The correct option is 4 mL.
Explanation
The transport of respiratory gases involves the movement of oxygen ($O_2$) and carbon dioxide ($CO_2$) between the alveoli, blood, and tissues. This process is driven by partial pressure gradients and the solubility of gases in the blood.
Detailed Analysis:
- CO2 Transport: Carbon dioxide is transported in the blood in three primary forms: dissolved in plasma (approx. 7%), bound to haemoglobin as carbamino-haemoglobin (approx. 20-25%), and as bicarbonate ions (approx. 70%).
- Exchange at Alveoli: Deoxygenated blood reaching the pulmonary capillaries has a higher partial pressure of carbon dioxide ($pCO_2 \approx 45$ mm Hg) compared to the alveoli ($pCO_2 \approx 40$ mm Hg). This gradient facilitates the diffusion of $CO_2$ from the blood into the alveoli.
- Quantitative Value: Under normal physiological conditions, every 100 mL of deoxygenated blood delivers approximately 4 mL of carbon dioxide to the alveoli to be exhaled.
Comparative Note:
In contrast, every 100 mL of oxygenated blood delivers approximately 5 mL of oxygen to the tissues.
Key Takeaway:
The standard physiological value for carbon dioxide delivery is 4 mL of $CO_2$ per 100 mL of deoxygenated blood.