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2 out of 3

Renal medullar hyperosmolarity is due to: (LQ)

A Increased interstitial Na

B Increased interstitial K

C Increased interstitial Na & urea

D All of the above

Ans. D All of the above

I. Urine concentration:

The Basic requirement for forming a concentrating urine are :

a. A high level of ADH (vassopressin) increases the permeadility of the DT and collecting ducts (main site) to water and

b. A high osmolarily of the renal medullary interstitial fluid, which provides the osmotic gradient necessary for water reabsorption to occur in the presence of high levels of ADH.

Counter current Mechanism produces a hyperosmotic Renal Medullary interstitium.

1. The osmolarity of the interstitial fluid in the medulla of the kidney is much high. Increasing progressively to about 1200-1400 mOsm/L in the pelvic tip of the medulla. Q

The counter current mechanism depends on the special anatomical arrangement of the loops of Henle and the Vasarecta, the specialized peritubular capillaries of renal medulla. Q

c. Mechanism:

i. Active transport of Na+ and co-transport of K+, CI- and other ions out of the thick portion of the ascending limb of the loop of Henle into the medullary interstitium. .

ii. Active transport of ions from the collecting ducts into the medullary interstitium.

iii. Passive diffusion of large amounts of Urea from inner medullary collecting ducts into the medullary interstitium.

iv. Diffusion of only small amount of water from medullary tubules into medullary interstitium.

Operation of the vasa recta as counter-current exchangers :NaCI and Urea diffuse out of the ascending limb of the vessel and into the descending limb, whereas water diffuses out of the descending and into the ascending limb of the vascular loop.

II. In presence of Vasopressin, maximal antidiuresis, urine conc. is 1400 mOsm/Kg H2O (in 0.5 L/d urine) whereas in the absence of vasopressin, urine conc. is 30 mOsm/Kg H2O (in 23.3 L/d urine) Water balance:

a. Water intake: regulated by the thirst, via osmoreceptor, located in the antero-lateral hypothalamus, mainly regulated by toxicity of plasma.

b. Water excretion: The principal determinant of renal excretion is argeninevassopressin (AVP or ADH). The major stimulus for AVP secretion is hypertoxicity (i.e. plasma Na+ conc.) AVP binds on to V2 receptor on the basolateral membrane of principal cells in the collecting duct activates adenylyl cyclase insertion of water channels (Aqua porin-2) on luminal membrane →↑passive water reabsorption along a osmotic gradient from the lumen of the collecting duct to hypertonic medullary interstitium.

III. Regulation of ADH (AVP secretion

Increase ADH

Decrease ADH

(= ↑↑ urine conc.)

(=↓↓ urine conc, i.e. dilute urine)

Plasma osmolarity (M.Imp)

Blood volume

( = ECF volume contraction)




Drugs –




Plasma osmolarity

Blood volume


Drugs –




(Dopamine blockers)

IV. Control of thirst

Increase thirst

Decrease thirst

Osmolarity (M.Imp)


Blood volume

Angiotensin II

Dryness of mouth


Blood volume


Angiotensin II

Gastric distention