Handouts 1st Nov Part 2

Regulation of Extracellular Fluid Oxmolarity and Sodium Concentration

Regulation of Extracellular Fluid Osmolarity and Sodium ConcentrationConcept of Osmolarity/OsmolalityOsmole: Number of osmotically active particles in a solutionOsmolarity/osmolality: Osmolal concentration of a solution (in oSm/L and oSm/kg)In the body, 80% of total osmolarity of plasma and intersitial fluid is due to sodium and chlorideOsmolarity of plasma: 282 mOsm/LOsmolarity of Interstitial fluid: 281 mOsm/LOsmolarity of Intracellular fluid: 281 mOsm/LRole of KidneysKidneys excrete excess water by forming dilute urineExcess water when plasma osmolarity < 282 mOsm/LKidneys conserve water by excreting concentrated urine Too little water when plasma osmolarity > 282 mOsm/L

Excretion of Concentrated UrineRequires:High Levels of ADHHigh osmolarity of renal medullary interstitial fluidIf above two conditions are present: Water will flow out of the permeable tubular membrane via osmosis (due to osmole gradient between tubules and renal medulla)Water will be carried by blood vessels back into the blood

Formation of concentrated urine

Anti-Diuretic HormoneDiuresis = urine productionAnti-diuretic hormone = decreased urine productionAction of ADH: Increased permeability to water molecules at the Distal collecting tubuleCortical collecting tubuleInter medulary collecting duct Hyperosmotic Renal MedullaDue to pumping of ions (sodium, chloride, potassium) and urea into interstitium (remember the ascending loop of henle!)Enhanced by countercurrent mechanism Maintained by countercurrent exchange in the surrounding blood vesslesExcreting Dilute UrineNo ADHWater does not flow out of distal tubules Resulting urine is dilute

Renal regulation of PotassiumPotassium HomeostasisExtracellular potassium = 4.2 mEq/LHyperkalaemia = Too much potassiumCan result in cardiac arrest or fibrillationHypokalaemia = Too little potassium Can result in cardiac arrhythmias

Role of kidneysPotassium will always be filtered through glomerular membranesHyperkalaemia Secrete extra potassiumHypokalaemia Absorb potassium back Renal Potassium ExcretionDetermined by:Potassium filtrationPotassium reabsorptionPotassium secretion65% of filtered potassium will be reabsorbed in the proximal tubule25% to 30% of filtered potassium will be reabsorbed in the loop of HenleOnly site at which potassium secretion/reabsorption can be regulated is at the principal cells of the distal and cortical collecting tubules

Renal Potassium Excretion

On normal days, potassium is secretedPotassium is lost because you consume potassium through foodWhat happens at the principal cells

Secretion of potassiumUptake into principal cells from renal intersitium by active transportSecretion into tubules from principal cells by passive diffusionFactors stimulating potassium secretionIncreased extracellular potassiumAldosteroneIncreased tubular flow rateAcid Base RegulationAcid-Base BalanceExtracellular fluid contains H+ ionsConcentration of H+ ions determine pH of extracellular fluidNormal pH of extracellular fluid = 7.4 (range from 7.35 to 7.45)Acidosis = pH < 7.35Alkolosis = pH > 7.45

Acid Base equations of body fluids

AcidBase

When an acid is added, carbon dioxide is produced

When a base is added, bicarbonate salts are used to produced weak basesCarbon dioxide in the blood is used to replace bicarbonate salts

3 systems of regulating H+Chemical Acid-base buffers of body fluidsBody stores of BicarbonatePhosphateProteins (intracellular)LungsKidneysLungsIncrease in breathing (ventilation) eliminates carbon dioxide (acid)Decrease in breathing (ventilation) decreases elimination of carbon dioxide Works fast Can respond to changes in minutesRenal Control of Acid-BaseExcreting acidic or basic urineAcidic urine: Large quantities of H+In acidosis, kidneys reabsorb filtered HCO3- and produce more HCO3-Basic urine: Large quantities of HCO3-In alkalosis, kidneys do not reabsorb any filtered HCO3-3 methods:Secretion of H+Reabsorption of filtered HCO3-Production of HCO3-Secretion and reabsorption of HCO3-80 90% of HCO3- reabsorption and secretion occurs in proximal tubule10% of HCO3- is reabsorp in thick ascending loop of henleRemainder takes place in distal tubule and collecting ductImportant concept: HCO3- reabsorption and H+ secretion always occurs simultaneouslySecretion and reabsorption of HCO3-

Creation of new bicarbonatePhosphate buffer

Ammonia buffer