Acid - base equilibrium

Transcription

1 Acid base equilibrium

2 ph concept ph = log [H + ] ph [H+] mmol/l D = 90 mmol/l 2 10 mmol/l D = 9 mmol/l 3 1 mmol/l 2

3 ph = log [H + ] 3

4 ph ph = log [H + ] ph of capillary blood norm: 7,35 7,45 Sorensen unit 44,7 nmol/l 35,5 nmol/l pathology: 6,9 7,7 Sorensen unit 126 nmol/l 20 nmol/l ph [H + ] 7,4 7,1 (0,3 unit) nmol/l (D = 40 nmol/l) 7,4 7,7 (0,3 unit) nmol/l (D = 20 nmol/l) 4

5 Buffer solutions Mixtures: weak acid and its salt with strong base CH 3 COOH + CH 3 COO weak base and its salt with strong acid NH 3 aqueous + NH 4 + two salts of polyprotic acids H 2 PO 4 + HPO 4 2 5

6 Henderson Hasselbalch equation AH A + H + K a = [A ] [H + ] [AH] log K a = log [A ] [H + ] [AH] because log K a = pk a pk a = log [A ] [H + ] [AH] 6

7 Henderson Haselbalch equation AH A + H + pk a = log [A ] [H + ] [AH] pk a = log [A ] log [H + ] + log [AH] log [H + ] = pk a + log[a ] log [AH] log [H + ] = ph ph = pk a + log [A ] [AH] 7

8 Henderson Hasselbalch equation ph = pk a + log [A ] [AH] ph of the buffer mixture depends on: kind of the acid ratio of the concentrations of buffer components ph of the buffer mixtures does not change with dilution of the solution! 8

9 Buffer solutions mechanism of action acetate buffer: CH 3 COOH i CH 3 COO at equilibrium: CH 3 COOH + H 2 O CH 3 COO + H 3 O + acetic acid is a weak acid, acetate ions come entirely from salt (acetate) CH 3 COOH CH 3 COO + H + add some amount of strong acid: +H + CH 3 COOH CH 3 COO + H + strong acid removes weak acid from its salt 9

10 Buffer capacity b buffer capacity Dc b = DpH Dc amount of strong acid or strong base added to the buffer solution mol/l DpH observed change of ph Buffer capacity depends on concentration of components: increases with their increment decreases with dilution of the buffer 10

11 Buffer capacity Dc b = DpH ph = pk a + log [A ] [AH] is the highest when ph = pk Excess of acid in the buffer better buffering of bases Excess of a salt better buffering of acids. During addition of acid or base buffer capacity decreases is equal to zero when whole salt in the buffer is converted to weak acid or whole weak acid is converted to salt. 11

12 Intracellular ph cytoplasm 6,0 mitochondria endoplasmatic reticulum 7,0 7,4 nucleus average ph of intracellular fluid: 6,95 (112 nmol/l) differences in ph of extracellular fluid between cells from different organs: erytrocytes 7,20 renal tubular epithelial cells 7,32 cells of skeletal muscles. 6,9 12

13 Mechanisms of ph regulation in organism Organ s regulation regulation by kidney regulation by lungs regulation by bones Buffer regulation proteinate buffer Hproteins Proteinates phosphate buffer H 2 PO 4 HPO 4 2 bicarbonate buffer HCO 3 H 2 CO 3 Hemoglobin buffer 13

14 Hemoglobin buffer is the most important proteinate buffer 1. Hemoglobin makes about ¾ of all blood s protein 2. Hemoglobin has an acidic character because of the presence of majority of acidic groups of hem over basic groups of globins. Therefore, hemoglobin (Hb) has a large capacity for base binding. 3. Acidity of hemoglobin changes significantly with degree of oxidation. Hemoglobin transports not only oxygen from lungs to cells but also carbon dioxide from cells to lungs. 14

15 Proteinate bufferhemoglobin buffer Hemoglobine buffer is an intracellular buffer. The Hb and oxygen connection is reversible and the Hb molecule changes its conformation when it binds O 2. HHb (Hb without oxygen) and HHbO 2 (Hb connected with oxygen, oxyhemoglobin) differ in their ability to donate or accept H + ions. HbO 2 + H+ HHb + O 2 In tissues stronger acid weaker acid HHb + O 2 + HCO 3 HbO 2 + H 2 O + CO 2 In Lungs Buffering properties depend on the equilibrium between oxyhemoglobin and hemoglobin. Hemoglobin is less acidic than oxyhemoglobin. With increasing CO 2 pressure and H + concentration, the amount of HHb increases and buffering capacity increases as well. 15

16 Proteinate buffer In acidic environment: carboxyl groups of amino acids do not dissociate basic groups (amine, imidasole) are proton acceptors In basic environment: carboxyl groups are proton donors and they neutralize hydroxide ions in weak basic environment ph 7,4, proteins are in form of anions. concentration of proteins in the blood is 16 meq/l Buffer capacity of proteinate buffer is: 5 mmols/ l/one ph unit 16

17 Proteinate Buffer hemoglobin buffer I system HHbO 2 KHbO 2 II system HHb KHb hemoglobin is an important buffer in blood hemoglobin makes ¾ of whole proteins in the blood. hemoglobin is acidic because of excess of acidic groups of heme over basic groups from globin. acidity of hemoglobin can change depending on oxygenation It is indispensable for full effectiveness of carbonate buffer in an open system 17

18 Phosphate buffer important urinary buffer phosphate buffer in blood : H 2 PO 4 HPO H + KH 2 PO 4 = 6,2 x 10 8, pk 2 = 7,21 HPO 2 4 / H 2 PO 4 = 4/1 phosphate buffer in urine: HPO 2 4 / H 2 PO 4 = 1/4 HPO H + H 2 PO 4 main intracellular buffer optimal pk for this buffer is 6,8. in phosphate buffer in urine (ph about 6,0) ratio of hydrogen phosphate to dihydrogen phosphate is 1/4 Difference between ratios of phosphates in blood and urine is because hydrogen phosphate of urine binds protons secreted by distal tubules of kidneys and is converted to dihydrogen phosphate. 18

19 Bicarbonate buffer an extracellular buffer; in equilibrium with atmospheric air. The most important buffer sytem in the blood is: HCO 3 /H 2 CO 3 organism removes by lungs a product of dehydration of carbonic acid carbon dioxide. This buffer acts in an open system. H 2 CO 3 i CO 2 dissolved in water phase remain at equilibrium with CO 2 which is in gasous phase. CO 2 in blood circulating in lungs remains at equilibrium with CO 2 in lung vesicles. buffer in an open system has several times higher capacity in comparison to a closed system 19

20 Bicarbonate buffer is responsible for the physiological ph of blood, CO 2 + H 2 O H 2 CO 3 H + + HCO 3 99% 1% gas liquid liquid liquid liquid % expresses amounts in equilibrium state ph = pkh 2 CO 3 + log [A] [ AH] ph = pk H2 CO 3 + log [ HCO 3 ] [CO 2 ] 20

21 Gasometric parameters of blood (physiological levels): pk H2 CO 3 = 6,11 [HCO 3 ] = 24 mmol/l [CO 2 ] = a x p a coefficient of solubility of CO 2 in plasma a = 0,225 mmol/l p partial pressure of CO 2 in lung vessels pco 2 = 5,32 kpa 21

22 Bicarbonate buffer CO 2 + H 2 O H 2 CO 3 H + + HCO 3 99% 1% ph = pk H2 CO 3 + log [ HCO 3 ] [CO 2 ] pkh 2 CO 3 = 6,11 [HCO 3 ] = 24 mmol/l [CO 2 ] = a x p a = 0,225 mmol/l pco 2 = 5,32 kpa ph of bicarbonate buffer in blood 24 ph = 6,11 + log = 7,4 0,225 x 5,32 22

23 Bicarbonate buffer CO 2 + H 2 O H 2 CO 3 H + + HCO 3 99% 1% Bicarbonate buffer mechanism of action +H + CO 2 + H 2 O H 2 CO 3 H + + HCO 3 The kidneys regulate HCO 3 ion concentration in blood plasma and protect the organism against metabolic acidosis. The role of kidneys is: reabsorption of HCO 3 ions in kidney tubules excretion of HCO 3 ions when their level changes in extracellular fluid regeneration of lost HCO 3 ions or in reactions with H+ derived from water (in proximal tubular cells and collecting tubule cells) 23

24 Bicarbonate buffer most impotant buffer in blod, in acidbase balance acts in open system in normal conditions HCO 3 /CO 2 is 20:1 metabolic component ph = pk H2 CO 3 + log [HCO 3 ] pco 2 x a respiratory component 24

25 Disturbances of acidbase balance ph = pk H2 CO 3 + log [HCO 3 ] pco 2 x a metabolic component respiratory component Too much acid in the body resulting from accumulation of acid or depletion of alkaline reserves leads to acidosis abnormally low ph. May be caused by : diabetic ketoacidosis, lung disease, kidney disease. The condition opposite to acidosis is alkalosis ph is to high due to excess base or insufficient acid in the body. 25

26 Disorder ph HCO 3 pco 2 Metabolic acidosis Respiratory acidosis Metabolic alkalosis Respiratory alkalosis primary changes secondary changes ph = pk H2 CO 3 + log [HCO 3 ] pco 2 x a 26

27 Bicarbonate buffer To 1 liter of plasma 10mmols of strong acid were added. Calculate ph change when the system is: open closed. pk H2 CO 3 = 6,11; [HCO 3 ] = 24 mmol/l; a = 0,225 mmol/l /kpa pco 2 = 5,32 kpa; ph=7,4 Open system Closed system 24 HCO H + > 10 CO H 2 O + 14 HCO ph = 6,11 + log = 6,11 + log = 7,17 0,225 x 5,32 1,2 24 HCO H + > X 10 CO H 2 O + 14 HCO ph = 6,11 + log = 6,11 + log = 6,2 1, ,2 27

28 Red blood cells role in acidbase regulation of blood. Reaction in capillary tissues Erythrocyte O 2 HbO 2 + H + HHb + O 2 To tissues CO 2 + H 2 O H 2 CO 3 H + + HCO 3 CO 2 From tissues carbonic anhydrase Cl Cl Offset chloride HCO 3 28

29 Red blood cells role in blood s acidbase regulation; reaction in lung capillaries O 2 from lungs Erythrocyte HHb + O 2 HbO 2 + H + HCO 3 + H + H 2 CO 3 H 2 O + CO 2 Cl carbonic anhydrase HCO 3 Cl CO 2 Exhaled 29

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