Urea concentration gradients in liver and kidney blood vessels Steps: - Urea enters blood from protein metabolism and is highest in vessels delivering unprocessed blood to detoxifying organs. - Renal artery (likely 1) carries systemic blood with moderate-high urea to kidneys for filtration. - Renal vein (likely 2) exits kidneys with reduced urea after glomerular filtration. - Hepatic portal vein (3) delivers gut-absorbed blood with highest urea to liver for processing. - Hepatic vein (4) carries blood from liver with lowest urea after hepatic metabolism. Why B is correct: - Hepatic portal vein (3) has the highest urea from intestinal protein breakdown, while hepatic vein (4) has the lowest after liver urea conversion to ammonia and excretion support. Why the others are wrong: - A: Renal artery (1) has high but not peak urea; renal vein (2) reduces urea but not to the lowest level. - C: Renal artery (1) is not the highest; hepatic vein (4) is lowest, but pairing mismatches extremes. - D: Hepatic portal vein (3) is highest, but renal vein (2) is not the lowest compared …