Page 27

LSS Urinary System   

Alexandra Burke-Smith

Pathologically – hypoxia, carbohydrates + fats, diabetes, ketoacids Volatile acids – produced from metabolism of carbohydrates + fats, result in carbon dioxide production which is lost through respiration Non-volatile acids – derived from metabolism of proteins, result in H+ ions that are excreted by the kidneys

Renal H+ excretion  The kidneys must excrete 50-100mmol of non-carbonic acids generated each day  Involves different mechanisms at various parts of the nephron (PCT, thick ascending LoH, CD)  Overview: o All bicarbonate ions filtered into urine reabsorbed o Secreted H+ ions are then excreted, either with:  Filtered buffers e.g. phosphates + creatinine  Manufactured buffer e.g. ammonia; manufactures from glutamine in the PCT Renal H+ pumps  In the PCT, ions are secreted into the lumen by Na/H exchanger o Bicarbonate ions are then returned to the systemic circulation by the Na/HCO3 cotransporter  In the collecting duct, the luminal pump is mediated by active H+-ATPase and CL-HCO3 exchanger in basolateral membrane  The net effect is the excretion of 1H+ ion (buffered by phosphate) + addition of 1 HCO3- to the plasma Regulation of excretion To increase or decrease H+ secretion, there are both primary + secondary mechanisms (secondary stimuli/mechanisms are not directed at maintaining acid-base balance) 

To increase H+ secretion: o Primary – decrease plasma bicarbonate concentration + increase pp of arterial CO2 o Secondary – increase filtered load of bicarbonate, decrease ECF volume, increase Ang II, increase aldosterone, hypokalaemia To decrease H+ secretion o Primary – increase plasma bicarbonate concentration + decrease pp of arterial CO2 o Secondary – decrease filtered load of bicarbonate, increase ECF volume, decrease aldosterone + hyperkalaemia

Bicarbonate reabsorption  

~80% is reabsorbed in the PCT, with the remaining being reabsorbed in the thick ascending loop of Henle + outer medullary collecting duct – this is known as segmental reabsorption There is a net reabsorption of 1 filtered Na+ and 1HCO3-

New Bicarbonate formation    

Reason for new formation = bicarbonate reabsorption < bicarbonate lost during buffering of non-volatile acids In the liver, amino acids are broken down into glutamine + urea; this glutamine is converted to ammonium ions + alpha-ketoglutarate in the kidney The alpha-ketoglutarate is then converted to bicarbonate ions in the kidney

Ammonium excretion  Ability to excrete H+ ions as ammonium adds important degree of flexibility to renal acid-base regulation  NH3 produced in tubular cells predominantly from glutamine 27

Alex's Urinary  

Alex's Urinary

Advertisement