Renal vein blood reflects kidney filtration and metabolism

Steps:

• Renal artery delivers unfiltered blood to kidneys with baseline urea and CO2 levels.
• Kidneys filter blood, removing urea via glomerular filtration and excretion into urine.
• Kidney tissues respire, adding CO2 to blood as a metabolic waste product.
• Result: renal vein blood exits with elevated CO2 but reduced urea compared to renal artery.

Why B is correct:

• Kidney function involves urea removal through filtration (per nephron physiology), lowering its concentration, while cellular respiration produces CO2, raising it in venous blood.

Why the others are wrong:

• A: Urea concentration decreases due to filtration, not increases.
• C: CO2 increases from tissue metabolism; urea decreases, not increases.
• D: CO2 increases and urea decreases, so neither is lower.

Final answer: B