Question

The partial pressure of carbon dioxide ($ppCO_2$) of blood entering the pulmonary capillaries is ________ .

• A. 45 mm of Hg
• B. 65 mm of Hg
• C. 85 mm of Hg
• D. 104 mm of Hg

Hint:

Memory: Blood entering lungs → $CO_2 = 45$, leaving lungs → $CO_2 = 40$ mm Hg.

Answer 1

The Correct Option is A

Concept:
Gas exchange in lungs depends on partial pressures of gases. Deoxygenated blood entering pulmonary capillaries has higher $CO_2$ and lower $O_2$.

Step 1: Recall standard values.
• $ppCO_2$ in deoxygenated blood (entering lungs) $\approx 45$ mm Hg
• $ppCO_2$ in alveoli $\approx 40$ mm Hg

Step 2: Understand gradient. $CO_2$ diffuses from blood (45 mm Hg) to alveoli (40 mm Hg) due to pressure gradient.

Step 3: Conclusion. \[ \text{$ppCO_2$ of incoming blood = 45 mm Hg} \]