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 Flashcard 1 of 5

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 5

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

Answer: hypo

Acid-Base Balance Flashcard 3 of 5

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

Answer: phosphate

Acid-Base Balance Flashcard 4 of 5

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 5

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

Answer: acidic