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. 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. 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.
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. The glucose molecule then diffuses across the basal membrane by facilitated diffusion into the interstitial space and from there into peritubular capillaries. 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. Figure 2. Substances Reabsorbed and Secreted by the proximal convoluted tubule. The renal corpuscle filters the blood to create a filtrate that differs from blood mainly in the absence of cells and large proteins. From this point to the ends of the collecting ducts, the filtrate is undergoing modification through secretion and reabsorption before true urine is produced.
The first point at which the filtrate is modified is in the proximal convoluted tubule. Here, some substances are reabsorbed, whereas others are secreted. Water and substances that are reabsorbed are returned to the circulation by the peritubular capillaries and vasa recta capillaries that surround the nephron tubules.
Movement of water into the peritubular capillaries and vasa recta will be influenced primarily by osmolarity and concentration gradients. Sodium is pumped out as an act of active transport of the proximal convoluted tubule into the interstitial spaces between cells and diffuses down its concentration gradient into the peritubular capillary.
As it does so, water will follow passively t o maintain an isotonic fluid environment inside the capillary. More substances move across the membranes of the proximal convoluted tubule than any other portion of the nephron. Recall that cells have two surfaces: apical and basal. The apical surface is the one facing the lumen or open space of a cavity or tube, in this case, the inside of the proximal convoluted tubule.
The basal surface of the cell faces the connective tissue base to which the cell attaches basement membrane or the cell membrane closer to the basement membrane if there is a stratified layer of cells. The numbers and particular types of pumps and channels vary between the apical and basilar surfaces.
Most of the substances transported by a symport mechanism on the apical membrane are transported by facilitated diffusion on the basal membrane. The proximal convoluted tubule is where a majority of reabsorption occurs. In addition, almost percent of glucose, amino acids, and other organic substances such as vitamins are normally recovered here. We will now discuss the process of reaborption of a few important molecules in detail. The transport of glucose from the lumen of the proximal convoluted tubule to the interstitial space is similar to the way it is absorbed by the small intestine.
Sodium moves down its electrochemical and concentration gradient into the cell and takes glucose with it. Glucose leaves the cell to enter the interstitial space by facilitated diffusion. Glucose should normally not be found in urine, as it should all be recovered in the proximal convoluted tubule.
Some glucose may appear in the urine if circulating glucose levels are high enough that all the glucose transporters in the proximal convoluted tubule are saturated, so that their capacity to move glucose is exceeded transport maximum, or T m. Though an exceptionally high sugar intake might cause sugar to appear briefly in the urine, the appearance of glycosuria usually points to type I or II diabetes mellitus. Recovery of bicarbonate HCO 3 — is vital to the maintenance of acid—base balance, since it is a very powerful and fast-acting buffer.
An important enzyme is used to catalyze this mechanism: carbonic anhydrase. This same enzyme and reaction is used in red blood cells in the transportation of CO 2 , in the stomach to produce hydrochloric acid, and in the pancreas to produce HCO 3 — to buffer acidic chyme from the stomach. In the kidney, most of the carbonic anhydrase is located within the cell, but a small amount is bound to the brush border of the membrane on the apical surface of the cell.
This is enzymatically catalyzed into CO 2 and water, which diffuse across the apical membrane into the cell. Water can move osmotically across the lipid bilayer membrane due to the presence of aquaporin water channels.
Inside the cell, the reverse reaction occurs to produce bicarbonate ions HCO 3 —. Note how the hydrogen ion is recycled so that bicarbonate can be recovered. Early studies that determined transport rates of ions by the DCT are described, as are the channels and transporters expressed along the DCT with the advent of molecular cloning. Regulation of expression and activity of these channels and transporters is also described; particular emphasis is placed on the contribution of genetic forms of DCT dysregulation to our understanding.
Abstract The distal convoluted tubule DCT is a short nephron segment, interposed between the macula densa and collecting duct. Publication types Review. Almost percent of glucose, amino acids, and other organic substances such as vitamins are normally recovered here.
Some glucose may appear in the urine if circulating glucose levels are high enough that all the glucose transporters in the PCT are saturated, so that their capacity to move glucose is exceeded transport maximum, or T m. Though an exceptionally high sugar intake might cause sugar to appear briefly in the urine, the appearance of glycosuria usually points to type I or II diabetes mellitus. The transport of glucose from the lumen of the PCT to the interstitial space is similar to the way it is absorbed by the small intestine.
Sodium moves down its electrochemical and concentration gradient into the cell and takes glucose with it. Glucose leaves the cell to enter the interstitial space by facilitated diffusion. Recovery of bicarbonate HCO 3 — is vital to the maintenance of acid—base balance, since it is a very powerful and fast-acting buffer. An important enzyme is used to catalyze this mechanism: carbonic anhydrase CA.
This same enzyme and reaction is used in red blood cells in the transportation of CO 2 , in the stomach to produce hydrochloric acid, and in the pancreas to produce HCO 3 — to buffer acidic chyme from the stomach. In the kidney, most of the CA is located within the cell, but a small amount is bound to the brush border of the membrane on the apical surface of the cell. This is enzymatically catalyzed into CO 2 and water, which diffuse across the apical membrane into the cell.
Water can move osmotically across the lipid bilayer membrane due to the presence of aquaporin water channels. Inside the cell, the reverse reaction occurs to produce bicarbonate ions HCO 3 —. Note how the hydrogen ion is recycled so that bicarbonate can be recovered. The significant recovery of solutes from the PCT lumen to the interstitial space creates an osmotic gradient that promotes water recovery.
As noted before, water moves through channels created by the aquaporin proteins. These proteins are found in all cells in varying amounts and help regulate water movement across membranes and through cells by creating a passageway across the hydrophobic lipid bilayer membrane. Changing the number of aquaporin proteins in membranes of the collecting ducts also helps to regulate the osmolarity of the blood.
The movement of many positively charged ions also creates an electrochemical gradient. This charge promotes the movement of negative ions toward the interstitial spaces and the movement of positive ions toward the lumen.
The loop of Henle consists of two sections: thick and thin descending and thin and thick ascending sections. The loops of cortical nephrons do not extend into the renal medulla very far, if at all. Juxtamedullary nephrons have loops that extend variable distances, some very deep into the medulla. These changes are accomplished by osmosis in the descending limb and active transport in the ascending limb.
Solutes and water recovered from these loops are returned to the circulation by way of the vasa recta. The majority of the descending loop is comprised of simple squamous epithelial cells; to simplify the function of the loop, this discussion focuses on these cells. This increase results in reabsorption of up to 15 percent of the water entering the nephron. Most of the solutes that were filtered in the glomerulus have now been recovered along with a majority of water, about 82 percent.
As the forming urine enters the ascending loop, major adjustments will be made to the concentration of solutes to create what you perceive as urine. The ascending loop is made of very short thin and longer thick portions. Once again, to simplify the function, this section only considers the thick portion. The thick portion is lined with simple cuboidal epithelium without a brush border. These are found between cells of the ascending loop, where they allow certain solutes to move according to their concentration gradient.
Therefore, in comparison to the lumen of the loop, the interstitial space is now a negatively charged environment. The structure of the loop of Henle and associated vasa recta create a countercurrent multiplier system.
The countercurrent term comes from the fact that the descending and ascending loops are next to each other and their fluid flows in opposite directions countercurrent. In addition, collecting ducts have urea pumps that actively pump urea into the interstitial spaces. Ammonia NH 3 is a toxic byproduct of protein metabolism. It is formed as amino acids are deaminated by liver hepatocytes. That means that the amine group, NH 2 , is removed from amino acids as they are broken down. Most of the resulting ammonia is converted into urea by liver hepatocytes.
Urea is not only less toxic but is utilized to aid in the recovery of water by the loop of Henle and collecting ducts. At the same time that water is freely diffusing out of the descending loop through aquaporin channels into the interstitial spaces of the medulla, urea freely diffuses into the lumen of the descending loop as it descends deeper into the medulla, much of it to be reabsorbed from the forming urine when it reaches the collecting duct.
The amino acid glutamine can be deaminated by the kidney. Ammonia and bicarbonate are exchanged in a one-to-one ratio. This exchange is yet another means by which the body can buffer and excrete acid.
The presence of aquaporin channels in the descending loop allows prodigious quantities of water to leave the loop and enter the hyperosmolar interstitium of the pyramid, where it is returned to the circulation by the vasa recta.
As the loop turns to become the ascending loop, there is an absence of aquaporin channels, so water cannot leave the loop. At the transition from the DCT to the collecting duct, about 20 percent of the original water is still present and about 10 percent of the sodium.
If no other mechanism for water reabsorption existed, about 20—25 liters of urine would be produced. Now consider what is happening in the adjacent capillaries, the vasa recta. They are recovering both solutes and water at a rate that preserves the countercurrent multiplier system.
In general, blood flows slowly in capillaries to allow time for exchange of nutrients and wastes.
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