AP BIOLOGY

Detailed explanation-1: -Amino acids may also be metabolized through the liver to form urea, which is then excreted in the urine. Body protein stores can be converted back to essential and nonessential amino acids or may be metabolized, forming waste products and ions, which, as previously detailed, are excreted in the urine.

Detailed explanation-2: -Urea apparently permeates the red cell membrane via a facilitated diffusion system, which plays an important role when red blood cells traverse the renal medulla; rapid urea transport helps preserve the osmotic stability and deformability of the cell, and it helps prevent dissipation of extracellular osmotic gradients.

Detailed explanation-3: -Vasopressin, upon binding to V2 receptors in the inner medullary collecting ducts, increases urea permeability through activation of UT-A1 molecules, thus enabling urea to diffuse into the inner medullary interstitium.

Detailed explanation-4: -Urea is the chief end product of nitrogen metabolism in mammals. Most of the urea produced is excreted by the kidneys.