Biology - Kidney
A kidney is made up of nephrons. It filters the blood, gets rid of the waste material, and keeps nutrients in the blood. Nephrons keep our body fluid at the right balance of solutes and water. Bowman’s capsule looks like a baseball glove. It captures all of the blood that passes within its enclosure. Glomerulus is a bunch of blood vessels that sit within Bowman’s capsule pocket of space. Therefore, the blood passing through these vessels ends up getting through the Bowman’s capsule.
Note: Blood filtration through the Bowman’s capsule means that all sorts of things that were in the blood before reaching the Bowman’s capsule are now out of the blood and inside the nephron of the kidney. Things that do get filtered are ions, water, other molecules, and small proteins. Things that do not get filtered are cells and large proteins.
The fluids and solutes that have been filtered through the Bowman’s capsule travel along the entire nephron. From the Bowman’s capsule, it passes through some convolutions in the tubule then it goes down one side of a long loop and back up the other side. Next, it passes through some more convolutions.
The section of convolutions closest or most proximal to the Bowman’s capsule is the proximal convoluted tubule. The long loop that goes down and then up is the loop of Henle. The section of convolutions furthest from the Bowman’s capsule is the distal convoluted tubule.
Once the filtrate makes it from the Bowman’s capsule all the way to the distal convoluted tubule and passes the distal convoluted tubule, it enters a collecting duct. The collecting duct collects all of the filtrate that completed the tour of the nephron and the filtrate coming from other nephrons. The filtrate is now called urine. Urine enters the ureter and gets stored in the urinary bladder and it leaves the body through the urethra.
Just outside the loop of Henle, we find that salt concentration is relatively low near the top of the loop and relatively high near the bottom. Therefore, the tissue outside the loop of Henle shows a salt concentration gradient. Concentration is lowest toward the top of the loop and highest toward the bottom. As the filtrate moves down the loop on Henle, the concentration of salt and the fluid outside the loop is increasing. Therefore, the filtrate wants to equalize the concentrations on each side of the loop. In order for this process to occur, it must take salt into the loop and/or move water out.
Note: The walls of the descending portion of the loop are permeable both to water and sodium, so as the filtrate descends the loop, water diffuses passively from the loop outward into the surrounding medium and passively from the surrounding medium inward into the loop.
As the filtrate moves up the loop of Henle, the concentration of salt and the fluid outside the loop progressively decreases, so the filtrate tries to equalize the concentrations on each side of the loop again. But now, the walls of the ascending portion of the loop are impermeable to water and permeable to salt. Therefore, as the filtrate moves up the ascending loop, salt diffuses passively from the loop outward to the surrounding medium, but water cannot move in either direction.
At corresponding portions of the ascending and descending limbs of the loop, the concentration of the filtrate is pretty much equal. The filtrate concentration is 300 moles per liter at the top of the descending loop, and it is 300 moles per liter also at the top of the ascending loop.
Something important does happen in the loop of Henle. On the descending side of the loop, the filtrate takes in sodium and ejects water. On the ascending side of the loop, the filtrate ejects salt but does not take in any water. Therefore, the trip up and down the loop of Henle has increased the water volume outside the loop and decreased the water volume inside the loop. Concentrations are equal at corresponding points of the ascending and descending limbs of the loop, but volumes are not. The trip down and up the loop of Henle has decreased filtrate volume. Water is taken out of the filtrate and put back into the tissues outside the tubules.
The filtrate is ultimately tossed out of the body as urine. As you see, the loop of Henle maintains water concentration in the body. Otherwise, most of the water that enter the nephron would still be in the urine when it is kicked out of the body. The body would be losing a lot of water all the time. The loop of Henle causes the body to take a lot of water out of the filtrate to keep it in the tissues outside the nephron and hence to prevent it from leaving the body as urine. That is why it has been said that the loop of Henle promotes bodily conservation of water.
Remember that in addition to the passive diffusion of salt outward into the surrounding tissue; the ascending limb does some active pumping of salt, also outward. The active pumping of salt is done to reduce the concentration of the filtrate at the top of the ascending limb so that it is a little less than that of the fluid at the top of the descending limb. It is this active transport of salt that ultimately produces the concentration gradient outside the loop of Henle.
Salt concentration in the surrounding tissue increases as the filtrate moves from top to bottom of the collecting duct. Keep in mind that in the collecting duct, filtrate is now called urine. As any other fluid, the urine wants to equalize the concentration across the tubular wall. Unfortunately for urine, the tubular wall is impermeable to salt so it cannot achieve this goal by moving salt.
One of the body’s glands secretes a hormone called antidiuretic hormone (ADH). ADH makes the collecting duct very permeable to water. If there is not so much ADH around, the collecting duct is not very permeable to water. ADH regulates the collecting duct’s permeability to water. In the presence of relatively large amounts of ADH, water diffuses passively out of the collecting duct in large quantities and the urine becomes quite concentrated. In the presence of relatively small amounts of ADH, water diffuses passively out of the collecting duct only in small quantities and the urine is relatively dilute. Therefore, ADH decides whether the urine is going to be concentrated or dilute because ADH regulates the collecting duct’s permeability to water. Lots of ADH means concentrated urine. Small amounts of ADH mean dilute urine.
After passing through the collecting duct, the urine has become more concentrated, a lot more concentrated if there is lots of ADH around and only a little more concentrated if there is not so much ADH around. From the collecting duct, the urine is passed into the ureter, into the bladder, into the urethra, and out of the body.
Note: Blood filtration through the Bowman’s capsule means that all sorts of things that were in the blood before reaching the Bowman’s capsule are now out of the blood and inside the nephron of the kidney. Things that do get filtered are ions, water, other molecules, and small proteins. Things that do not get filtered are cells and large proteins.
The fluids and solutes that have been filtered through the Bowman’s capsule travel along the entire nephron. From the Bowman’s capsule, it passes through some convolutions in the tubule then it goes down one side of a long loop and back up the other side. Next, it passes through some more convolutions.
The section of convolutions closest or most proximal to the Bowman’s capsule is the proximal convoluted tubule. The long loop that goes down and then up is the loop of Henle. The section of convolutions furthest from the Bowman’s capsule is the distal convoluted tubule.
Once the filtrate makes it from the Bowman’s capsule all the way to the distal convoluted tubule and passes the distal convoluted tubule, it enters a collecting duct. The collecting duct collects all of the filtrate that completed the tour of the nephron and the filtrate coming from other nephrons. The filtrate is now called urine. Urine enters the ureter and gets stored in the urinary bladder and it leaves the body through the urethra.
Just outside the loop of Henle, we find that salt concentration is relatively low near the top of the loop and relatively high near the bottom. Therefore, the tissue outside the loop of Henle shows a salt concentration gradient. Concentration is lowest toward the top of the loop and highest toward the bottom. As the filtrate moves down the loop on Henle, the concentration of salt and the fluid outside the loop is increasing. Therefore, the filtrate wants to equalize the concentrations on each side of the loop. In order for this process to occur, it must take salt into the loop and/or move water out.
Note: The walls of the descending portion of the loop are permeable both to water and sodium, so as the filtrate descends the loop, water diffuses passively from the loop outward into the surrounding medium and passively from the surrounding medium inward into the loop.
As the filtrate moves up the loop of Henle, the concentration of salt and the fluid outside the loop progressively decreases, so the filtrate tries to equalize the concentrations on each side of the loop again. But now, the walls of the ascending portion of the loop are impermeable to water and permeable to salt. Therefore, as the filtrate moves up the ascending loop, salt diffuses passively from the loop outward to the surrounding medium, but water cannot move in either direction.
At corresponding portions of the ascending and descending limbs of the loop, the concentration of the filtrate is pretty much equal. The filtrate concentration is 300 moles per liter at the top of the descending loop, and it is 300 moles per liter also at the top of the ascending loop.
Something important does happen in the loop of Henle. On the descending side of the loop, the filtrate takes in sodium and ejects water. On the ascending side of the loop, the filtrate ejects salt but does not take in any water. Therefore, the trip up and down the loop of Henle has increased the water volume outside the loop and decreased the water volume inside the loop. Concentrations are equal at corresponding points of the ascending and descending limbs of the loop, but volumes are not. The trip down and up the loop of Henle has decreased filtrate volume. Water is taken out of the filtrate and put back into the tissues outside the tubules.
The filtrate is ultimately tossed out of the body as urine. As you see, the loop of Henle maintains water concentration in the body. Otherwise, most of the water that enter the nephron would still be in the urine when it is kicked out of the body. The body would be losing a lot of water all the time. The loop of Henle causes the body to take a lot of water out of the filtrate to keep it in the tissues outside the nephron and hence to prevent it from leaving the body as urine. That is why it has been said that the loop of Henle promotes bodily conservation of water.
Remember that in addition to the passive diffusion of salt outward into the surrounding tissue; the ascending limb does some active pumping of salt, also outward. The active pumping of salt is done to reduce the concentration of the filtrate at the top of the ascending limb so that it is a little less than that of the fluid at the top of the descending limb. It is this active transport of salt that ultimately produces the concentration gradient outside the loop of Henle.
Salt concentration in the surrounding tissue increases as the filtrate moves from top to bottom of the collecting duct. Keep in mind that in the collecting duct, filtrate is now called urine. As any other fluid, the urine wants to equalize the concentration across the tubular wall. Unfortunately for urine, the tubular wall is impermeable to salt so it cannot achieve this goal by moving salt.
One of the body’s glands secretes a hormone called antidiuretic hormone (ADH). ADH makes the collecting duct very permeable to water. If there is not so much ADH around, the collecting duct is not very permeable to water. ADH regulates the collecting duct’s permeability to water. In the presence of relatively large amounts of ADH, water diffuses passively out of the collecting duct in large quantities and the urine becomes quite concentrated. In the presence of relatively small amounts of ADH, water diffuses passively out of the collecting duct only in small quantities and the urine is relatively dilute. Therefore, ADH decides whether the urine is going to be concentrated or dilute because ADH regulates the collecting duct’s permeability to water. Lots of ADH means concentrated urine. Small amounts of ADH mean dilute urine.
After passing through the collecting duct, the urine has become more concentrated, a lot more concentrated if there is lots of ADH around and only a little more concentrated if there is not so much ADH around. From the collecting duct, the urine is passed into the ureter, into the bladder, into the urethra, and out of the body.
