Download Urine formation function of kidney

Urine formation function of
kidney.
Urine formation
Structures responsible
for the urine formation:
 glomeruli,
 proximal canaliculi,
 distal canaliculi.
Mechanism of urine
formation:
 filtration
 reabsorption
 secretion
Mechanisms of elimination:
 filtration
 reabsorption
 excretion
Filtration
Takes place in glomeruli.
Substances with molecular mass below 40,000 Da pass
through the membrane of glomerulus into capsula.
About 120 mL/min
or 180 L/day of
blood is filtrated.
Filtration – passive
process.
After filtration –
primary urine (180
L/day)
Filtration is caused by:
-hydrostatic pressure of blood in
capillaries of glomeruli (70 mm
Hg)
-oncotic pressure of blood plasma
proteins (30 mm Hg)
-hydrostatic pressure of
ultrafiltrate in capsule (20 mm
Hg)
70 mm Hg-(30 mm Hg+20 mm
Hg)=20 mm Hg
Hydrostatic pressure in glomeruli
is determined by the ratio
between diameter of ascendant
and descendant arteriole
Reabsorption
Takes place in proximal and distal canaliculi.
What is reabsorbed? Glucose (100%), amino acids (93%),
water (98%), NaCl (70%) etc.
The urine is concentrated (toxins damages the
proximal canaliculi)
Reabsorption:
active
passive.
Lipophilic substances
- passive.
Na/K АТP-аse is very
active
Secretion
Transport of substances from blood into filtrate.
Takes place in proximal and
distal canaliculi.
Secretion:
active
passive.
Passive secretion depends on
the pH.
What is secreted?
•Ions of K, аmmonia, H+
•drugs
•xenobiotics
CLEARANCE
Clearance of any substance is expressed in ml of blood
plasma that is purified from this substance for 1 min while
passing through the kidneys.
About 180 L of primary urine is formed for 1 day, about
125 mL of primary urine for 1 min.
Glucose is reabsorbed completely; clearance = 0
Inulin is not reabsorbed absolutely; clearance = 125 mL/min
If clearance is more than 125 mL/min the substance is
secreted actively.
Clearance = (C urine/C plasma) * V