acid-base balance in the body

Transcription

1 acid-base balance in the body 1 why acid/base balance is important a major homeostatic challenge is keeping [H+] (ph) of body fluids at an appropriate level. maintenance of acidbase balance is of critical importance to normal cellular function. ex/ a diet containing a large amount of protein, causes cellular metabolism to produce more acids than bases, which tends to acidify blood. 2 ph values in the body arterial blood: 7.40 venous blood/interstitial fluid: 7.35 intracellular fluid:

2 ph values alkalosis: arterial blood ph > 7.45 acidosis: arterial blood ph < review time 5 acids proton (H + ) donors in solution: dissociate to release free H + types strong acids weak acids 6

3 strong acid completely dissociate to free all of its H+ have a large effect on ph 7 weak acid partially dissociates liberates fewer H + ions less effect on fluid ph used to prevent dramatic changes in ph important buffers 8 bases H + acceptors: in solution, dissociate + pick up free H + NaOH Na + + OH - OH - then binds with H + in solution to form water OH - + H + H2O 9

4 strong bases/weak bases strong base: quickly pick up any and all free H + big impact on ph weak bases: slow to accept H + little impact on ph prevents dramatic ph changes (buffers) ex/ (HCO3 - ), ammonia (NH3) 10 abnormalities of acid-base balance 11 abnormalities of acid-base balance Respiratory acidosis alkalosis Metabolic acidosis alkalosis 12

5 respiratory acidosis (pco2 > 45 mm Hg) most common cause of acid-base imbalances cause: decrease in ventilation or gas exchange characterized by falling blood ph rising PCO2 13 respiratory alkalosis (pco2 < 35 mm Hg) a common result of hyperventilation due to stress pain 14 causes of metabolic acidosis ingestion of too much alcohol excessive loss of HCO3 accumulation of metabolic byproducts shock ketosis (diabetic crisis) starvation kidney failure 15

6 metabolic alkalosis much less common indicated by rising blood ph and HCO3 caused by vomiting intake of excess base (e.g., antacids) 16 depression of CNS coma death excitation of nervous system muscle tetany extreme nervousness convulsions respiratory arrest consequences of acid/base imbalances 17 how is ph regulated in the body? 18

7 1 2 ph regulation 3 1. chemical buffer systems 2. respiratory mechanisms 3. renal mechanisms chemical buffers: first line of defense because they act quickly for strong acids or bases utilize weak acids/ bases 1. bind up H + when ph 2. release H + when ph 20 major chemical buffers work together to resist changes in ph in ALL body compartments 1. bicarbonate system 2. phosphate system 3. protein system 21

8 a. bicarbonate system 22 1a. bicarbonate system important buffer in the ECF uses a mixture of H2CO3 carbonic acid or NaHCO3 sodium bicarbonate 23 how does the bicarbonate system work? 24

9 what happens when we introduce a strong acid like HCl? HCl strong acid + NaHCO3 H2CO3 + weak base weak acid NaCl salt 25 weak acid (H2CO3) changes ph only slightly partially dissociates to form H + + HCO3- if need arises, H2CO3 can split into CO2 + H2O CO2 stim. ventilation CO2 removed ph raised 26 what happens when we introduce a strong base? NaOH + strong base H2CO3 weak acid NaHCO3 + weak base H2O water 27

10 bicarbonate reserve quantity/availability of HCO3- ion in body levels closely regulated by kidneys if too much acid enters and all HCO3 - is tied up buffering power is lost so system has limited effectiveness 28 what about carbonic acid? [H2CO3] is 1/20th but its supply is limitless obtaining it is not a problem why? 29 b. phosphate buffer system 30

11 1b phosphate buffer important buffer in ICF similar to bicarbonate system; composed of 2 salts: sodium dihydrogen phosphate NaH2PO4 weak acid sodium monohydrogen phosphate Na2HPO4 weak base 31 adding strong acid/base if a strong acid is introduced HCl + Na2HPO4 NaH2PO4 + NaCl strong acid weak base weak acid salt if a strong base is added: NaOH + NaH2PO4 Na2HPO4 + H2O strong base weak acid weak base water 32 c. protein buffer system 33

12 protein buffer system uses intracellular and plasma proteins recall proteins have carboxyl group amine group R H2N C H C O OH free carboxylic group R H2N C C H O O + H + weak acid part free amine end + H3N weak base part R C H C O OH 36

13 amphoteric molecules + H3N weak base part R C C H O O H + + weak acid part 37 respiratory control of ph 38 acidosis short term control of blood ph regulates CO2 to modify acid content of blood recall equation: CO2 + H2O H2CO3 H + + HCO3 - when blood CO2 is high, equation is shifted to right 39

14 alkalosis when blood CO2 is low equation pushed to left CO2 + H2O H2CO3 H + + HCO3-40 however respiratory system impairment can cause acid-base imbalances Hypoventilation respiratory acidosis Hyperventilation respiratory alkalosis 42

15 renal control of ph 43 purpose 1. slow correction of acid/ base imbalances and long term control of ph 2. eliminate metabolic acids 3. renew chemical buffers 44 most important renal mechanism for controlling ph A. reabsorbing or generating new bicarbonate (HCO3-) ions (from filtrate) ph B. excreting bicarbonate (HCO3-) ions (into filtrate) ph in blood happens in Type A cells 45

16 A. reabsorption of HCO3 - (via renal secretion of H + ) 46 reabsorption of HCO3 - Na + H + H + H + HCO 3 Na + Na + H 2 O H + Na + HCO 3 + H + H 2 CO 3 H + URINE tubule lumen CA CO 2 H + A T P A CO 2 + CA H 2 O H 2 CO 3 H + + HCO 3 3Na + tubule cell HCO 3 Cl 2K + peritubular cap 47 generating new bicarbonate dietary H + must be balanced by generating new HCO3 most filtered HCO3 is used up before filtrate reaches collecting duct 2 mechanisms exist in PCT and CD (in type A cells) to do this both involve renal excretion of acid 1. H + 2. NH4 + 48

17 1. active secretion of H+ HPO 4 2 HPO 4 2 HPO 4 2 HPO 4 2 HPO H + H 2 PO 4 URINE tubule lumen A T P A CO 2 + H 2 O CA (NEW) H 2 CO 3 H + + HCO 3 tubule cell HCO 3 Cl peritubular cap ammonium ion excretion involves metabolism of glutamine in PCT cells each glutamine produces 2 NH4 + and 2 new HCO3 HCO3 moves to the blood and NH4 + is excreted in urine ammonium excretion GLUTAMINE 2NH 4 + 2NH 4 + Na + URINE tubule lumen A Deamination, oxidation, and acidification (+H + ) GLUTAMINE 2NH 4 + (NEW) 2 HCO 3 HCO 3 A T P 3Na + tubule cell HCO 3 Na + 2K + peritubular cap 51

18 B. excreting (secreting) bicarbonate ion secretion when the body is in alkalosis type B cells secrete HCO3 reclaim H+ and acidify the blood mechanism is opposite of bicarbonate ion reabsorption process by type A intercalated cells 52 secretion of HCO3 - H + HCO 3 + H + H 2 CO 3 H 2 O HCO 3 HCO 3 URINE tubule lumen CA CO 2 B CO 2 + CA H 2 O H 2 CO 3 HCO 3 H + H + A T P Cl H + Na + tubule cell peritubular cap 53