The most superior portion of the pharynx is the:

A. The external exchange of gases refers to the process of oxygen diffusing from the alveoli into the blood and carbon dioxide diffusing from the blood into the alveoli in the lungs. This occurs during pulmonary ventilation.
B. Pulmonary ventilation involves the movement of air in and out of the lungs (breathing), not the exchange of gases.
C. The internal exchange of gases refers to the diffusion of oxygen from the blood to the tissues and carbon dioxide from the tissues to the blood. This happens at the cellular level throughout the body.
D. This answer is incorrect because internal exchange of gases is the correct phase for oxygen to diffuse out of the blood.

A. A small amount of carbon dioxide is transported dissolved in plasma, but this accounts for only about 5–10% of the total CO₂ in the blood.
B. Some carbon dioxide binds to hemoglobin to form carbaminohemoglobin, but this form carries only about 20–30% of CO₂.
C. Albumin primarily transports hormones, fatty acids, and other substances—not carbon dioxide.
D. The majority of carbon dioxide (about 60–70%) is carried in the blood as bicarbonate ions (HCO₃⁻). This occurs through a reaction where CO₂ combines with water to form carbonic acid, which then dissociates into bicarbonate and hydrogen ions.