Urea is reabsorbed and secreted in the nephron by diffusion,

Urea

  1. Urea is reabsorbed and secreted in the nephron by diffusion, either simple or facilitated, depending on the segment of the nephron.
  2. Fifty percent of the filtered urea is reabsorbed in the proximal tubule by simple diffusion.
  3. Urea is secreted into the thin descending limb of the loop of Henle by simple diffusion (from the high concentration of urea in the medullary interstitial fluid).
  4. The distal tubule, cortical collecting ducts, and outer medullary collecting ducts are impermeable to urea; thus, no urea is reabsorbed by these segments.
  5. ADH stimulates a facilitated diffusion transporter for urea (UT1) in the inner medullary collecting ducts. In the presence ofADH, urea reabsorption from inner medullary collecting ducts contributes to urea recycling in the inner medulla and to the addition of urea to the corticopapillary osmotic gradient
  6. Urea excretion varies with urine flow rate. At high levels of water reabsorption (low urine flow rate), there is greater urea reabsorption and decreased urea excretion. At low levels of water reabsorption (high urine flow rate), there is less urea reabsorption and increased urea excretion.