The main elements of the phosphate buffer system are:

RRB Nsg. Superintendent - 29 April 2025 (Shift-3rd)

Applied Physiology

Easy

Appeared in: RRB Nsg. Superintendent - 29 April 2025 (Shift-3rd)

Explanation

The phosphate buffer system is composed of a weak acid and its conjugate base.
Sodium dihydrogen phosphate (NaH₂PO₄) acts as the weak acid (proton donor).
Disodium hydrogen phosphate (Na₂HPO₄) acts as the weak base or salt (proton acceptor).
This pair works together to resist changes in pH, particularly within cells and in the renal tubules.

Option A: This option incorrectly combines a neutral salt (NaCl) with a component of the hemoglobin buffer system (HHb, reduced hemoglobin). Neither is a primary element of the phosphate buffer system.
Option B: While NaH₂PO₄ is the correct weak acid component, NaCl (sodium chloride) is a neutral salt and not part of the buffer pair. It is a byproduct when the buffer neutralizes a strong acid like HCl.
Option D: This option incorrectly mixes a component from the hemoglobin buffer system (HHb) with a component of the phosphate buffer system (Na₂HPO₄, though written as NaHPO₄). A buffer system consists of a conjugate acid-base pair from the same chemical family.

Visual 1: Diagram illustrating the chemical equilibrium of the phosphate buffer system, showing the reversible conversion between H₂PO₄⁻ and HPO₄²⁻ as it accepts or donates H⁺ ions.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Physiology of the phosphate buffer system as background academic context rather than a clinical decision trigger.
The phosphate buffer system is critical for renal function. It allows the kidneys to excrete excess hydrogen ions (acids) in the urine, which is essential for preventing metabolic acidosis.
Inside cells, where phosphate levels are high, this system is a primary defense against pH shifts that could otherwise damage enzymes and disrupt metabolic processes.
What if? If a patient has chronic kidney disease, their ability to excrete acid via the phosphate buffer in urine is impaired. This contributes to the development of chronic metabolic acidosis, a common complication of renal failure.

How to Approach the Question

First, identify the core concept of the question, which is the 'phosphate buffer system'.
Recall the definition of a chemical buffer: a mixture of a weak acid and its conjugate base (or salt).
Analyze each option to see which one represents a weak acid and its corresponding conjugate base from the phosphate family.
Eliminate options that contain components from other buffer systems (e.g., HHb for hemoglobin) or substances that are not part of a buffer pair (e.g., NaCl).
Select the option that correctly pairs the weak acid (NaH₂PO₄) and the weak base (Na₂HPO₄).

Concept Tested & Keywords

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