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The process of producing urine occurs in three stages: filtration, reabsorption, and secretion. The physiologic goal is to modify the composition of the blood plasma and, in doing so, eliminate only waste in the form of urine. In the last section, we discussed filtrate formation. Now, we will examine how most nutrients are selectively returned into the blood, and how the composition of urine is regulated.
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. Reabsorption occurs in the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and to a lesser degree, the collecting ducts.
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. Most water is recovered in the proximal convoluted tubule, loop of Henle, and distal convoluted tubule. About 10 percent (about 18 L) reaches the collecting ducts. Antidiuretic hormone and aldosterone are responsible for regulating how much water is retained in urine. The collecting ducts, under the influence of antidiuretic hormone, 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.
Figure 1. Locations of Secretion and Reabsorption in the Nephron. Arrows pointing away from the tubule indicate substances that are returning to the blood. Arrows pointing towards the tubule indicate additional substances being removed from the blood and moved into the filtrate.
|Glucose||Almost 100 percent reabsorbed; secondary active transport with Na+|
|Oligopeptides, proteins, amino acids||Almost 100 percent reabsorbed; symport with Na+|
|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 antidiuretic hormone; osmosis||Variable amounts reabsorbed, controlled by antidiuretic hormone, 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|
|NH4+||Secreted; diffusion||Secreted; diffusion||Secreted; diffusion|
|HCO3–||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|
Mechanisms of Recovery
Mechanisms by which substances move across membranes for reabsorption or secretion include simple diffusion, facilitated diffusion, active transport, secondary active transport, and osmosis.
Simple diffusion moves a substance from a higher to a lower concentration down its concentration gradient. It requires no energy and only needs to be soluble.
Facilitated diffusion is similar to simple diffusion in that it moves a substance down its concentration gradient. The difference is that it requires specific membrane transporters or channel proteins for movement. The movement of glucose and, in certain situations, Na+ ions, is an example of facilitated diffusion. In some cases of facilitated diffusion, two different substances share the same channel protein port; these mechanisms are described by the terms symport and antiport. Symport mechanisms move two or more substances in the same direction at the same time, whereas antiport mechanisms move two or more substances in opposite directions across the cell membrane.
Active transport is when a membrane transporter 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. The membrane transporteris very specific and must have an appropriately shaped binding pocket for the substance to be transported. An example would be the active transport of Na+ out of a cell and K+ into a cell by the Na+/K+ pump. Both ions are moved in opposite directions from a lower to a higher concentration.
Both symport and antiport may utilize concentration gradients maintained by ATP pumps. This is a mechanism described by the term secondary active transport. For example, a Na+ ATPase pump on the basilar membrane of a cell may constantly pump Na+ out of a cell, maintaining a strong electrochemical gradient. On the opposite (apical) surface, a Na+/glucose symport protein channel assists both Na+ and glucose into the cell as Na+ moves down the concentration gradient created by the basilar Na+ ATPase pumps. The glucose molecule then diffuses across the basal membrane by facilitated diffusion into the interstitial space and from there into peritubular capillaries.
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Most of the Ca2+, Na+, glucose, and amino acids must be reabsorbed by the nephron to maintain homeostatic plasma concentrations. Other substances, such as urea, K+, ammonia (NH3), creatinine, and some drugs are secreted into the filtrate as waste products. Acid–base balance is maintained through actions of the lungs and kidneys: The lungs rid the body of H+, whereas the kidneys secrete or reabsorb H+ and HCO3– . In the case of urea, about 50 percent is passively reabsorbed by the proximal convoluted tubule. More is recovered by in the collecting ducts as needed. Antidiuretic hormone induces the insertion of urea transporters and aquaporin channel proteins.
|Water||180 L||179 L||1 L|