Of the following blood vessels, which pair have the highest and lowest urea concentration in the blood?
A. (highest) hepatic portal vein (lowest) hepatic vein
B. (highest) renal artery (lowest) renal vein
C. (highest) hepatic vein (lowest) renal vein
D. (highest) hepatic artery (lowest) hepatic vein
答案是C,但為甚麼不是B?
renal artery 和 hepatic vein 的 urea concentration 不是應該一樣嗎?