acid-base balance disturbances
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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 (H2CO3)
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 = No pH change
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) Buffer concentrations are abnormal
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 & HCO3-
Sample: Arterial Blood using Procedure: Blood gas analysis
Acid-Base Balance Disturbances
1-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). 2-Ingestion of acids (or substance that produces an acid) - Poisons: as salicylate (aspirin) overdose - Methanol ingestion - High protein diet. 3-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
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
CausesImpaired 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
2-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)
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)
Compensation: by kidney via
↑ HCO3- reabsorption
↑ H+ excretion
The primary abnormality in metabolic alkalosis is the increased plasma bicarbonate
level. (HCO3-). Causes:Less common1- Intake of a large amounts of alkali as sodium bicarbonate: (if intake is more than 1000 mmol/day)More common2- Loss 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 and gastric wash
3-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)
LABORTORY INVESTIGATION: Sample: Arterial Blood Equipment: Blood Gas Analyzer pH: High
HCO3: High
PCO2 : High (due to compensatory respiratory depression)
Compensation: by lungs via inhibition of respiration which leads to ↑ PCO2
4- 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- pulmonary embolism.3- Fevers4- Hepatic failure
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)
Compensation : by kidney by ↓ HCO3 reabsorption and ↓ H+ secretion
LABORTORY INVESTIGATION: Sample: Arterial Blood Equipment: Blood Gas Analyzer pH: High
HCO3: Low
PCO2 : Low
Respiratory Alkalosis
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