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Course: MCAT > Unit 7
Lesson 14: Renal system- Renal system questions
- Mini MCAT passage: Effects of pressure on GFR
- Mini MCAT passage: Hair dye chemicals and cancer
- Renal physiology: Glomerular filtration
- Tubular reabsorption article
- Renal physiology: Counter current multiplication
- Meet the kidneys!
- Kidney function and anatomy
- Glomerular filtration in the nephron
- Changing glomerular filtration rate
- Countercurrent multiplication in the kidney
- Secondary active transport in the nephron
- Urination
- The kidney and nephron
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Renal physiology: Counter current multiplication
What is countercurrent multiplication?
Your kidneys have a remarkable mechanism for reabsorbing water from the tubular fluid, called countercurrent multiplication.
- Countercurrent multiplication in the kidneys is the process of using energy to generate an osmotic gradient that enables you to reabsorb water from the tubular fluid and produce concentrated urine. This mechanism prevents you from producing litres and litres of dilute urine every day, and is the reason why you don’t need to be continually drinking in order to stay hydrated.
Where does it happen?
The kidneys contain two types of nephrons, superficial cortical nephrons (70-80%) and juxtamedullary nephrons (20-30%). These names refer to the location of the glomerular capsule, which is either in the outer cortex of the kidney, or near the corticomedullary border. Nephrons can be thought of in sections, each with a different structure and function. These are the glomerulus, the proximal tubule, the loop of Henle, the distal tubule, and the collecting duct. The loop of Henle is a hairpin-like structure comprised of a thin descending limb, a thin ascending limb and a thick ascending limb. While the loops of Henle of cortical nephrons penetrate only as far as the outer medulla of the kidney, those of the juxtamedullary nephrons penetrate deeply within the inner medulla.
Although both cortical and juxtamedullary nephrons regulate the concentrations of solutes and water in the blood, countercurrent multiplication in the loops of Henle of juxtamedullary nephrons is largely responsible for developing the osmotic gradients that are needed to concentrate urine. Fluid leaving the ascending limb of the loop of Henle enters the distal convoluted tubule, where its composition is further adjusted, and then drains into collecting tubules. These tubules empty into collecting ducts that descend back through the medulla, and eventually connect to the ureter, which transports urine to the bladder.
Although the loops of Henle are essential for concentrating urine, they do not work alone. The specialized blood capillary network (the vasa recta) that surrounds the loops are equally important. The vasa recta capillaries are long, hairpin-shaped blood vessels that run parallel to the loops of Henle. The hairpin turns slow the rate of blood flow, which helps maintain the osmotic gradient required for water reabsorption.
How does countercurrent multiplication work?
The three segments of the loops of Henle have different characteristics that enable countercurrent multiplication.
- The thin descending limb is passively permeable to both water and small solutes such as sodium chloride and urea. As active reabsorption of solutes from the ascending limb of the loop of Henle increases the concentration of solutes within the interstitial space (space between cells), water and solutes move down their concentration gradients until their concentrations within the descending tubule and the interstitial space have equilibrated. As such, water moves out of the tubular fluid and solutes to move in. This means, the tubular fluid becomes steadily more concentrated or hyperosmotic (compared to blood) as it travels down the thin descending limb of the tubule.
- The thin ascending limb is passively permeable to small solutes, but impermeable to water, which means water cannot escape from this part of the loop. As a result, solutes move out of the tubular fluid, but water is retained and the tubular fluid becomes steadily more dilute or hyposmotic as it moves up the ascending limb of the tubule.
- The thick ascending limb actively reabsorbs sodium, potassium and chloride. this segment is also impermeable to water, which again means that water cannot escape from this part of the loop. This segment is sometimes called the “diluting segment”.
Countercurrent multiplication moves sodium chloride from the tubular fluid into the interstitial space deep within the kidneys. Although in reality it is a continual process, the way the countercurrent multiplication process builds up an osmotic gradient in the interstitial fluid can be thought of in two steps:
- The single effect. The single effect is driven by active transport of sodium chloride out of the tubular fluid in the thick ascending limb into the interstitial fluid, which becomes hyperosmotic. As a result, water moves passively down its concentration gradient out of the tubular fluid in the descending limb into the interstitial space, until it reaches equilibrium.
- Fluid flow. As urine is continually being produced, new tubular fluid enters the descending limb, which pushes the fluid at higher osmolarity down the tube and an osmotic gradient begins to develop.
As the fluid continues to move through the loop of Henle, these two steps are repeated over and over, causing the osmotic gradient to steadily multiply until it reaches a steady state. The length of the loop of Henle determines the size of the gradient - the longer the loop, the greater the osmotic gradient.
Absorbed water is returned to the circulatory system via the vasa recta, which surrounds the tips of the loops of Henle. Because the blood flow through these capillaries is very slow, any solutes that are reabsorbed into the bloodstream have time to diffuse back into the interstitial fluid, which maintains the solute concentration gradient in the medulla. This passive process is known as countercurrent exchange.
The concentration of urine is controlled by antidiuretic hormone, which helps the kidneys to conserve water. Its main effects in the renal tubules is to increase water permeability in the late distal tubule and collecting ducts, increase active transport of sodium chloride in the thick ascending limb of the loop of Henle, and enhance countercurrent multiplication and urea recycling, all of which increase the size of the osmotic gradient.
Urea recycling
Urea recycling in the inner medulla also contributes to the osmotic gradient generated by the loops of Henle. Antidiuretic hormone increases water permeability, but not urea permeability in the cortical and outer medullary collecting ducts, causing urea to concentrate in the tubular fluid in this segment. In the inner medullary collecting ducts it increases both water and urea permeability, which allows urea to flow passively down its concentration gradient into the interstitial fluid. This adds to the osmotic gradient and helps drive water reabsorption.
Consider the following:
The kidneys are able to separate the reabsorption of water and solutes in the loop of Henle, distal nephron and collecting ducts. This means urine can be made more concentrated or more dilute than plasma, depending on how hydrated you are. This process is mainly controlled by antidiuretic hormone, a hormone that is made in the hypothalamus of the brain and stored in the pituitary gland. The release of antidiuretic hormone by the pituitary gland is controlled by sensors in your heart and blood vessels that detect drops in blood pressure, or increased concentrations of salt in your bloodstream that may occur when you are dehydrated. If you have ever felt dehydrated after having a few glasses of wine or beer, this is because alcohol inhibits the release of antidiuretic hormone, which increases urine production.
Want to join the conversation?
0ne way to remember and not confuse about the afferent arteriole and efferent arteriole is that A comes before E the blood first enters the glomerulus through the afferent arteriole and the leaves through the efferent arteriole(12 votes)
I was always under the impression the descending limb was impermeable to solutes. Is that just a matter of semantics because its unlikely to see much diffusion with the gradients that exist around it?(10 votes)
The descending limb is only permeable to small solutes (diffuse in) and water (diffuse out).
In the descending limb, solutes (like sodium) can diffuse into the tubule because of the reabsorption of these solutes that occur in the ascending limb. (Note: ascending limb can reabsorb small solutes but it is impermeable to water, thus tubule becomes more hypoosmotic.) Some of reabsorbed solutes from ascending limb will diffuse into the descending limb while others will remain in the interstitial fluid. Because of the increase of sodium in the interstitial fluid, water is driven out of the tubule at the descending limb and water will continue flowing out until it reaches steady state. Also because urine is continually being produced, new tubule fluid can enter the descending limb. This will push fluids of higher osmolarity down the tube and further increase this osmotic gradient. Thus, longer loops will have higher osmotic gradients.
-Hope this makes sense!(2 votes)
- which is larger afferent arteriole or efferent arteriole and why so?(2 votes)
The afferent arteriole is the blood vessel that carries blood towards the glomerulus. The efferent arteriole is the blood vessel that carries blood out and away from the glomerulus.
The afferent arteriole is larger in the sense ,it has a larger diameter than the efferent arteriole and this helps to build up high blood pressure in the glomerulus for ultrafiltration to take place. To help understand how this works ,imagine garden hose with holes in it just before the spray. Water pours out of these holes with pressure because the spray at the end has a lower diameter than the diameter of the hose. This will build up the pressure which is fundamental for ultrafiltration.
One way to remember and not confuse about the afferent arteriole and efferent arteriole is that A comes before E. The blood first enters the glomerulus through the Afferent arteriole and then leaves through the Efferent arteriole.
Hope I helped you(4 votes)
- In the urine concentration mechanism, what changes in cortical nephrons compared to juxstamedullary nephrons?(2 votes)
- Nothing truly changes. The process is the same, the main difference would be the length (or positioning) of the Juxtamedullary Nephrons. Because the Juxtamedullary Nephrons are:
(1) longer which means that there will be a greater osmotic gradient (thus leading to a greater occurrence of water reabsorption)
(2) within the medulla which allows for the solutes to be freely released into the interstitium. This occurs when tubular fluid goes down the Descending loop of Henle (water is passively reabsorbed along with small solutes; hyperosmotic) and then ascends via the Ascending Segments of the Loops of Henle (water can't escape, but solutes can).
At this point, the reabsorbed water is passed to the Vasa Recta which later enters into the circulatory system but solutes can still escape back into the interstitium thus allowing for maintenance of the medulla's concentration gradient and later allowing for this process to continue and repeat.
I could be wrong, but from my understanding, these are the main differences. Hopefully it helps :)(2 votes)
- difference between heartwood and sapwood??(2 votes)
- "The thick ascending limb actively reabsorbs sodium, potassium and chloride. this segment is also impermeable to water, which again means that water cannot escape from this part of the loop. This segment is sometimes called the “diluting segment”.
Is this true? If the thick ascending limb actively reabsorbed these molecules wouldn't the osmolarity increase? Did you mean actively pumps sodium, potassium and cloride into the interstitial fluid?(2 votes) 
If filtrate leaves the counter current multiplier system at 100mosm/L hypoosmotic to blood (300mosm/L) how does this system concentrate urine? From what I understand, it itself does not concentrate urine, but rater decreases the amount so much that the Collecting Duct is able to concentrate it or dilute it based on vasopressin. If this system did not exist the Collecting Duct would be overrun with so much fluid there would be tons of urine and it would all be around 300mosm/L.(2 votes)- if cortical nephrons do not have vasa recta how do they maintain the hyperosmolality? is it only by the urea recycling method?(2 votes)
I dont think cortical nephron even have a urea recycling process. Since it doesnt extend into the inner medullary part, and only inner medullary collecting duct is permeable to urea. My guess is that the cortical nephron is basically maintain its gradient by the flow of new fluid plus the anatomy of the tubular system. Plus the cortical nephron osm concentration is much lower than JM nephron (peak 600 vs peak 1200) so it doesnt require much maintenance(1 vote)
- isit helpful for producing hypotonic urine?(1 vote)
- Our body needs salts ,more accurately ions to carry out various metabolic processes. For instance ,sodium ions ,potassium ions and chloride ions are required for the transmission of nerve impulses . This is one of their functions among many.
Hypotonic solution means that a lower concentration of salts are present in the solution relative to something else ,which is usually the concentration of salts in a cell. Having hypotonic urine means that the urine has a relatively low concentration of salts so less salt is lost from the body. When an organism is not getting much salts from it's diet ,either because it is in a habitat like a desert ,it's body should try to conserve salts and this is achieved by producing hypotonic urine.
The other part of the hypotonic indicates that lots of water is present in the urine. When an organism has consumed large amounts of water, to prevent the red blood cells from just taking in the water by osmosis and bursting, this extra water has to be removed usually. This is achieved by producing hypotonic urine.
Remember that kidney is an organ which carries out excretion and osmoregulation. In osmoregulation, in order to maintain the water potential of the blood plasma, they will produce hypertonic or hypotonic urine depending on the conditions ,to attain a constant normal level of water potential . These two types of urine are helpful when suited to the conditions that the organisms needs to survive.
Hope I helped you(2 votes)
- Is it true that aldosteron has no effect on sodium concentration in ecf as a whole ?(1 vote)