Mucous membranes prevent most pathogens from entering the body because of the stickiness of the mucus and the presence of lysozymes.

The liver plays a central role in iron metabolism by storing excess iron in the protein ferritin. Ferritin safely binds iron, preventing it from catalyzing harmful free radical reactions, and releases it when the body needs it for processes such as hemoglobin synthesis. This storage mechanism helps maintain iron homeostasis and prevents iron deficiency or overload.

A. Carbaminohemoglobin:About 20–30% of CO₂ binds to hemoglobin forming carbaminohemoglobin. While this contributes to CO₂ transport, it is significantly less than the amount carried as bicarbonate ions.

B. Bicarbonate ions:Most CO₂ (around 70%) in the blood is converted to bicarbonate ions (HCO₃⁻) via carbonic anhydrase in red blood cells. This allows efficient transport from tissues to lungs and facilitates rapid CO₂ release during exhalation.

C. bisphosphocarbonate:This is not a recognized form of CO₂ transport in blood. It does not play a role in carrying CO₂ from tissues to the lungs and is not physiologically relevant.

D. Carboxyhemoglobin:Carboxyhemoglobin forms when carbon monoxide binds to hemoglobin, not CO₂. It does not participate in normal CO₂ transport and is mainly a concern in CO poisoning.

E. Dissolved CO₂ gas:Only a small fraction (about 7–10%) of CO₂ is dissolved directly in plasma. This amount is minimal compared to the majority transported as bicarbonate ions.