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

About 120 mL/min or 180 L/day of blood is filtrated.

Filtration – passive process.

After filtration – primary urine (180 L/day)

Filtration

Takes place in glomeruli.

Substances with molecular mass below 40,000 Da pass through the membrane of glomerulus into capsula.

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: activepassive. Lipophilic substances - passive.

Na/K АТP-аse is very active

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)

Takes place in proximal and distal canaliculi.

Secretion: activepassive.

Passive secretion depends on the pH.

What is secreted?•Ions of K, аmmonia, H+

•drugs•xenobiotics

Secretion

Transport of substances from blood into filtrate.

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