Blood picks up CO2 from tissues

Steps:

• Blood flows from arteries to capillaries in body tissues (point 1).
• Cells in tissues perform respiration, consuming O2 and glucose while producing CO2.
• CO2 diffuses from cells into capillary blood, raising its concentration.
• Blood then flows to veins (point 2) with higher CO2 levels.

Why B is correct:

• Carbon dioxide is a waste product of cellular respiration that diffuses into blood per Fick's law of diffusion, increasing its concentration in systemic capillaries.

Why the others are wrong:

• A. Amino acids are transported to tissues for use or storage, not produced there.
• C. Glucose is metabolized by cells, decreasing its blood concentration.
• D. Oxygen is extracted by tissues for respiration, lowering its concentration.

Final answer: B