Under normal circumstances, concentration of oxygen in inspired air is about percent?

Appeared in: AIIMS Patna NO - 2020

Explanation

Under normal conditions, the air we breathe is atmospheric air.
The atmosphere consists of approximately 20.93% oxygen, which is rounded to 21% for clinical and general purposes.
This percentage is essential for normal gas exchange and cellular respiration in the human body.
The remaining primary component of air is nitrogen (about 78%), with trace amounts of other gases.

Option A: 100% oxygen is not found in normal inspired air. It is a therapeutic concentration administered for specific medical conditions.
Option B: An oxygen concentration of 6% is extremely low and incompatible with sustaining human life, as it would cause severe hypoxia.
Option C: 29% is an incorrect value. The concentration of oxygen in the atmosphere is stable and does not reach this level under normal circumstances.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Composition of Inspired Air as background academic context rather than a clinical decision trigger.
Understanding that room air is 21% oxygen is fundamental in nursing, especially in respiratory care. This value is the baseline known as the Fraction of Inspired Oxygen (FiO₂) of 0.21.
When a patient requires supplemental oxygen, the nurse administers it to increase the FiO₂ above 0.21 to improve blood oxygen levels. The amount of supplemental oxygen is titrated based on the patient's needs, often monitored by pulse oximetry.
What if a patient on room air has an oxygen saturation of 88%? The nurse's first action would be to apply supplemental oxygen to increase the FiO₂ above 0.21, aiming to raise the saturation to a target range (e.g., above 92%), while also assessing the underlying cause of the hypoxia.

How to Approach the Question

This is a factual recall question testing fundamental knowledge in respiratory physiology.
First, identify the core of the question: 'concentration of oxygen in inspired air'.
Recall the basic composition of the air we breathe under 'normal circumstances' (i.e., room air).
Access your knowledge about the primary gases in the atmosphere: nitrogen is the most abundant, followed by oxygen.
Compare the known percentage of oxygen (~21%) with the given options.
Select the option that matches this fundamental scientific fact.

Concept Tested & Keywords

QCnNKCh85FDr2RrZoGV6Om

E6 Nelson Textbook of Pediatrics(2024) — Volume 2 p. 298-300

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