Countercurrent Multiplication and Exchange – Urine Concentration and Volume (Nursing)

00:01 The counter-current multiplier involves the loops and is going to depend on the field trip flow in the opposite directions either descending and ascending.

00:12 And also what's really important is the difference in the permeabilities between the two loops.

00:19 Active transport of solutes out of the ascending limb are what is going to drive the counter-current multiplication.

00:28 Limbs of the nephron loops are not actually in direct contact with each other.

00:34 However, they're in close enough proximity to each other to influence each other's exchanges with the surrounding interstitial fluid in the renal medulla.

00:46 In the counter-current multiplier, the ascending limb of the nephron loop is going to be impermeable to water and selectively permeable to certain solutes.

00:57 These include sodium and chloride which are actively reabsorbed in the thicker segment of the ascending limb.

01:05 There's also some passive reabsorption in the thin segment.

01:10 The other limb the descending limb of the nephron loop is freely permeable to water but completely impermeable to the solutes.

01:19 Therefore water is going to pass out of the filtrate into the hyperosmotic medullary interstitial fluid that has been established due to the actions of the sodium and chloride in the ascending limb.

01:34 This causes the remaining filtrate osmolality to increase to 12 Milliosmoles inside the tube of the descending and ascending limb.

01:48 So the way the counter-current mechanism works is the more sodium chloride the ascending limb actively transports out into the interstitial fluid.

01:58 The more water in the descending limb will also diffuse out.

02:03 The more water that diffuses out of the descending limb, the saltier, the filtrate inside of the loop is going to become.

02:12 So then the ascending limb is in going to use this salty filtrate to pump those solutes out into the medullary area and further raise the osmolality of the medullary interstitial fluid.

02:28 So let's look at this in a diagram.

02:31 There's a constant difference of about 200 Milli osmoles that's going to exist between the two limbs of the nephron loop and also between the ascending limb and the interstitial fluid.

02:45 This difference of 200 is multiplied along the length of the loop from 300 to 1200 milliosmoles so that there is a difference from the beginning.

02:58 Of the from the top of the medulla toward the bottom of the medulla of about 900 milliosmoles.

03:08 So taking a closer look, we have active transport of the sodium and chloride into the medullary interstitium on the ascending side of the nephron loop.

03:19 This will then cause the passive transport of water out of the descending side of the nephron loop.

03:27 Inside of the tube you'll as the filtrate moves through it's going to become increasingly concentrated as it descends because water is moving out.

03:38 Conversely the filtrate is now going to become increasingly dilute as it goes up the ascending limb because solute is being pumped out.

03:47 The filtrate entering the distal convoluted tubule is going to be less concentrated than the filtrate going into the nephron loop at about 100 milliosmoles.

04:00 So we are actually hypotonic to our blood osmolality.

04:08 The counter current exchanger is going to use the vasa recta.

04:12 The vase erecta is made up of blood capillaries that are going to extend from the efferent arteriole and surround the tubules of the Nephron.

04:23 The vase erecta is highly permeable to both water and solutes.

04:28 So the flow of blood in the vasa erecta is also counter-current and contains a hairpin turn.

04:37 This means that the blood can exchange sodium chloride and water with the surrounding interstitium as it moves through the adjacent parallel sections of the gradient.

04:48 So blood inside the vase erecta is going to remain osmotic with the surrounding interstitial fluid.

04:57 The vasa erecta is able to reabsorb water and solute without undoing the osmotic gradient that was created by the counter-current multiplier of the nephron loops.

05:11 So the counter-current exchanger is going to preserve this medullary gradient that we created by the nephron loops by preventing the rapid removal of salt from the interstitial space and also by removing reabsorbed water.

05:29 So water found in the ascending Vasa recta is going to come from the descending vasa recta or is going to be reabsorbed from the nephron loop and the collecting duct.

05:41 This means that the volume of blood at the end of the vase erecta is a little bit more than that at the beginning of the vasa recta.

05:52 So putting it all together the medullary gradient will be maintained by the capillaries of the vasa recta.

06:00 It is iso-osmotic with the tubules of the nephron loop.