Countercurrent mechanism in kidney | Concentration of urine | Renal system Physiology mbbs 1st year

Physiology lecture on renal system physiology details countercurrent mechanism for the concentration and dilution of urine.
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Kidneys need to excrete concentrated or dilute urine depending on the hydration status of the body. The concentration or dilution of urine depends on the presence of a gradient of hyper osmolarity in medullary interstitium. Basic function of countercurrent mechanism is to make the medullary interstitium hypertonic. Ultimately, it will help in excretion of a concentrated or dilute urine as required.
So, there are aspects
1) Generation of hypertonic gradient in medullary interestitium
2) Maintenance of hypertonic gradient in medullary interstitium
Generation of gradient occurs by countercurrent multiplier and maintenance of gradient is done by countercurrent exchanger
Countercurrent multiplier: Occurs in juxtamedullary nephrons
Ascending limb of LOH is permeable to solutes only and descending limb is permeable to water only. Permeability in thick ascending limb to solutes is by active transport by transporter Na+K+2Cl- present on the apical membrane of cells of thick ascending limb.
Permeability to water in descending limb is passive.
The fluid which filters through the Bowman’s capsule has the same osmolarity as that of blood i.e 300 mOsm/L of fluid. Now we are considering the first time as fluid passes through the nephron.. So everywhere osmolarity is 300 mOSm/L. Now as the filtered fluid of 300 mosm/l osmolarity passes through the nephron, even though it passes through the descending limb, there will be no net movement of water because the osmolarity is same everywhere.
As the filtrate passes through the ascending limb, the Na+K+2Cl- transporter, starts throwing out these ions into the interstitium since it is active transport. Because of this, the osmolarity of interstitium increases and osmolarity of fluid in the nephron decreases. So fluid in the nephron becomes hypotonic.
Now this effect on osmolarity of medullary interstitium by passing of fluid in the nephron is known as Single Effect.
As second cycle of fluid passes through the descending limb, because of the first cycle there is a gradient for water to move. So passive movement of water occurs from descending limb into the medullary interstitium and the osmolarity of tubular fluid increases.
Now as the hypertonic fluid of nephron passes through the ascending limb , Na K+2cl- throws out more and more ions, decreasing the osmolarity of the tubular fluid while increasing the osmolarity in medullary interstitium. So this will create a gradient of osmolarity in medullary interstitium.
This medullary osmolarity keeps on increasing with each cycle of fluid passing. So this is multiplication of the single effect and the osmolarity keeps on increasing until 1200 mOsm/L.
Countercurrent exchanger: Role of vasa recta
The osmolarity of the blood is 300 mOsm/L. The capillary bents into a U shape. In vasa recta, water moves out of the capillary while salt moves into the capillary in descending limb while opposite movement of water and salt occurs in ascending limb.
So, this is how gradient in medullary interstitium is maintained.
Because of generation of medullary hyperosmolarity, end result is presence of hypoosmolar tubular fluid in DCT, is hypo-osmotic , so a dilute urine is excreted.
However to concentrate urine, we need ADH
ADH increases the permeability of cells to water in last part of distal tubules, collecting tubules and collecting ducts. Only if medullary interstitium is hyperosmolar, water will move from nephrons into the medullary interstitium and then into the blood.
So for concentration urine, ADH and hyperosmolar medullary gradient is needed.
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