Acid and base balance
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Acid and Base Balance. Zhihong Li (李志红) Department of Biochemistry. 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. CO 2. CO 2.

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Acid and Base Balance

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Acid and base balance

Acid and Base Balance

Zhihong Li(李志红)

Department of Biochemistry


The body and ph

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


The body produces more acids than bases

CO2

CO2

The body produces more acids than bases

  • Acids take in with foods.

  • Cellular metabolism produces CO2.

  • Acids produced by metabolism of lipids and proteins.

CO2

H2CO3

CO2+ H2O

Volatile acid

(H+ 15 –20 mol /d)

H2SO4 H3PO4

Uric acid

Lactic acid

Ketone body

(H+ < 0.05 –0.10 mol /d)

Fixed acid


Maintenance of blood ph

Maintenance of blood pH

  • Three lines of defense to regulate the body’s acid-base balance

    • Blood buffers

    • Respiratory mechanism

    • Renal mechanism


Buffer systems

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


Principal buffers in blood

Principal buffers in blood


Bicarbonate buffer

[HCO3-]

pH=pKa+lg

[H2CO3]

H2CO3 H+ + HCO3-

24

= 6.1+ lg

1.2

20

= 6.1+ lg

1

= 6.1+1.3 = 7.4

Bicarbonate buffer

  • Predominant buffer system

  • Sodium Bicarbonate (NaHCO3) and carbonic acid (H2CO3)

  • HCO3- : H2CO3: Maintain a 20:1 ratio


Bicarbonate buffer1

Bicarbonate buffer

  • HCl + NaHCO3↔ H2CO3 + NaCl

  • NaOH + H2CO3 ↔ NaHCO3 + H2O


Phosphate buffer

Phosphate buffer

  • Major intracellular buffer

  • NaH2PO4-Na2HPO4

  • H+ + HPO42-↔ H2PO4-

  • OH- + H2PO4- ↔ H2O + HPO42-


Protein buffers

Protein Buffers

  • Include plasma proteins and hemoglobin

  • Carboxyl group gives up H+

  • Amino Group accepts H+


2 respiratory mechanisms

CO2

CO2

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


3 kidney excretion

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


Rates of correction

Rates of correction

  • Buffers function: almost instantaneously

  • Respiratory mechanisms: take several minutes to hours

  • Renal mechanisms: may take several hours to days


Acid base imbalances

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.


Acid base imbalances1

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. ↓


Acid and base balance

pH

acidosis

alkalosis

metabolic

respiretory

metabolic

respiretory

[HCO3-]↓

PaCO2↑

[HCO3-]↑

PaCO2↓

HCO3-


Compensation

Compensation

  • If underlying problem is metabolic, hyperventilation or hypoventilation can help: respiratory compensation.

  • If problem is respiratory, renal mechanisms can bring about metabolic compensation.


Metabolic acidosis

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).


Compensation for metabolic acidosis

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


Respiratory acidosis

Respiratory Acidosis

  • Carbonic acid excesscaused 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


Compensation for respiratory acidosis

Compensation for Respiratory Acidosis

  • Kidneys eliminate hydrogen ion (H+ and NH4+) and retain bicarbonate ion


Metabolic alkalosis

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


Compensation for metabolic alkalosis

Compensation for Metabolic Alkalosis

  • Hypoventilation to retain CO2 (hence H2CO3↑)

  • Renal excretes more HCO3-, retain H+.


Respiratory alkalosis

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.


Compensation of respiratory alkalosis

Compensation of Respiratory Alkalosis

  • Kidneys conserve hydrogen ion

  • Excrete bicarbonate ion


Mixed acid base disorders

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.


Points

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 ↓


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