Respiratory System Gas Exchange: TEAS Practice Question with Answer & Explanation

Understanding Oxygen Diffusion in the Bloodstream

To answer this question correctly, you must understand the factors that govern gas exchange in the respiratory system. Oxygen enters the blood through a process called passive diffusion, which occurs at the alveolar-capillary membrane. Anything that hinders this process or increases the demand beyond the body's supply will lead to lower blood oxygen levels.

Why Choice A is Correct

An increase in the thickness of the alveolar-capillary membrane is the correct answer. According to Fick's Law of Diffusion, the rate of gas transfer is inversely proportional to the thickness of the membrane.

• In a healthy lung, the "blood-air barrier" is extremely thin to allow oxygen to pass easily into the capillaries.
• If this membrane thickens—due to conditions like pulmonary fibrosis, inflammation, or fluid buildup (edema)—oxygen molecules have a much harder time crossing into the blood. This directly results in lower arterial oxygen levels (hypoxemia).

Why the Other Options are Incorrect

• B: An increase in the surface area of the lungs. This would actually increase the amount of oxygen in the blood. More surface area means more "doorways" for oxygen to enter the bloodstream. This is why some athletes train to improve lung efficiency.
• C: An increase in the partial pressure of oxygen. If you increase the amount of oxygen in the air you breathe (like using an oxygen mask), the pressure gradient becomes steeper, forcing more oxygen into the blood.
• D: An increase in the total number of red blood cells. Red blood cells contain hemoglobin, which carries oxygen. Increasing the "transport trucks" (RBCs) generally increases the blood's oxygen-carrying capacity, not decreases it.

TEAS Concept: Factors Affecting Gas Exchange

On the TEAS, you are expected to know that gas exchange depends on:

1. Pressure Gradients: Gas moves from high pressure to low pressure.
2. Surface Area: More area = more diffusion.
3. Membrane Thickness: Thinner = faster diffusion; Thicker = slower diffusion.
4. Blood Flow and Ventilation: The lungs must be well-ventilated and the capillaries must be well-perfused.

Nursing Student Tip

When you see a question about "decreased oxygen," think of the "barrier." If the wall gets thicker or the surface area gets smaller (like in emphysema), the blood loses out on oxygen!

Memory Aid

Think of diffusion like people trying to get into a concert.

• Surface Area: More open doors (faster entry).
• Membrane Thickness: A very long, narrow hallway (slower entry). If the hallway gets longer/thicker, fewer people make it to the stage!