Page 11 - LN- EXCRETION

the urine more concentrated.
This can be achieved by a special mechanism known as counter current mechanism and also
known as urine concentration mechanism.
Basic Concept
(i) Henles loop and vasa recta (capillary loop) play an important role in this mechanism. The
flow of filtrate in the limbs of Henle’s loop is in opposite directions and thus, forms a
counter current. The flow of blood with in the two limbs of vasa recta also occur in the
counter current pattern.
(ii) The osmolarity (i.e., number of Osmols of solute per litre) of renal cortical interstitium is
the same (300 m Osmol/ L) as in other tissues, but that of the interstitium of renal medulla
is hypertonic with a gradient of hyperosmolarity from renal cortex to the tips of medullary
papillae.
The hyperosmolarity of medullary interstitium near the tips of the papillae is as high as
1200-1450 m Osmol/L.
The Mechanism
The gradient of increasing hyperosmolarity of medullary interstitium is maintained by a
counter current mechanism and the proximity between the Henle’s loop and vasa recta.
This gradient is mainly caused by NaCl and urea. The transport of these substances is
facilitated by the special arrangement of Henle’s loop and vasa recta.
There are two aspect of this mechanism
(i) Counter current multiplication (by the Henle’s loop).
(ii) Counter current exchange (by the vasa recta).
NaCl is transported by the ascending limb of Henle’s loop, which is exchanged with the
descending limb of vasa recta.