In: Anatomy and Physiology
Outline how a concentrated urine (what enters the renal pelvis) is formed as the filtrate flows through the Loop of Henle and then the Collecting Duct.
This involves the movement of filtrate back into the circulation. The volume of filtrate formed per day around 170 to 180 L and the urine released per day is around 1.5 L. Nearly 99% of the glomerular filtrate that has formed is reabsorbed by the renal tubules.
Loop of Henle:
The descending limb of the Loop of Henle is permeable to water due to the presence of Aquaporins, but not permeable to salts. Water is lost in the descending limb. Hence sodium and chloride ions get concentrated in the filtrate.
Ascending limb of Henle's loop is impermeable to water but permeable to solutes such as sodium chloride and potassium ions.
The collecting duct is permeable to water. Potassium ions are actively transported into the tubule and there is absorption of sodium to produce concentrated urine. the segment allows the passage of a small amount of urea into the medullary interstitium to keep the osmolarity.
Formation of concentrated urine:
The capability of concentrating the urine is largely related to the length of the loop of Henle. Vasa recta also helps in concentrating the urine. The flow of filtrate in two limbs of the loop of Henle is in the opposite direction and does form countercurrent. The flow of blood through the two limbs of the vasa recta is also known as a counter-current pattern.
The proximity between Henle's loop, Vasa recta as well as a counter-current mechanism helps in maintaining an increasing osmolarity towards the inner medullary interstitium (from 300 mOs/L in the cortex about 1200 mOs/L in the inner medulla). This gradient is mainly caused by sodium chloride and urea.
Sodium chloride is transported by the ascending limb of the loop of Henle which is exchanged with the descending branch of Vasa recta. Sodium chloride is returned to the interstitium by the ascending portion of Vasa recta.
Similarly, a small amount of urea enters the thin segment of the ascending loop of Henle is which is transported back to the interstitium by the collecting tubule.
This transport of substance facilitated by the special arrangement of Loop of Henle and Vasa recta is called the countercurrent mechanism.
It helps to maintain the concentration gradient in the medullary interstitium, which helps in easy passage of water from the collecting tubule thereby concentrating the urine. Human kidneys can produce urine nearly four times concentrated than the initial filtrate formed.