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By the end of this section, you will be able to:
  • List specific transport mechanisms occurring in different parts of the nephron, including active transport, osmosis, facilitated diffusion, and passive electrochemical gradients
  • List the different membrane proteins of the nephron, including channels, transporters, and ATPase pumps
  • Compare and contrast passive and active tubular reabsorption
  • Explain why the differential permeability or impermeability of specific sections of the nephron tubules is necessary for urine formation
  • Describe how and where water, organic compounds, and ions are reabsorbed in the nephron
  • Explain the role of the loop of Henle, the vasa recta, and the countercurrent multiplication mechanisms in the concentration of urine
  • List the locations in the nephron where tubular secretion occurs

With up to 180 liters per day passing through the nephrons of the kidney, it is quite obvious that most of that fluid and its contents must be reabsorbed. That recovery occurs in the PCT, loop of Henle, DCT, and the collecting ducts ( [link] and [link] ). Various portions of the nephron differ in their capacity to reabsorb water and specific solutes. While much of the reabsorption and secretion occur passively based on concentration gradients, the amount of water that is reabsorbed or lost is tightly regulated. This control is exerted directly by ADH and aldosterone, and indirectly by renin. Most water is recovered in the PCT, loop of Henle, and DCT. About 10 percent (about 18 L) reaches the collecting ducts. The collecting ducts, under the influence of ADH, can recover almost all of the water passing through them, in cases of dehydration, or almost none of the water, in cases of over-hydration.

Locations of secretion and reabsorption in the nephron

This diagram shows the different ions and chemicals that are secreted and reabsorbed along the nephron. Arrows show the direction of the movement of the substance.
Substances Secreted or Reabsorbed in the Nephron and Their Locations
Substance PCT Loop of Henle DCT Collecting ducts
Glucose Almost 100 percent reabsorbed; secondary active transport with Na +
Oligopeptides, proteins, amino acids Almost 100 percent reabsorbed; symport with Na +
Vitamins Reabsorbed
Lactate Reabsorbed
Creatinine Secreted
Urea 50 percent reabsorbed by diffusion; also secreted Secretion, diffusion in descending limb Reabsorption in medullary collecting ducts; diffusion
Sodium 65 percent actively reabsorbed 25 percent reabsorbed in thick ascending limb; active transport 5 percent reabsorbed; active 5 percent reabsorbed, stimulated by aldosterone; active
Chloride Reabsorbed, symport with Na + , diffusion Reabsorbed in thin and thick ascending limb; diffusion in ascending limb Reabsorbed; diffusion Reabsorbed; symport
Water 67 percent reabsorbed osmotically with solutes 15 percent reabsorbed in descending limb; osmosis 8 percent reabsorbed if ADH; osmosis Variable amounts reabsorbed, controlled by ADH, osmosis
Bicarbonate 80–90 percent symport reabsorption with Na + Reabsorbed, symport with Na + and antiport with Cl ; in ascending limb Reabsorbed antiport with Cl
H + Secreted; diffusion Secreted; active Secreted; active
NH 4 + Secreted; diffusion Secreted; diffusion Secreted; diffusion
HCO 3 Reabsorbed; diffusion Reabsorbed; diffusion in ascending limb Reabsorbed; diffusion Reabsorbed; antiport with Na +
Some drugs Secreted Secreted; active Secreted; active
Potassium 65 percent reabsorbed; diffusion 20 percent reabsorbed in thick ascending limb; symport Secreted; active Secretion controlled by aldosterone; active
Calcium Reabsorbed; diffusion Reabsorbed in thick ascending limb; diffusion Reabsorbed if parathyroid hormone present; active
Magnesium Reabsorbed; diffusion Reabsorbed in thick ascending limb; diffusion Reabsorbed
Phosphate 85 percent reabsorbed, inhibited by parathyroid hormone, diffusion Reabsorbed; diffusion

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Source:  OpenStax, Anatomy & Physiology. OpenStax CNX. Feb 04, 2016 Download for free at http://legacy.cnx.org/content/col11496/1.8
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