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Animation: Nephron Function

by Pearson
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>> Nephrons regulate the composition of blood by a combination of three processes that transfer materials between the nephron tubules and the capillaries that serve them: filtration, secretion, and reabsorption. The diagram shows a nephron tubule, and the blood vessels surrounding it have been straightened and simplified. The gradation of color (lighter on top, darker on the bottom) corresponds to an increasing concentration of solutes in the interstitial fluid surrounding the nephron towards the center of the kidney. As blood flows through the glomerulus (a knot of porous capillaries), water, and virtually all other molecules small enough are forced by blood pressure out of the glomerulus and into Bowman's capsule. This process is called filtration. Filtration is the fluid that accumulates in Bowman's capsule. Blood contains blood cells, plasma proteins, water, sodium chloride, hydrogen ions, bicarbonate ions, urea, glucose, amino acids, and possibly some drugs and poisons. Blood cells and plasma proteins stay in the blood because they are just too big to pass through, but all the other substances are filtered from the blood and form the filtrate that accumulates in the nephron tubule. As the filtrate moves along the proximal tubule, certain substances are transported from the blood and added to the filtrate through a process called secretion. Secretion of hydrogen ions helps to regulate body pH. Certain poisons are also secreted to remove them from the blood. This list summarizes substances contained in the filtrate: water, sodium chloride, hydrogen ions, bicarbonate ions, urea, glucose, amino acids, drugs, and poisons. The filtration process is not selective, so some of these substances are useful to the body and it is important that they be returned to the blood. The kidneys generally need to recover water, sodium chloride, bicarbonate ions, and nutrients, such as glucose and amino acids. This reclamation of valuable solutes and water is called reabsorption. Let's look at the process of reabsorption. In the proximal tubule reabsorption of bicarbonate ions (HCO3) helps regulate the blood's pH. Sodium chloride, NaCl, glucose, and amino acids are actively transported out of the filtrate. This leaves the filtrate more dilute than the surrounding interstitial fluid, so water follows by osmosis. These solutes and water re-enter the blood. Next, as the filtrate moves down the loop of Henle, the concentration of solutes increases in the interstitial fluid surrounding the nephron tubule. Since this portion of the tubule is water-permeable, the water leaves by osmosis and is reabsorbed into the blood. This concentrates the filtrate. In the ascending loop, the surrounding fluid becomes more dilute. This portion of the loop is impermeable to water, but not sodium chloride. Sodium chloride diffuses out, lowering the solute concentration of the filtrate and adding to the solute concentration of the surrounding fluid. Near the top of the loop, sodium chloride is actively transported out, further diluting the filtrate. More substances are being reabsorbed from the distal tubule. Sodium chloride is being actively transported out of the filtrate. Bicarbonate ions may be reabsorbed, helping to regulate body pH. Some drugs and poisons are secreted from the blood into the filtrate at this point, and this is another place where hydrogen ions may also be secreted into the filtrate to further adjust pH. There is a lot going on here. The main point is that the nephron is able to expel unneeded or harmful substances from the blood by filtration and secretion into the filtrate, and the nephron can reabsorb substances useful to the body. The filtrate enters the collecting duct. As it moves down, it passes through a region where the surrounding fluid has a higher and higher solute concentration. Water leaves the collecting duct by osmosis, concentrating the remaining filtrate into urine. Urea is one of the main body wastes in urine, but here some urea actually diffuses out of the filtrate. The urea performs a useful function by adding to the solute concentration of the interstitial fluid and causing even more water to be absorbed from the filtrate. Even though most of the water has been reabsorbed from the filtrate, urine is still 95 percent water. The most abundant solute is urea, along with other nitrogenous wastes. There is some sodium chloride and traces of other ions, such as calcium and potassium. Hydrogen ions make urine acidic, and drugs and toxins may also be present.
>> Nephrons regulate the composition of blood by a combination of three processes that transfer materials between the nephron tubules and the capillaries that serve them: filtration, secretion, and reabsorption. The diagram shows a nephron tubule, and the blood vessels surrounding it have been straightened and simplified. The gradation of color (lighter on top, darker on the bottom) corresponds to an increasing concentration of solutes in the interstitial fluid surrounding the nephron towards the center of the kidney. As blood flows through the glomerulus (a knot of porous capillaries), water, and virtually all other molecules small enough are forced by blood pressure out of the glomerulus and into Bowman's capsule. This process is called filtration. Filtration is the fluid that accumulates in Bowman's capsule. Blood contains blood cells, plasma proteins, water, sodium chloride, hydrogen ions, bicarbonate ions, urea, glucose, amino acids, and possibly some drugs and poisons. Blood cells and plasma proteins stay in the blood because they are just too big to pass through, but all the other substances are filtered from the blood and form the filtrate that accumulates in the nephron tubule. As the filtrate moves along the proximal tubule, certain substances are transported from the blood and added to the filtrate through a process called secretion. Secretion of hydrogen ions helps to regulate body pH. Certain poisons are also secreted to remove them from the blood. This list summarizes substances contained in the filtrate: water, sodium chloride, hydrogen ions, bicarbonate ions, urea, glucose, amino acids, drugs, and poisons. The filtration process is not selective, so some of these substances are useful to the body and it is important that they be returned to the blood. The kidneys generally need to recover water, sodium chloride, bicarbonate ions, and nutrients, such as glucose and amino acids. This reclamation of valuable solutes and water is called reabsorption. Let's look at the process of reabsorption. In the proximal tubule reabsorption of bicarbonate ions (HCO3) helps regulate the blood's pH. Sodium chloride, NaCl, glucose, and amino acids are actively transported out of the filtrate. This leaves the filtrate more dilute than the surrounding interstitial fluid, so water follows by osmosis. These solutes and water re-enter the blood. Next, as the filtrate moves down the loop of Henle, the concentration of solutes increases in the interstitial fluid surrounding the nephron tubule. Since this portion of the tubule is water-permeable, the water leaves by osmosis and is reabsorbed into the blood. This concentrates the filtrate. In the ascending loop, the surrounding fluid becomes more dilute. This portion of the loop is impermeable to water, but not sodium chloride. Sodium chloride diffuses out, lowering the solute concentration of the filtrate and adding to the solute concentration of the surrounding fluid. Near the top of the loop, sodium chloride is actively transported out, further diluting the filtrate. More substances are being reabsorbed from the distal tubule. Sodium chloride is being actively transported out of the filtrate. Bicarbonate ions may be reabsorbed, helping to regulate body pH. Some drugs and poisons are secreted from the blood into the filtrate at this point, and this is another place where hydrogen ions may also be secreted into the filtrate to further adjust pH. There is a lot going on here. The main point is that the nephron is able to expel unneeded or harmful substances from the blood by filtration and secretion into the filtrate, and the nephron can reabsorb substances useful to the body. The filtrate enters the collecting duct. As it moves down, it passes through a region where the surrounding fluid has a higher and higher solute concentration. Water leaves the collecting duct by osmosis, concentrating the remaining filtrate into urine. Urea is one of the main body wastes in urine, but here some urea actually diffuses out of the filtrate. The urea performs a useful function by adding to the solute concentration of the interstitial fluid and causing even more water to be absorbed from the filtrate. Even though most of the water has been reabsorbed from the filtrate, urine is still 95 percent water. The most abundant solute is urea, along with other nitrogenous wastes. There is some sodium chloride and traces of other ions, such as calcium and potassium. Hydrogen ions make urine acidic, and drugs and toxins may also be present.