- The lacis cells which are interstitial cells in continuity with
mesangial cells. - The peripolar cells which are present at the vascular pole of the
glomerulus, separating the podocytes from the flat parietal
epithelial cells of Bowman's capsule.
Concentration And Dilution Of Urine:
This function is very important to regulate body water and tissue
osmolarity. Normal body tissue and fluid osmolarity is 280-
mosmol/Liter. This is maintained despite the wide variation in fluid intake
(increased intake decreases osmolarity and vice versa) and load of
osmotically active substances e.g. salt. Through biologic activity, there is
a basal production of 600 mosmol/day. This can increase to over 1200
mosmol/day in states of severe catabolism as in patients with extensive
burns.
The kidney is responsible for the control of secretion of water and
solutes through process of urine formation so as to keep normal plasma
osmolarity. The normal urine volume is around 1.5 liter/day but can vary
from 400 ml to over 20 liter/day according to water and solute intake.
The urine osmolarity may vary from 30 mosmol/liter (when urine is
maximally diluted) to 1400 mosmol/liter (when urine is maximally
concentrated). The minimum urine output to maintain adequate excretion
of waste products (600 mosmol/day) is 400 ml with maximum osmolarity
of 1400 mosmol/liter.
Under normal circumstances, over 99% of filtered water is
reabsorbed. Water is reabsorbed in an iso-osmotic fashion with sodium
chloride i.e. as NaCl is reabsorbed, water flows back into the circulation.