KIDNEYS BIOCHEMISTRY
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: active passive. 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: active passive. 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
Three mechanisms: -Conversion of two substituted phosphates into one substituted in the cavity of canaliculi - Formation of carbonic acid in the cells with the following dissociation to Н + and НСО ammonia excretion MAINTAINING OF ACIDIC-BASE BALANCE BY KIDNEYS
Cells of canaliculus BloodCavity of canaliculus Na 2 HPO 4 Na + HPO 4 2- Na + NaH 2 PO 4 Na + H+H+ H+H+ MAINTAINING OF ACIDIC-BASE BALANCE BY KIDNEYS
NaHCO 3 Na + HCO 3 - Na + H 2 CO 3 Na + H+H+ H+H+ H2OH2OCO 2 H2OH2O + H 2 CO 3 HCO 3 - Blood Cells of canaliculus Cavity of canaliculus MAINTAINING OF ACIDIC-BASE BALANCE BY KIDNEYS
NH 3 +H + Glutamine NH 4 + NH 3 Glutamic acid MAINTAINING OF ACIDIC-BASE BALANCE BY KIDNEYS Blood Cells of canaliculus Cavity of canaliculus
COLLECTION OF URINE: For biochemical investigations, urine can be collected as a single specimen or for 24 hours. Single specimens of urine, normally collected in the morning, are useful for qualitative tests e.g., sugar, proteins. For biochemical investigations, urine can be collected as a single specimen or for 24 hours. Single specimens of urine, normally collected in the morning, are useful for qualitative tests e.g., sugar, proteins.
Urine specimens tend to deteriorate. Urine specimens tend to deteriorate. The changes include: The changes include: 1. destruction of glucose by bacteria; 1. destruction of glucose by bacteria; 2. conversion of urea to ammonia, by bacteria, with fall in [H+] and precipitation of phosphates; 2. conversion of urea to ammonia, by bacteria, with fall in [H+] and precipitation of phosphates; 3. oxidation of urobilinogen to urobilin and porphobilinogen to porphyrins. 3. oxidation of urobilinogen to urobilin and porphobilinogen to porphyrins.
Preservatives for urine : The preservatives are used (1) to reduce bacterial action; (2) to minimise chemical decomposition, and (3) to decrease atmospheric oxidation of unstable compounds. Preservatives for urine : The preservatives are used (1) to reduce bacterial action; (2) to minimise chemical decomposition, and (3) to decrease atmospheric oxidation of unstable compounds. Formalin, thymol, chloroform, toluene, concentrated HCI and glacial acetic acid are the commonly used urine preservatives. Formalin, thymol, chloroform, toluene, concentrated HCI and glacial acetic acid are the commonly used urine preservatives.
PROPERTIES OF URINE Amount – mL/day Poliuria – diabetes mellitus and insipidus Oliguria – heart failure, nephritis, vomiting, fever Anuria – kidney failure, acute intoxication by heavy metals Color – straw-yellow Pale – poliuria (diabetes insipidud) Dark – jaundice, concentrated urine Red – blood available Green-blue – decay of proteins in the intestine Transparency – transparent Cloudy – pus or mucin available
Density – 1,003-1,035 g/mL Increased density – organic or inorganic substances available (diabetes mellitus) Decreased density – diabetes insipidus Isostenuria – continuously low density in oliguria (kidney failure) pH – Acidic – meat food, diabetes mellitus, starvation, fever Alkaline – plant food, cystitis, pyelitis