Download High osmotic gradient along the renal medullary interstitial fluid

A. High osmotic gradient along the renal medullary interstitial
fluid:
Which means that 300 mOsm/l at the cortex,
about 800 mOsm/l at outer medulla,
and high 1200-1400 mOsm/l at inner medulla.
This is produced by:
i. Countercurrent Multiplier of Loop of Henle:
This high interstitial medullary osmotic is maintained by the operation of
the Vasa recta as countercurrent exchanges and by the slight medullar
blood flow.
This depend on the following:
 Active transport of Na and Cl out of thick ascending limb from the
tubular lumen to interstium.
 This creates a high osmotic gradient between interstitial fluid and the
fluid in the thin descending loop of Henle which is permeable to water.
 Movement of water by osmosis from thin descending loop of Henle
to the interstitial fluid.
 The continues inflow of tubular fluid from proximal tubule without
flow into the distal tubule.
 All of the above processes are essential to produce the increasing
osmotic gradient along the medullar interstitial fluid.
ii. Role of urea:
 Urea moved passively out the proximal tubule.
 Then urea increase in tubular fluid as water is removed in loop and
distal tubule.
 At inner medullary portion of the collecting duct, urea moves into
interstitial fluid, this adding to the hyper-osmolarity of the interstitium.
The role of urea contributing to urine concentrating ability is evidenced
by fact that people who fed a high protein diet, yielding large amount of
urea as a nitrogenous waste product, can concentrate their urine much
better than people whose protein intake and urea production are low.
***Excretion of urea, Na, K and H ions:
1. Excretion of urea: The rate of excretion determined by
a. Concentration of urea in the plasma.
b. GFR.
 The quantity of urea that passes into the urine average between 40-60%
of urea filtered.
 In renal diseases the GFR of two kidneys falls below normal and
excretion of urea is decreased.
 The body continues to form large quantities of urea, collect in the body
fluid until the plasma concentration very high, urea filtered in
glomerular filtrate eventually will become great enough to allow
excretion of urea as rapidly as it formed
2. Na excretion: The quantities of Na that pass through filtrate each day
is so huge, keeping the Na concentration in the body fluid constant.
The reabsorption of Na by tubular system and excretion the excess of it
is conducted by the following mechanism:
 About 65% of Na is reabsorbed in addition to the Cl and water in
proximal tubule by active transport process.
 About 25% of Na is reabsorbed by active process in thick portion of
ascending limb of Henle.
 The rest 10% of Na will pass into distal tube. Na reabsorption in the
distal tubule, cortical collecting tubule, and collecting duct is
controlled by aldosterone by active transport process coupled with
active transport of K into the cell in opposite direction K-Na exchange.
3. Potassium excretion: To maintain normal body K balance, only 1/8
of total daily tubular load of K can be excreted. The reabsorption of K
by tubular system and excretion the excess of it is conducted by the
following mechanism:
 About 65% are reabsorbed by passive, active, counter and co-transport
from proximal tubule.
 About 25% from thick portion of ascending limb of loop of Henle.
 Leaving only 10% of the original filtered K to enter the distal tubule.
 The remaining small amount of K is actively reabsorbed in the distal
tubule and cortical collecting tubules. K secretion in the distal segment
of tubular system under effect of aldosterone in an exchange to Na ions
by which the rate of K loss in urine.
There are 3 factors that determine the rate of tubular secretion of K:
 Aldosterone: By which the Na ions reabsorbed from the tubular fluid
in an exchange with K ions.
 Plasma K concentration: High plasma K concentration, the rate of K
transport from interstitial fluid to the tubular lumen increased
according to electrochemical gradient.
 Distal tubular Na load:
High Na ions entered the distal tubules,
increases the rate of Na reabsorption in an exchange to K ions.
4- H ions secretion: In the epithelium cells of the:
 Proximal tubules
 Thick segment of the ascending limb of the L.H.
 Distal tubules
 Collecting tubules and collecting duct
All secrete H ions into the tubular fluid in an exchange with Na ions
and HCO3 ions into extracellular fluid. This is an example of secondary
active transport. For each H ion secreted, one Na ion and one HCO3
ion enter the interstitial fluid.