Question

In mammals, which blood vessel would normally carry the largest amount of urea?
A. Renal vein
B. Dorsal Aorta
C. Hepatic Vein
D. Hepatic Portal Vein

Answer 1

Terrestrial adaptation necessitated the production of lesser toxic nitrogenous wastes like urea and uric acid for conservation of water. Urea can be tolerated in much more concentrated form because it is 100,000 times less toxic than ammonia Mammals, many terrestrial amphibians and marine fishes mainly excrete urea and are called ureotelic Animals.

Complete answer: Excretion of urea by animals is known as ureotelism. Ammonia is produced by metabolism and it is converted into urea in the liver of animals and then gets released into the blood which is filtered and excreted out by the kidneys. Some amount of urea may be retained in the kidney matrix of some of these animals to maintain a desired osmolarity. Most urea is produced inside the liver so the maximum amount of urea will be carried.
Now let us match this with given options:
Renal vein: renal vein is the tubular part present in the kidney. It is used to reabsorb the filtrate formed by the kidney. 180 litres of filtrate is formed per day. Substances like glucose are absorbed actively by this segment.
Dorsal Aorta: the aorta carries oxygenated blood to heart muscles and returns the deoxygenated blood from heart muscles to the right atrium. The ascending aorta supplies oxygenated blood to heart muscles. It has no role with urea circulation.
Hepatic vein: it is the vein present in the liver. Urea is produced in the liver so this vein carries maximum amount urea. Thus ,this option is correct. It also acts as bile duct.
Hepatic portal vein: Liver receives blood from two sources. The hepatic artery supplies oxygenated blood to the liver and the hepatic portal vein brings deoxygenated blood from the digestive organs to the liver. The flow of deoxygenated blood from the digestive organs to the liver before returning to the systemic circulation is called hepatic portal circulation.

Our required answer is C) Hepatic vein..

Note: Urea Synthesis (The Ornithine Cycle) is the biochemical aspect of excretion. Also called Kreb-Henseleit cycle, it occurs in the liver and includes Formation of carbamoyl phosphate by the combination of ammonia, CO, and ATP.
Carbamoyl phosphate combines with ornithine to form citrulline joins aspartic acid and changes to argininosuccinic acid. The latter breaks into fumaric acid and arginine. With the help of enzyme arginase, arginine is hydrolysed to urea and ornithine (which is thus regenerated and re-used in the cycle).