It requires no energy and only needs to be soluble.įacilitated diffusion is similar to diffusion in that it moves a substance down its concentration gradient. Simple diffusion moves a substance from a higher to a lower concentration down its concentration gradient. Both ions are moved in opposite directions from a lower to a higher concentration. An example would be the active transport of Na + out of a cell and K + into a cell by the Na +/K + pump. It is very specific and must have an appropriately shaped receptor for the substance to be transported. These were discussed in an earlier chapter, and you may wish to review them.Īctive transport utilizes energy, usually the energy found in a phosphate bond of ATP, to move a substance across a membrane from a low to a high concentration. Mechanisms by which substances move across membranes for reabsorption or secretion include active transport, diffusion, facilitated diffusion, secondary active transport, and osmosis. Reabsorbed if parathyroid hormone present activeĨ5 percent reabsorbed, inhibited by parathyroid hormone, diffusion Reabsorbed in thick ascending limb diffusion Secretion controlled by aldosterone active Reabsorbed, symport with Na + and antiport with Cl – in ascending limbĢ0 percent reabsorbed in thick ascending limb symport Variable amounts reabsorbed, controlled by ADH, osmosisĨ0–90 percent symport reabsorption with Na + Reabsorbed in thin and thick ascending limb diffusion in ascending limbĦ7 percent reabsorbed osmotically with solutesġ5 percent reabsorbed in descending limb osmosis Reabsorption in medullary collecting ducts diffusionĢ5 percent reabsorbed in thick ascending limb active transportĥ percent reabsorbed, stimulated by aldosterone active Substances Secreted or Reabsorbed in the Nephron and Their LocationsĪlmost 100 percent reabsorbed secondary active transport with Na +Īlmost 100 percent reabsorbed symport with Na +ĥ0 percent reabsorbed by diffusion also secreted 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. About 10 percent (about 18 L) reaches the collecting ducts. Most water is recovered in the PCT, loop of Henle, and DCT. This control is exerted directly by ADH and aldosterone, and indirectly by renin. 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. Various portions of the nephron differ in their capacity to reabsorb water and specific solutes. That recovery occurs in the PCT, loop of Henle, DCT, and the collecting ducts ( and ). 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. List the locations in the nephron where tubular secretion occurs.Explain the role of the loop of Henle, the vasa recta, and the countercurrent multiplication mechanisms in the concentration of urine.Describe how and where water, organic compounds, and ions are reabsorbed in the nephron.Explain why the differential permeability or impermeability of specific sections of the nephron tubules is necessary for urine formation.Compare and contrast passive and active tubular reabsorption.List the different membrane proteins of the nephron, including channels, transporters, and ATPase pumps.List specific transport mechanisms occurring in different parts of the nephron, including active transport, osmosis, facilitated diffusion, and passive electrochemical gradients.By the end of this section, you will be able to:
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