Acid-Base Balance Disturbances

Acids are produced continuously during normal metabolism. (provide H+ to blood)

H+ ion concentration of blood varies between narrow limits pH of the extracellular fluid = 7.35 – 7.45

Constant H+ concentration within physiological limits is physiologically important to preserve the enzyme activity and metabolism

Hydrogen ion homeostasis

1- Volatile acids: - Carbon Dioxide CO2

2- Nonvolatile acids: 1- Organic acids: - Lactic Acid - Ketone bodies

2- Inorganic acids - Sulphuric Acid - Phosphoric Acid

Sources of acids of blood

H+ is generated during intracellular metabolism from several sources (~ 150 000 mmol H+ is produced every day)

They are continuously neutralized by buffers resulting in no gain of H+ ions

Buffering of acids (H+) in blood

The equilibrium reactions of the buffer system

H+ + HCO3

- H2CO3 (Reaction 1)

H2CO3 CO2 + H2O (Reaction 2)

excreted by the lungs

Accordingly, the addition of H+ causes the equilibrium to be shifted to the right (towards CO2 production and excretion by the lungs)

Carbonic Acid - Bicarbonate Buffer System ( H2CO3 – HCO3

- )

H+ + HCO3- H2CO3 (Reaction 1)

H2CO3 CO2 + H2O (Reaction 2)

First Stage: In this case, pH of blood may be within normal range i.e. not much affectedHowever, amount of buffer (HCO3-) is reduced (COMPENSATED ACIDOSIS)

End Stage:Continuous reduction of H+ will lead to continuous reduction of buffer (HCO3-) will finally end in lowering pH of blood to below normal limits i.e. acidemia (UNCOMPENDSATED ACIDOSIS)

In case of increased H+ production, H2CO3 - HCO3- buffer will reduce H+ as follows:

Carbonic Acid - Bicarbonate Buffer System ( H2CO3 – HCO3

- ) cont.

Disorders of Acid-Base Balance

Increase in H+ concentrations results in a decrease in pH of blood (acidosis) Decrease in H+ concentrations results in an increase in pH of blood (alkalosis)

  Alkalosis or Acidosis describes any abnormality in H+ balance whether :

1- Compensated Alkalosis or Acidosis

No blood pH changes (pH of blood is within normal range). Buffer concentrations are abnormal Compensatory mechanisms try to restore pH to normal if pH is changed. 2- Uncompensated alkalosis or acidosis (alkalaemia or acidaemia)

Abnormal pH of blood (above or below normal range)

Relation between pH & buffer

Henderson-Hasselbach Equation

[HCO3-]

pH = 6.1 + log --------------------------------- pCO2 + 0.225

Assessment of Acid-Base Balance

Normal pH of blood is not an indication of acid-base balance.

Accordingly, in order to assess acid-base balance (status) of blood , we should assess pH & buffer concentration of blood

Blood pH & bicarbonate buffer are to be measured

Bicarbonate buffer measurement:

1- INDIRECTLY From arterial blood sample

Using blood gas analyzer to measure pH & PCO2 in arterial blood Accordingly, bicarbonate in blood can be measured indirectly by applying

Henderson- Hasselbach Equation

2- DIRECTLY From venous blood Samples: used to measure HCO3

- directly

Assessment of Acid-Base Balance cont.

[HCO3-]

pH = 6.1 + log --------------------------------- PCO2 + 0.225

Acid-base

1- Acidosis: - Metabolic - Respiratory

2- Alkalosis: - Metabolic - Respiratory

DIAGNOSIS IS CONFIRMED BY LABORATORY INVESTIGATIONS OF

pH, pCO2 & pO2

Sample: Arterial Blood using Procedure: Blood gas analysis

Acid-Base Balance Disturbances

Metabolic Acidosis

Causes:

 

I- Increased production of H+

 Common Causes of increased H+ (acids) in the blood: 1- Increased endogenous acid production. - Diabetic ketoacidosis (increased ketone bodies in blood) - Lactic acidosis (increased lactic acid in blood). 3-Ingestion of acids (or substance that produces an acid) - Poisons: as salicylate (aspirin) overdose - Methanol ingestion - Ethylene glycol poisoning. 4-decreased acid (H+) excretion by the kidney: in renal failure.

II- Loss of bicarbonate: e.g. in diarrhea

Mechanism:

The more decrease in HCO3- in blood leads to finally end in an decrease in pH as

follows: [HCO3

-]

pH = 6.1 + Log ---------------------- PCO2 X 0.225

Metabolic Acidosis cont.

H+ is increased. It reacts with HCO3-. HCO3- is reduced.CO2 is produced {then exhaled by lungs (increase respiration)

H+ + HCO3- H2CO3 (Reaction 1)

H2CO3 CO2 + H2O (Reaction 2)

CO2 is produced (increased) & then exhaled by lungs (increase respiration)

Compensatory mechanisms of metabolic acidosis

1- Exhaustion of bicarbonate buffer with shift of reactions to CO2 production.

Stimulation of the respiratory centre to eliminate excess CO2 formed

2- Increase in renal acid excretion of H+

Metabolic Acidosis cont

in metabolic acidosis

LABORTORY INVESTIGATION: Sample: Arterial Blood Equipment: Blood Gas Analyzer pH : Low

HCO3: Low

PCO2 : Low: as CO2 is produced then exhaled by lungs by increasing respiration

PO2: Normal

H+ is increased. It reacts with HCO3. HCO3 is reduced.CO2 is produced {then exhaled by lungs (increase respiration)}

H+ + HCO3- H2CO3 (Reaction 1)

H2CO3 CO2 + H2O (Reaction 2)

CO2 is produced (increased) & then exhaled by lungs (increase respiration)

  Causes

Impaired carbon dioxide excretion and thus blood pCO2 increases. caused by any pulmonary (lung) cause resulting in hypoventilation. 

1-Chronic respiratory acidosis: occurs due to chronic obstructive airway diseases. Chronic bronchitis Emphysema Bronchial asthma 2-Acute respiratory acidosis: occurs due to acute respiratory failure Cardiac arrest Neuromuscular disorders of chest wall Depression of the respiratory centre in the brain by: cerebral disease or drugs

Respiratory Acidosis

Mechanism:

The increase in pCO2 in blood leads to an decrease in pH as follows:

[HCO3-]

pH = 6.1 + Log ---------------------- PCO2 X 0.225

Respiratory Acidosis

CO2 is increased in blood (due to respiratory disease). So, the reaction is directed as follows

CO2 + H2O H2CO3 (Reaction 1)H2CO3 H

+ + HCO3- (Reaction 2)

H+ is produced & pH is decreased (acidosis)

in respiratory acidosis

LABORTORY INVESTIGATION: Sample: Arterial Blood Equipment: Blood Gas Analyzer

pH: Low

HCO3: High

PCO2: High (due to the respiratory problem) PO2 : Low (due to the respiratory problem)

CO2 is increased in blood (due to respiratory disease). So, the reaction is directed as follows

CO2 + H2O H2CO3 (Reaction 1)

H2CO3 H+ + HCO3

- (Reaction 2)

H+ is produced & pH is decreased (acidosis)

 

The primary abnormality in metabolic alkalosis is the increased plasma bicarbonate level. (HCO3-). 

Causes:

Less commonIntake of a large amounts of sodium bicarbonate: (if intake is more than 1000 mmol/day)

More commonLoss of H+ (acids) from the body:   1- From the kidneys (increased excretion of acids, H+ ions):

a- Mineralcorticoid (aldeserone) excess b- Severe potassium deficiency

2- From the GIT (increased loss acids, H+ ions): vomiting

Metabolic ALkalosis

Mechanism:

The increase in HCO3- in blood leads to an increase in pH as follows:

[HCO3

-]

pH = 6.1 + Log ---------------------- PCO2 X 0.225

Metabolic ALkalosis

H+ is reduced. So, the reaction is directed as followsCO2 production is increased by respiratory depression (compensatory)

CO2 + H2O H2CO3 (Reaction 1)

H2CO3 H+ + HCO3

- (Reaction 2)

HCO3- is produced & pH is increased (alkalosis)

H+ is reduced. So, the reaction is directed as followsCO2 production is increased by respiratory depression (compensatory)

CO2 + H2O H2CO3 (Reaction 1)

H2CO3 H+ + HCO3

- (Reaction 2)

HCO3- is produced & pH is increased (alkalosis)

in metabolic alkalosis

LABORTORY INVESTIGATION: Sample: Arterial Blood Equipment: Blood Gas Analyzer

pH: High

HCO3: High

PCO2 : High (due to compensatory respiratory depression)

Respiratory Alkalosis

Causes:

The PCO2 is reduced due to: Hyperventilation which may be due to:

1- Respiratory centre stimulation as in cases of: Anxiety Salicylate overdose Cerebral disease (infection, tumour)2- Mechanical ventilation. 

Mechanism The decrease in PCO2 leads to an increase in pH as follows:

  [HCO3

-]

pH = 6.1 + Log ---------------------- PCO2 X 0.225

CO2 is decreased.So, the reaction is as follows to produce CO2:

H+ + HCO3- H2CO3 (Reaction 1)

H2CO3 CO2 + H2O (Reaction 2)

LABORTORY INVESTIGATION: Sample: Arterial Blood Equipment: Blood Gas Analyzer pH: High

HCO3: Low

PCO2 : Low

Respiratory Alkalosis