Urea Removal by Kidneys Determines Concentrations

Steps:

• Urea is a nitrogenous waste produced by the liver and circulated in blood.
• Kidneys filter urea from blood, so incoming renal artery has high concentration.
• Outgoing renal vein has low urea after glomerular filtration.
• Liver vessels (hepatic artery/vein/portal) have similar urea levels, as liver produces but does not remove it.
• Diagram labels: 3 as renal artery (highest urea), 4 as renal vein (lowest urea).

Why B is correct:

• Renal artery (3) delivers systemic blood with accumulated urea for filtration; renal vein (4) returns filtered blood with reduced urea, per kidney excretion function.

Why the others are wrong:

• A: Vessel 1 (likely hepatic portal) has moderate urea, not highest; 2 (hepatic vein) similar, not lowest.
• C: Vessel 1 moderate, not highest; 4 is lowest but pairs wrong.
• D: Vessel 3 highest correct, but 2 (hepatic) not lowest—renal vein is.

Final answer: B