5. Concentration And Dilution Of Urine

CONCENTRATION AND DILUTION OF URINE Professor Harbindar Jeet Singh Faculty of Medicine Universiti Teknologi MARA

Objectives 1. Explain the role of the counter-current multiplier and exchanger 2. Describe the generation of the cortico-medullary gradient and its role in urine concentration. 3. Mechanism of water reabsorption 4. Describe the role of ADH and urea in the concentration of urine 5. Describe the intra-renal urea recycling 6. List the osmoprotective osmolytes

a)

The tubular fluid in the proximal convoluted tubule is always approximately iso-osmotic with plasma,

e)

The fluid in the early distal convoluted tubule is always hypotonic regardless of the osmolality of the urine

j)

The earliest site along the nephron where differences in tubule fluid osmolality between antidiuresis and water diuresis could be detected is the late distal tubule, where the tubule fluid becomes iso-osmotic with plasma during antidiuresis but remains hypotonic during water diuresis

d) Between the late distal tubule and the final urine, the tubule fluid rises to an osmolality level greater than that of plasma during antidiuresis but remains hypotonic during water diuresis

Urine osmolality varies widely in response to changes in water intake e.g. in antidiuresis and diuresis. Typically, urine osmolality can vary from about 50 mOsm/kg H2O during maximum diuresis to about 1,200 mOsm/kg H2O during maximum antidiuresis in humans. The critical regulatory capabilities to vary osmolality of the urine and hence the rate of water excretion are provided by the kidney’s urine concentrating mechanism. Five very important pre-requisites or features are required in the kidney for this purpose

1. Countercurrent arrangement of the entire nephron and

the collecting duct (The ‘S’ shape) 2.

The wall of the ascending limb of Henle that is impermeable to water and capable of sodium chloride transport 3.

In addition to this there is also a need for an osmotic gradient that is increasing from the cortex to the medulla or the papillary tip (Cortico-medullary osmotic gradient). 4. Walls of the distal tubule and the collecting duct whose water permeability could be regulated 5.

A countercurrent arrangement of blood flow (Vasa recta)

The ‘S’ shape of the nephron

1. Countercurrent arrangement of the entire nephron and

the collecting duct (The ‘S’ shape) 2.

The wall of the ascending limb of Henle that is impermeable to water and capable of sodium chloride transport 3.

In addition to this there is also a need for an osmotic gradient that is increasing from the cortex to the medulla or the papillary tip (Cortico-medullary osmotic gradient). 4. Walls of the distal tubule and the collecting duct whose water permeability could be regulated 5.

A countercurrent arrangement of blood flow (Vasa recta)

From the example note the following points. The events described occur in the thick ascending limb and in the outer medulla. By step 8 the interstitial concentration near the bend of the loop of Henle is nearly 400 mOsm/Kg/H2O higher than that near the beginning of the descending limb. The longer the loop of Henle, the greater will be the concentration of the interstitium

The whole process of urine concentration consists of two relatively independent components

Countercurrent multiplication, which generates a hypertonic interstitium Osmotic equilibration of the tubule fluid with the hypertonic medullary interstitium

Osmolality measured in rabbit plasma, urine and several kidney slices from two kidneys during antidiuresis

The medullary interstitium osmotic gradient is set up through two main mechanisms. •

The gradient in the outer medulla is due to the active transport of NaCl by the thick ascending limb of Henle. At any particular point the thick ascending limb of Henle cells are able to generate an osmolality gradient of approximately 200 mOsm/kg/H20 through active transport of NaCl.

2) The osmotic gradient in the inner medulla is primarily

due to passive movement of urea and NaCl. The thin ascending limb has no active NaCl transport high permeabilities to NaCl and Urea impermeable to water VP

Role of vasopressin (ADH) Water excretion is regulated by vasopressin. In the presence of vasopressin the entire collecting duct becomes highly permeable to water. In addition to that vasopressin also has additional effects: 1. It increases the urea permeability in the terminal part of the inner medullary collecting duct 2. Vasopressin increases the rate of active sodium chloride absorption in the medullary thick ascending limb of the loops of Henle. 3. Vasopressin increases the rate of active NaCl and fluid reabsorption in the cortical collecting duct

4. Vasopressin also increases the rate of K+ secretion in the cortical collecting duct and distal tubule (This effect balances the decrease in potassium that would occur because of the decreased rate of tubule fluid flow during antidiuresis)

Vasopressin (ADH) increases water permeability by binding to V2 receptors in the basolateral membrane of cells in the collecting ducts, stimulated adenyl cyclase to produce c’AMP, which then activates protein kinase A that leads to insertion of aquaporin-2 (AQP2) water channels into the apical membrane

Water permeability of different parts of the nephron

AQP1 - found across the ascending limb of the loop of Henle. Not sensitive to vasopressin AQP2 - found on the apical surface of the collecting duct. Sensitive to vasopressin AQP3 - found on the basolateral membrane of the cortical and outer medullary collecting duct AQP4 - found on the basolateral membrane of the inner medullary collecting duct.

Urea diffuses down its concentration gradient from the permeable inner medullary collecting duct into the medullary interstitium. Fluid entering the thin ascending limb has a high NaCl concentration relatively to urea and relatively higher than its concentration in the interstitium, Na Cl diffuses down the concentration gradient and into the interstitium.

Role of the Vasa Recta

Urea Recycling

The kidney has several urea recycling pathways, which help maintain high urea concentration within the inner medullary interstitium. 1. Ascending limb, distal tubule and collecting duct The major recycling pathway involves urea absorption from the terminal inner medullary collecting duct and secretion into the thin ascending limb. This is carried through the thick ascending limbs, the distal convoluted tubule and the early part of the collecting duct by the flow of the tubule fluid.

Urea Recycling

The kidney has several urea recycling pathways, which help maintain high urea concentration within the inner medullary interstitium. •

Through the vasa recta and short loop of Henle and collecting duct.

Urea exiting from the inner medullary collecting duct enters the ascending vasa recta and then enters the descending limb of the short loops of Henle. From here through the superficial distal tubule it goes back to the inner medullary collecting duct.

Urea Recycling The kidney has several urea recycling pathways, which help maintain high urea concentration within the inner medullary interstitium. 1. Between ascending limb and descending limb. Urea reabsorbed from the thick ascending limb in the outer stripe of the outer medulla enters the neighbouring proximal straight tubule of both the short-looped and long-looped nephrons. The urea that enters the short - looped nephrons is returned to the inner medulla by the flow of fluid through the collecting duct. Whereas that in the long-looped nephrons returns to the inner medulla directly through the descending limb.

Osmoprotective Osmolytes The osmolality in the renal medulla can vary from 300 – 1200 m Osm/kg H2O in humans. In addition to that tubule fluid or urine osmolality can vary from 50 – 1200 m Osm/kg H2O The intracellular osmolality of cells in the medulla must be close to that in the medullary interstitium and in the urine.

Two main processes account for the overall osmoregulatory response of medullary cells Short-term regulation of ion transport Volume regulatory increase (VRI) due to entry of Na+ and ClVolume regulatory decrease (VRD) due to exit of K+ Cl- ions Long-term regulation by accumulating intracellularly similarly osmotically active substances, (osmolytes) as are found extracellularly, in the medullary interstitium and urine but do not perturb protein function Medullary cells accumulate non-perturbing or organic osmolytes such as Sorbitol Glycerophosphocholine Inositol Betaine Taurine

These organic osmolytes are divided into two general categories. Compatible osmolytes which have no effect on protein function (Sorbitol, inositol, taurine) Counteracting osmolytes which counteract the effects of urea on protein function (Glycerophosphocholine, and Betaine). The ratio of counteracting osmolyte to urea is usually 1:2.

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The thin descending limb of the inner medulla on the other hand is

highly permeable to water low sodium permeability low urea permeability Active absorption of NaCl from thick ascending limb and absorption of water from the cortical and outer medullary collecting ducts elevate the urea concentration of the collecting duct fluid

Urine concentration and dilution The sites of tubule fluid concentration and dilution along the mammalian nephron have been demonstrated by micropuncture studies in rats and other rodents.

Tubule fluid osmolalities in the rat kidney in antidiuresis (AD) and water diuresis (WD)