Producing a Concentrated or Dilute Urine

Question 1

What does the countercurrent mechanism in the kidneys mean?

Answer 1

It is the interaction between the flow of filtrate through the loops of henle of juxtamedullary nephrons and the flow of blood through adjacent vasa recta blood vessels. This establishes a osmotic gradient which allows the kidneys to vary the conc of urine produced. (Countercurrent because of AL and DL. It basically means that the reabsorption of ions in the AL drives the passive diffusion of H2O in the DL)

Question 2

What is the normal osmolality of plasma?

Answer 2

300 mOsm

Question 3

What are some factors that contribute to the build up of osmolarity in the medulla?

Answer 3

• Passive absorption of ions in the thin ascending limb,
• Active transport of sodium and co transport of K, Cl and other ions in thick ascending limb
• Active transport from collecting duct,
• Facilitated diffusion of urea into medullary interstitium,
• Diffusion of only small amounts of water into interstitium.

Question 4

Describe how the loop of Henle of juxtomedullary nephrons acts as a countercurrent multiplier to establish the osmotic gradient

Answer 4

• The descending limb is impermeable to solutes but freely permeable to water. Therefore by the time you reach the elbow of the loop, the osmolality is at 1200mOsm.
• The ascending limb is permeable to solutes but not water therefore more ions leave the tubule than water so the osmolality decreases.
• Collecting ducts in deep medulla are permeable to urea

Question 5

Describe the steps in the counter current mechanisms

Answer 5

1) Fluid enters loop of henle at 300 mOsm.
2) Active transport of ions from thick ascending limbs which increases medullary interstitum osmolality to 400 and decreases the osmolality of the TAL.
3) Tubular fluid in DL equilibrates with interstitual fluid as water leaves tubules leaving higher conc of ions within DL.
4) Hyperosomtic fluid in the DL moves to AL
5) Additional ions are pumped out of AL until a 200 mOsm/L gradient is established between AL and interstitum
6) Again, movement of water out of DL so to reach equilibrium with interstitum. Steps 4-6 repeated till elbow = 1200-1400 mOsm/L

Question 6

Describe the formation of concentrated urine

Answer 6

Release of ADH is enhanced by any event that raises plasma osmolality above 300 mOsm/L. ADH Increases the permeability of the collecting ducts and the fluid at the end of collecting ducts is the same as the renal medulla. Thus producing a more concentrated urine

Question 7

Describe the role of urea in producing a concentrated urine

Answer 7

In the DL the urea conc continues rises due to water reabsorption and in a dehydrated subject there is passive movement of urea into tubule from interstitum. The TAL, DCT and collecting ducts are relatively impermeably to urea so it doesn’t move back into the interstitium. In presence of ADH more water reabsorbed so conc of urea increases further. In the medullary collecting ducts the permeability to urea increases so urea can diffuse into the interstitum however it is recycled and excreted in the loop og Henle. Recirculation means there is high concs of urea in the distal tubular fluids in times of dehydrated. It also provides an additional mechanism for hyperosmotic fluid in renal medulla.

Question 8

Describe the function of blood flow through vasa recta

Answer 8

The looped structure of the vasa recta serves as a counter current mechanism to prevent the washout of solutions from the medullary interstitium. Blood becomes more hypertonic as it descends into medullary interstitium and then becomes less hypertonic as it ascends back towards the cortical regions.

Question 9

Describe the formation of dilute urine

Answer 9

In the absence of ADH the DCT is impermeable to water and so the tubular fluid becomes more dilute. The failure to reabsorb water and continue reabsorption of ions leads to a large volume of dilute urine.