PO2**, particularly when it drops significantly (below 60 mmHg), central chemoreceptors are not. - The primary role of central chemoreceptors is to monitor and respond to changes in CO2 and pH, rather than oxygen levels. *pH* - Central chemoreceptors are indirectly sensitive to **pH changes** in the cerebrospinal fluid (CSF), which result from blood PCO2 changes. - However, they are not directly or primarily sensitive to changes in **blood pH** because hydrogen ions do not readily cross the blood-brain barrier. *HCO3-* - Bicarbonate ions (**HCO3-**) are important in buffering pH, but central chemoreceptors do not directly sense bicarbonate levels. - Changes in HCO3- indirectly affect pH, and it is the resultant **H+ concentration** in the CSF, derived from CO2, that primarily stimulates central chemoreceptors.

Acid-Base Balance Indian Medical PG 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

Acid-Base Balance 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.

Acid-Base Balance Indian Medical PG Question 9: 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

Acid-Base Balance 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.

Acid-Base Balance Indian Medical PG Question 10: The lab reports of a patient given below: pH = 7.2, HCO3 = 10 mEq/L, PCO2 = 30 mmHg. This exemplifies which of the following disorders?

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

Acid-Base Balance Explanation: ***Metabolic acidosis*** - The pH of 7.2 is acidic, and the **bicarbonate (HCO3) of 10 mEq/L** is significantly low (normal: 22-28 mEq/L), indicating a primary metabolic disturbance causing acidosis. - The **PCO2 of 30 mmHg** is also low (normal: 35-45 mmHg), which represents **partial respiratory compensation** through hyperventilation to blow off CO2 and raise pH. - This is a classic example of **metabolic acidosis with respiratory compensation**. *Metabolic alkalosis* - This condition would be characterized by a **high pH** and a **high bicarbonate (HCO3)** level, which is the opposite of the given values. - The body would attempt to compensate by increasing PCO2 through hypoventilation. *Respiratory acidosis* - This would present with a **low pH** and a **high PCO2** (>45 mmHg), indicating a primary respiratory problem leading to CO2 retention and acid accumulation. - Metabolic compensation would show elevated HCO3, not the low HCO3 (10 mEq/L) seen here. *Respiratory alkalosis* - This condition is characterized by a **high pH** (>7.45) and a **low PCO2**, due to excessive ventilation causing CO2 elimination. - While PCO2 is low in the given scenario, the pH is acidic (7.2), not alkalotic, ruling out this diagnosis.

Acid-Base Balance Flashcard 1 of 10

Question: The primary disturbance in metabolic alkalosis is a(n) _____ in HCO3- OR a decrease in H+

Answer: increase

Acid-Base Balance Flashcard 2 of 10

Question: Increased arterial pH (metabolic alkalosis) causes compensatory _____-ventilation with consequent increased PCO2

Answer: hypo

Acid-Base Balance Flashcard 3 of 10

Question: Un-reabsorbed _____ (15%) serves as a urinary buffer for H+

Answer: phosphate

Acid-Base Balance Flashcard 4 of 10

Question: If the pH is reduced from 7.4 to 7.3 with the alveolar PO2 being constant at 100 mmHg, PCO2 would follow line _____.

Answer: 1

Acid-Base Balance Flashcard 5 of 10

Question: At _____ urine pH, the HA form of a weak acid exists, thus decreasing excretion

Answer: acidic

Acid-Base Balance Flashcard 6 of 10

Question: The arterial blood profile seen in compensated metabolic acidosis is: pH: _____[HCO3-]: decreasedPCO2: decreased

Answer: decreased

Acid-Base Balance Flashcard 7 of 10

Question: The arterial blood profile seen in compensated respiratory alkalosis is: pH: _____[HCO3-]: decreasedPCO2: decreased

Answer: increased

Acid-Base Balance Flashcard 8 of 10

Question: Renal correction of respiratory acidosis includes increased excretion of excess fixed H+ as _____ and NH4+

Answer: titratable acid

Acid-Base Balance Flashcard 9 of 10

Question: Decreased arterial pH (metabolic acidosis) causes compensatory _____-ventilation with consequent decreased PCO2

Answer: hyper

Acid-Base Balance Flashcard 10 of 10

Question: The primary disturbance in metabolic acidosis is a _____ in HCO3-

Answer: decrease