AP BIOLOGY

Detailed explanation-1: -According to our results urea diffuses very rapidly from RBC to plasma and is almost at equilibrium at the dialyser outlet; thus the extraction of urea during dialysis is from both plasma and RBC.

Detailed explanation-2: -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-3: -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-4: -The urea cycle or ornithine cycle converts excess ammonia into urea in the mitochondria of liver cells. The urea forms, then enters the blood stream, is filtered by the kidneys and is ultimately excreted in the urine.

Detailed explanation-5: -Although there has been some evidence that urea is transported actively (Kawamura et al. 1976; Kato & Sands, 1998), it is generally considered that urea is passively transported across biological membranes by diffusion.