Most of the carbon dioxide transported in the blood is carried:

A. Active transport requires energy and is used to move substances against their concentration gradient. Oxygen moves along its concentration gradient, so active transport is not involved.
B. Diffusion is the correct mechanism. Oxygen moves from an area of higher concentration in the alveoli to a lower concentration in the pulmonary capillaries through the alveolar-capillary membrane. This passive process does not require energy.
C. Endocytosis is a process cells use to engulf large particles or substances, not how gases like oxygen are transported.
D. Pinocytosis is a form of endocytosis specifically for fluid uptake and is not involved in the respiratory gas exchange process

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.