15. Metabolic acidosis is associated with decreased plasma level of
(A) bicarbonate
(B) lactate
(C) oxygen
(D) urea
Metabolic Acidosis Is Associated with Decreased Plasma Bicarbonate
Introduction
Maintenance of acid-base balance is essential for normal cellular metabolism and physiological homeostasis. The human body continuously produces acids through normal metabolic activities, and these acids must be neutralized or eliminated to maintain blood pH within the narrow physiological range of approximately 7.35–7.45. Even small deviations from this range can impair enzyme activity, alter membrane transport, and disrupt the normal functioning of multiple organ systems.
The bicarbonate buffer system is the most important extracellular buffering mechanism that maintains blood pH. This system works together with the lungs and kidneys to regulate hydrogen ion concentration. Disturbances in acid-base balance are broadly classified into metabolic and respiratory disorders. Metabolic acidosis is characterized by a primary decrease in plasma bicarbonate concentration, leading to a reduction in blood pH.
Correct Answer
Correct Option: (A) Bicarbonate
Detailed Explanation
Metabolic acidosis is an acid-base disorder in which the primary abnormality is a decrease in plasma bicarbonate (HCO₃⁻). Bicarbonate is consumed while buffering excess hydrogen ions produced during metabolic processes or retained because of impaired renal acid excretion. As bicarbonate concentration falls, the buffering capacity of blood decreases, resulting in a reduction in blood pH.
The bicarbonate buffer system can be represented by the following reversible equation:
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
During metabolic acidosis, excess hydrogen ions combine with bicarbonate ions to form carbonic acid, which is subsequently converted into carbon dioxide and water. This buffering reaction consumes bicarbonate, causing its plasma concentration to decrease. The lungs compensate by increasing ventilation (hyperventilation), thereby removing carbon dioxide and partially restoring blood pH. The kidneys also contribute by increasing hydrogen ion secretion and generating new bicarbonate molecules.
Common causes of metabolic acidosis include diabetic ketoacidosis, lactic acidosis, chronic kidney disease, severe diarrhea, poisoning by methanol or ethylene glycol, and renal tubular acidosis.
Explanation of Each Option
Option (A): Bicarbonate
This statement is correct. A decrease in plasma bicarbonate is the defining biochemical feature of metabolic acidosis. Bicarbonate is consumed while buffering excess acids, leading to reduced blood pH.
Option (B): Lactate
This statement is incorrect. Lactate usually increases in conditions such as tissue hypoxia, septic shock, and strenuous exercise. Elevated lactate is a cause of lactic acidosis rather than a decreased plasma component.
Option (C): Oxygen
This statement is incorrect. Plasma oxygen concentration is not the defining feature of metabolic acidosis. Although oxygen may decrease in some disease conditions, metabolic acidosis is diagnosed primarily by decreased bicarbonate and reduced blood pH.
Option (D): Urea
This statement is incorrect. Plasma urea concentration does not decrease as a characteristic feature of metabolic acidosis. In chronic kidney disease, metabolic acidosis is often accompanied by an increase in blood urea due to reduced renal excretory function.
Why Option (A) is Correct
The hallmark of metabolic acidosis is a primary reduction in plasma bicarbonate concentration. As bicarbonate buffers excess hydrogen ions, its concentration decreases, causing blood pH to fall. Therefore, measurement of plasma bicarbonate is one of the most important laboratory parameters for diagnosing metabolic acidosis.
Why the Other Options are Incorrect
Why Option (B) is Incorrect
Lactate generally increases rather than decreases in lactic acidosis and other conditions associated with anaerobic metabolism.
Why Option (C) is Incorrect
Oxygen concentration is not the primary abnormality used to diagnose metabolic acidosis.
Why Option (D) is Incorrect
Urea concentration is unrelated to the primary acid-base disturbance and may even increase in renal disorders causing metabolic acidosis.
Comparison of All Options
| Option | Parameter | Change in Metabolic Acidosis | Correct or Incorrect |
|---|---|---|---|
| A | Bicarbonate | Decreases | Correct |
| B | Lactate | Usually Increases | Incorrect |
| C | Oxygen | No characteristic decrease | Incorrect |
| D | Urea | No characteristic decrease | Incorrect |
Bicarbonate Buffer System
| Component | Function |
|---|---|
| Carbon Dioxide (CO₂) | Respiratory component regulated by lungs |
| Carbonic Acid (H₂CO₃) | Weak acid formed from CO₂ and water |
| Bicarbonate (HCO₃⁻) | Major extracellular buffer |
| Hydrogen Ion (H⁺) | Determines blood acidity |
Causes of Metabolic Acidosis
| Cause | Mechanism |
|---|---|
| Diabetic Ketoacidosis | Accumulation of ketone bodies |
| Lactic Acidosis | Accumulation of lactic acid |
| Severe Diarrhea | Loss of bicarbonate from the intestine |
| Chronic Kidney Disease | Reduced acid excretion and bicarbonate regeneration |
| Renal Tubular Acidosis | Defective bicarbonate handling or acid secretion |
Compensatory Responses in Metabolic Acidosis
| Organ | Compensation |
|---|---|
| Lungs | Hyperventilation reduces arterial CO₂ |
| Kidneys | Increase hydrogen ion secretion and regenerate bicarbonate |
| Buffers | Neutralize excess hydrogen ions |
Biological Significance of Plasma Bicarbonate
Plasma bicarbonate is the principal extracellular buffer responsible for maintaining blood pH within a narrow physiological range. It protects cells from harmful fluctuations in hydrogen ion concentration and ensures normal enzyme activity, protein function, nerve conduction, and muscle contraction. Alterations in bicarbonate concentration are among the earliest indicators of metabolic acid-base disorders and are routinely measured during arterial blood gas analysis.
Final Answer
Correct Option: (A) Bicarbonate
Metabolic acidosis is defined by a primary decrease in plasma bicarbonate (HCO₃⁻). As bicarbonate buffers excess hydrogen ions, its concentration falls, leading to a decrease in blood pH. Therefore, reduced plasma bicarbonate is the characteristic biochemical feature of metabolic acidosis.
