RRB Nsg. Superintende…: Osmotic pressure and acid-base balance of body fluids are maintained by?

RRB Nsg. Superintendent 21 July 2019 (shift 1st)

Applied Physiology

Easy

Appeared in: RRB Nsg. Superintendent 21 July 2019 (shift 1st)

Explanation

Sodium is the most abundant cation in the extracellular fluid (ECF) and is the primary determinant of ECF osmolality, which governs osmotic pressure.
The principle of 'water follows salt' means that water movement between fluid compartments is dictated by the sodium gradient, directly impacting fluid volume and pressure.
In the kidneys, sodium is reabsorbed in exchange for hydrogen ions (H⁺), a key mechanism for regulating blood pH and maintaining acid-base balance.
Sodium and its associated anions (like chloride) account for approximately 90% of the plasma osmolality.

Option B: Magnesium is primarily an intracellular cation and has a much smaller effect on overall osmotic pressure compared to sodium.
Option C: Fluorine is a trace element, not a major electrolyte, and its primary role is in bone and dental health.
Option D: Zinc is a trace element essential for immune function and wound healing, but it does not play a primary role in regulating overall osmotic pressure or acid-base balance.

Visual 1: Diagram illustrating the role of the Sodium-Potassium (Na⁺/K⁺) pump in maintaining electrolyte gradients across the cell membrane.
Visual 2: Infographic showing how the kidneys regulate acid-base balance through ion exchange, highlighting the Na⁺/H⁺ exchanger in the renal tubules.
Visual 3: Chart comparing the electrolyte composition of intracellular fluid (ICF) and extracellular fluid (ECF), showing sodium's dominance in the ECF.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Role of Electrolytes in Homeostasis as background academic context rather than a clinical decision trigger.
Nurses constantly monitor serum sodium levels as they are a critical indicator of a patient's fluid and electrolyte status. Abnormal levels (hyponatremia or hypernatremia) can lead to serious neurological complications like cerebral edema or seizures.
Understanding sodium's role is vital when administering IV fluids. For example, isotonic saline (0.9% NaCl) is used to expand ECF volume without causing major fluid shifts because its osmolality is similar to that of plasma.
What if? If a patient develops severe diarrhea, they lose significant amounts of sodium and water. This leads to ECF volume depletion (hypovolemia) and can cause hypotension and tachycardia. The nursing priority would be fluid resuscitation, often with sodium-containing solutions, to restore volume and prevent shock.

How to Approach the Question

First, identify the key physiological processes mentioned in the question: 'osmotic pressure' and 'acid-base balance'.
Recall the major electrolytes and their primary functions in the body.
Consider the location of each electrolyte (intracellular vs. extracellular). Osmotic pressure of the extracellular fluid (ECF) is mainly determined by the most abundant extracellular solute.
Evaluate each option: Sodium is the most abundant extracellular cation. Magnesium is mostly intracellular. Fluorine and Zinc are trace elements with different primary roles.
Connect the roles. Sodium's high concentration in the ECF makes it the main driver of osmotic pressure. Its exchange for H⁺ in the kidneys links it directly to acid-base balance, making it the best fit.

Concept Tested & Keywords

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Guyton & Hall Textbook of Medical Physiology 13e — Part 1 p. 387-389