Concentration of oxygen in expired air is about?

Appeared in: BHU NO-2019

Explanation

Inspired air, the air we breathe in, contains approximately 21% oxygen.
During gas exchange in the alveoli, oxygen moves from the lungs into the bloodstream to be used by the body's tissues.
The body's metabolic processes consume only a fraction of the inhaled oxygen (about 4-5%) per breath at rest.
Consequently, the air that is exhaled (expired air) still contains a significant amount of oxygen, which is approximately 16%.
Simultaneously, expired air becomes enriched with carbon dioxide (around 4%), a waste product of metabolism.

Option B: This is incorrect. The body's respiratory system is not 100% efficient at extracting oxygen. A significant portion of inhaled oxygen is exhaled with each breath.
Option C: This value is too low for the amount of oxygen in expired air under normal resting conditions. It would imply an oxygen consumption of 9%, which is much higher than the typical 4-5%.
Option D: This value is too low. An oxygen level of 10% in expired air would mean the body is extracting 11% of the oxygen from the atmosphere with each breath, which is an extremely high and unsustainable rate of oxygen consumption at rest.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Gas Exchange and Composition of Respired Air as background academic context rather than a clinical decision trigger.
The 16% oxygen concentration in expired air is the scientific basis for the effectiveness of rescue breathing (mouth-to-mouth resuscitation) in CPR.
A rescuer's exhaled breath provides enough oxygen to maintain vital organ function in a non-breathing victim until more advanced medical help arrives.
This principle underscores that even 'used' air from a rescuer is far better than no air for a victim of respiratory arrest.

How to Approach the Question

This is a factual recall question based on fundamental respiratory physiology.
First, recall the composition of the air we breathe (inspired air), which is approximately 21% oxygen.
Next, consider the purpose of breathing: to supply oxygen to the body. The body uses some, but not all, of this oxygen in one cycle.
Therefore, the expired air must contain less than 21% oxygen but significantly more than 0%. This helps eliminate the 0% option immediately.
Recall the standard physiological value for oxygen in expired air, which is around 16%. This corresponds to the body utilizing about 4-5% of the inhaled oxygen at rest.
Select the option that is closest to this known physiological fact.

Concept Tested & Keywords

QjpbLCC_4b4SKJQezngNcQ

E6 Nursing Fundamentals Taylor p. 1120-1122

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

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