In: Anatomy and Physiology
Describe why the vertebrate kidney medulla creates a strong osmotic gradient; and what physiological activities lead to the creation and maintenance of the gradient?
Counter current mechanism.
Mammals have the ability to produce a concentrated urine.
The Henle's loop and vasa recta play a significant role in
this.
The flow of filtrate in the two lombs of Henle's loop is in
opposite directions and thus forms a counter current.
The flow of blood through the two limb of vasa recta is also in a
counter current pattern.
The proximity between the Henle's loop and vasa recta ,as well as
the counter current in them help to maintaining an increase
osmolarity towards the inner medullary interstitium , that is from
300mOsmol/L inthe cortex to about 1200mOsmol/L in the inner
medulla.
This gradient is mainly caused by NaCl and urea.
NaCl is transported by the ascending limb of Henle's loop which is
exchanged with the descending limb of vasa recta.
NaCl is returned to the interstitium by the ascending portion of
vasa recta.
Similarly, small amount of urea enter the thin segment of the
ascending limb of Henle's loop which is transported back to the
interstitium by the collecting tubule.
The above described transport of substances facilitated by the
special arrangement of Henle's loop and vasa recta is called as
COUNTER CURRENT MECHANISM.