Acid and Base Balance
description
Transcript of Acid and Base Balance
1
Acid and Base Balance
Zhihong Li(李志红)Department of Biochemistry
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 CO2.
• Acids produced by metabolism of lipids and proteins.
Volatile acid H2CO3 CO2+ H2O
CO2
CO2 CO2
Fixed acidH2SO4 H3PO4
Uric acidLactic acid
Ketone body(H+ < 0.05 –0.10 mol /d)
(H+ 15 –20 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
H2CO3 / HCO3- 35% 18%
HHb / Hb- 35%
HPro / Pro- 7%
H2PO4- / HPO4
2- 5%
Total 42% 58%
8
Bicarbonate buffer• Predominant buffer system
• Sodium Bicarbonate (NaHCO3) and carbonic acid (H2CO3)
• HCO3- : H2CO3: Maintain a 20:1 ratio
= 6.1+1.3 = 7.4
pH=pKa+lgpH=pKa+lg[HCO[HCO33
-- ]]
[H[H22COCO33]]
= 6.1+ lg= 6.1+ lg2424
1.21.2
= 6.1+ lg= 6.1+ lg2020
11
H2CO3 H+ + HCO3-
9
Bicarbonate buffer
• HCl + NaHCO3 ↔ H2CO3 + NaCl
• NaOH + H2CO3 ↔ NaHCO3 + H2O
10
Phosphate buffer
• Major intracellular buffer
• NaH2PO4-Na2HPO4
• H+ + HPO42- ↔ H2PO4
-
• OH- + H2PO4- ↔ H2O + HPO4
2-
11
Protein Buffers
• Include plasma proteins and hemoglobin
• Carboxyl group gives up H+
• Amino Group accepts H+
12
2. Respiratory mechanisms
• Exhalation of CO2
• Rapid, powerful, but only works with volatile acids
• H+ + HCO3- ↔ H2CO3 ↔ CO2 + H20
• Doesn’t affect fixed acids like lactic acid
• Body pH can be adjusted by changing rate and depth of breathing
CO2 CO2
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 (HCO3-) (←HCO3
-)
• Excretion of ammonium ions(NH4+) (→NH4
+)
• 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 alkalosis
respiretory
[HCO3-]↓ [HCO3
-]↑PaCO2↑ PaCO2↓
metabolicmetabolic respiretory
HCOHCO33 --
21
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).
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
25
Respiratory Acidosis
• Carbonic acid excess caused by blood levels of CO2 above 45 mm Hg.
• Hypercapnia – high levels of CO2 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 NH4
+) and retain bicarbonate ion
28
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 CO2 (hence H2CO3↑)
• Renal excretes more HCO3-, retain H+.
31
Respiratory Alkalosis
• Carbonic acid deficit↓
• pCO2 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
34
Mixed acid-base disorders
• Sometimes, the patient may have two or
more acid-base disturbances occurring
simultaneously.
• In such instances, both HCO3- and H2CO3
are altered.
35
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 CO2
– Renal mechanism: eliminate acid, Reabsorption of HCO3-
• Acidosis- blood pH ↓(Causes, Compensation)– Metabolic acidosis: bicarbonate ↓– Respiratory acidosis: carbonic acid ↑
• Alkalosis- blood pH ↑ (Causes, Compensation)– Metabolic alkalosis: bicarbonate↑– Respiratory alkalosis : carbonic acid ↓