Concentration of Urine lecture note, different concentrations

Concentration of urine

1. DR. NAKIBONEKA R
PHYSIOLOGY DEPT.
CONCENTRATION OF URINE.

2. The Kidney is an Osmotic Machine
Kidney uses active transport (especially of Na) to set up
osmotic gradients.
Osmotic pressure in the cortex is isotonic (~300
milliosmoles/liter)
As you move toward the medulla the osmotic pressure rises
to about 1200 milliosmoles/liter (hypertonic).
This osmotic gradient is created by the loops of Henle in the
juxtamedullary nephrons.
Osmotic gradient is produced by a countercurrent
mechanism located in these loops of Henle

3. Osmotic gradient between the
renal cortex and the renal medulla

4. Renal osmolarity

5. Counter-current mechanism
A counter current system is a system of “U” shaped
tubules (tubes) in which, the flow of fluid is in
opposite direction in different limbs of the U”
shaped tubules.
In the kidney the structures that form the counter
current system are the loop of Henle and the vasa
recta.
In both the direction of flow of fluid in the
descending limb is just opposite to that in the
ascending limb.

6. Counter current multiplier Vs Counter
current exchanger
The loop of Henle creates the osmotic gradient and
the vasa recta helps maintain it.
The loop of Henle forms the counter current
multiplier and the vasa recta forms the counter
current exchanger.

7. Counter current multiplier
Na+ ions from outer medulla are carried to the inner medulla
by the long loops of Juxtamedullary nephrons in the following
manner.
 As the fluid flows from the outer medulla into the descending
limb more sodium ions are added from the successive
filtrations.
When the fluid passes through thin ascending segment, sodium
is pumped actively into the Medullary interstitium.
The large quantities of Na+ pumped into the interstitial fluid in
the medulla based upon the Na pump accumulate in the
medulla and increase the osmolarity and thus establishing a
Medullary osmotic gradient.
Loop of Henle multiplies the sodium concentration in the
medulla by retaining the new sodium ions coming from the
glomerular filtrate.

9. Counter current exchanger
When blood enters the vasa recta ,it is isotonic to systemic
plasma. But vasa recta is very permeable to water and sodium
salts.
There is an exchange of sodium and urea for water between the
ascending and descending limbs of vasa recta.
As the blood in vasa reta flows into the depths of the medulla, it
loses water and urea and gains salt and urea becoming
hypertonic.
As it emerges from the medulla in the ascending portion into
the cortex, the process is reversed (gains water and losses urea
and sodium ions).
Thus, the blood leaving the cortex and the blood entering the
cortex (both via the vasa recta) have the same solute
concentration.

10. Role of the vasa recta
1. Salt tends to go in on the way down
2. Water goes in on the way up
3. Volume of isotonic fluid out is larger than goes
in - i.e., returning salt and water to the body

11. Role of urea
1. Passively absorbed from proximal tubule
2. Enters thin limb to increase osmolarity (replaces
NaCl)
3. Leaves collecting duct to re-enter medulla
(promoted by ADH; goes with water to maintain
osmolarity).

12. Mechanism of urine concentration
Urine is concentrated by the counter current
mechanism and ADH.
The concentration of the urine is adjusted in the
collecting ducts of the kidney.
The collecting ducts pass through tissue (medulla) with
a very high osmotic pressure created and maintained by
counter-current multiplier and exchanger respectively.
Water will be sucked out of the tubules by osmosis if
the tubules are permeable

13. Mechanism of urine concentration
cont…
As the urine passes into the collecting duct it first passes through a
region of isotonic osmotic pressure (300 milliosmoles/liter) and
then through a region of hypertonic osmotic pressure (up to 1200
milliosmoles/liter)
If the collecting duct has low water permeability the dilute urine
in the kidney tubule passes through with little uptake of water
and this produces large amounts of dilute urine (diuresis)
If the collecting duct has high water permeability much of the
water will be reabsorbed from the collecting duct into the
interstitial fluid and this produces small amounts of concentrated
urine (antidiuresis)

15. Mechanism of urine concentration cont

16. Atrial Natriuretic Peptide (ANP)
It is a hormone released from the atrial wall.
If the blood volume is too high blood pressure goes up
and the atria are stretched more than normal as blood
enters the heart
This causes the release of a peptide called atrial
natriuretic peptide (ANP) by atrial cells
ANP causes the body to lose both Na and water,
restoring the blood volume to normal by inhibiting
secretion of:
i. Aldosterone
ii.Renin
iii.Anti-diuretic hormone (ADH)

17. Renal tubular Secretion.
Substances such as creatinine, ammonia, H+, K+,
and others are secreted from the peritubular
capillaries into the renal tubules if they are not
filtered at the glomerulus.
Secretion allows us to expedite waste removal and
helps us regulate blood levels of certain ions. For
example,
when blood pH dips too low, we'll secrete more
H+.
When blood potassium levels rise, we'll secrete
more K+.

18. Net Process Of urine formation
The processes of glomerular filtration, selective re-
absorption and tubular secretion, urine is formed in
the Nephron.
It is also concentrated by the counter current
mechanism and ADH.
Finally it passes through the ureter into the urinary
bladder and is stored there until voided out

19. THANK YOU