Acid and Base Balance 1
2
The Body and ph Homeostasis of ph is tightly controlled Extracellular fluid = 7.4 Blood = 7.35 7.45 < 7.35: Acidosis (acidemia) > 7.45: Alkalosis (alkalemia) < 6.8 or > 8.0: death occurs 3
4
The body produces more acids than bases Acids take in with foods. Cellular metabolism produces CO 2. Acids produced by metabolism of lipids and proteins. CO 2 Volatile acid H 2 CO 3 CO 2 + H 2 O (H + 15 20 mol /d) CO 2 CO 2 Fixed acid H 2 SO 4 H 3 PO 4 Uric acid Lactic acid Ketone body (H + < 0.05 0.10 mol /d) 5
Maintenance of blood ph Three lines of defense to regulate the body s acid-base balance Blood buffers Respiratory mechanism Renal mechanism 6
Buffer systems Take up H + or release H + as conditions change Buffer pairs weak acid and a base Exchange a strong acid or base for a weak one Results in a much smaller ph change 7
Principal buffers in blood in Plasma in RBC H 2 CO 3 / HCO - 3 35% 18% HHb / Hb - 35% HPro / Pro - 7% H 2 PO - 4 / HPO 2-4 5% Total 42% 58% 8
Bicarbonate buffer Predominant buffer system Sodium Bicarbonate (NaHCO 3 ) and carbonic acid (H 2 CO 3 ) HCO 3 - : H 2 CO 3 : Maintain a 20:1 ratio H 2 CO 3 H + + HCO 3 - ph=pka+log [HCO 3 - ] = 6.1+ log 24 1.2 [H 2 CO 3 ] = 6.1+ log 20 1 = 6.1+1.3 = 7.4 9
Bicarbonate buffer HCl + NaHCO 3 H 2 CO 3 + NaCl NaOH + H 2 CO 3 NaHCO 3 + H 2 O 10
Phosphate buffer Major intracellular buffer NaH 2 PO 4 -Na 2 HPO 4 H + + HPO 4 2- H 2 PO 4 - OH - + H 2 PO 4 - H 2 O + HPO 4 2-11
Protein Buffers Include plasma proteins and hemoglobin Carboxyl group gives up H + Amino Group accepts H + 12
2. Respiratory mechanisms Exhalation of CO 2 Rapid, powerful, but only works with volatile acids H + + HCO 3 - H 2 CO 3 CO 2 + H 2 0 CO 2 CO 2 Doesn t affect fixed acids like lactic acid Body ph can be adjusted by changing rate and depth of breathing 13
3. Kidney excretion Most effective regulator of ph The ph of urine is normally acidic (~6.0) H + ions generated in the body are eliminated by acidified urine. Can eliminate large amounts of acid ( H + ) Reabsorption of bicarbonate (HCO 3- ) ( HCO 3- ) Excretion of ammonium ions(nh 4+ ) ( NH 4+ ) If kidneys fail, ph balance fails 14
Rates of correction Buffers function: almost instantaneously Respiratory mechanisms: take several minutes to hours Renal mechanisms: may take several hours to days 15
16
17
Acid-Base Imbalances ph< 7.35: acidosis ph > 7.45: alkalosis The body response to acid-base imbalance is called compensation The body gears up its homeostatic mechanism and makes every attempt to restore the ph to normal level. May be complete if brought back within normal limits Partial compensation if range is still outside norms. 18
Acid-Base Imbalances Acidosis- a decline in blood ph Metabolic acidosis: due to a decrease in bicarbonate. Respiratory acidosis: due to an increase in carbonic acid. Alkalosis- a rise in blood ph Metabolic alkalosis: due to an increase in bicarbonate. Respiratory alkalosis : due to a decrease in carbonic acid. 19
ph acidosis HCO 3 - alkalosis metabolic respiretory metabolic [HCO 3- ] PaCO 2 [HCO 3- ] respiretory PaCO 2 20
Compensation If underlying problem is metabolic, hyperventilation or hypoventilation can help: respiratory compensation. If problem is respiratory, renal mechanisms can bring about metabolic compensation. 22
Metabolic Acidosis Bicarbonate deficit ( ) - blood concentrations of bicarb drop below 22mEq/L (milliequivalents / liter) Causes: Loss of bicarbonate through diarrhea or renal dysfunction Accumulation of acids (lactic acid or ketones) Failure of kidneys to excrete H + Commonly seen in severe uncontrolled DM (ketoacidosis-dka). 23
Compensation for Metabolic Acidosis Hyperventilation: increased ventilation Renal excretion of H + if possible K + exchanges with excess H + in ECF H + into cells, K + out of cells 24
Respiratory Acidosis Carbonic acid excess caused by blood levels of CO 2 above 45 mm Hg. Hypercapnia high levels of CO 2 in blood Causes: Depression of respiratory center in brain that controls breathing rate drugs or head trauma Paralysis of respiratory or chest muscles Emphysema 26
Compensation for Respiratory Acidosis Kidneys eliminate hydrogen ion (H + and NH 4+ ) and retain bicarbonate ion 27
Metabolic Alkalosis Bicarbonate excess - concentration in blood is greater than 26 meq/l Causes: Excess vomiting = loss of stomach acid Excessive use of alkaline drugs Certain diuretics Endocrine disorders: aldosterone Heavy ingestion of antacids 29
Compensation for Metabolic Alkalosis Hypoventilation to retain CO 2 (hence H 2 CO 3 ) Renal excretes more HCO 3-, retain H +. 30
Respiratory Alkalosis Carbonic acid deficit pco 2 less than 35 mm Hg (hypocapnea) Most common acid-base imbalance Primary cause is hyperventilation Hysteria, hypoxia, raised intracranial pressure, excessive artificial ventilation and the action of certain drugs (salicylate) that stimulate respiratory centre. 32
Compensation of Respiratory Alkalosis Kidneys conserve hydrogen ion Excrete bicarbonate ion 33
Mixed acid-base disorders Sometimes, the patient may have two or more acid-base disturbances occurring simultaneously. In such instances, both HCO - 3 and H 2 CO 3 are altered. 35
Anion Gap (AG) This is the difference between primary measured cations (sodium Na+ and potassium K+) and the primary measured anions (chloride Cl- and bicarbonate HCO3-) in serum. This test is most commonly performed in patients who present with altered mental status, unknown exposures, acute renal failure, and acute illnesses. 36
The normal value for the serum anion gap: With potassium The anion gap is calculated by subtracting the serum concentrations of chloride and bicarbonate (anions) from the concentrations of sodium and potassium (cations): = ([Na + ] + [K + ]) ([Cl ] + [HCO = 20 meq/l Without potassium (daily practice) Because potassium concentrations are very low, they usually have little effect on the calculated gap. Therefore, omission of potassium has become widely accepted.this leaves the following equation: = [Na + ] - ([Cl ] + [HCO normal AG= 6-12 meq/l. 37
For the urine anion gap, the most prominently unmeasured anion is ammonia. Healthy subjects typically have a gap of 0 to slightly normal (< 10 meq/l). A urine anion gap of more than 20 meq/l is seen in metabolic acidosis when the kidneys are unable to excrete ammonia (such as in renal tubular acidosis). If the urine anion gap is zero or negative but the serum AG is positive, the source is most likely gastrointestinal (diarrhea or vomiting). 38
39
Points Blood = 7.35 7.45; < 7.35: Acidosis, > 7.45: Alkalosis Three lines of defense to regulate the body s acid-base balance Blood buffers: Bicarbonate buffer, Phosphate buffer, Protein Buffers Respiratory mechanisms: Exhalation of CO 2 Renal mechanism: eliminate acid, Reabsorption of HCO - 3 Acidosis- blood ph (Causes, Compensation) Metabolic acidosis: bicarbonate Respiratory acidosis: carbonic acid Alkalosis- blood ph (Causes, Compensation) Metabolic alkalosis: bicarbonate Respiratory alkalosis : carbonic acid 40
END 41