1. LECTURE 2:
ACID-BASE BALANCE
Course: Clinical Biochemistry I
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2. Course outlines
• Buffer system
• Metabolic acidosis and alkalosis
• Respiratory acidosis and alkalosis
• Blood gases
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3. Acid and base
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 Acids are H+ donors.
• Bases are H+ acceptors or give up OH- in solution.
• Acids and bases can be:
• Strong – dissociate completely in solution
• Ex: strong acids such as HCl
strong Bases as NaOH
• Weak – dissociate only partially in solution
• Lactic acid, carbonic acid

4. PH of the human body
• The body's balance between acidity and alkalinity is
referred to as acid-base balance or acid-base homeostasis.
• Homeostasis: the body's ability to physiologically regulate
its inner environment to ensure its stability in response to
fluctuations in the outside environment.
• Body PH ranged from 7.35-7.45, any change in blood PH is
critical because it affects protein and enzyme stability and
function.
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6. Acid production in the body
• Human body produces acids more than bases.
• Acids are taken in with foods.
• Also, acids are produced by the metabolism of lipids and
proteins.
• Cellular metabolism produces CO2.
CO2 + H20 ↔ H2CO3 ↔ H+ + HCO3
-
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7. Body control against small change in PH
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The body regulates its PH through two systems:
A) Chemical buffer systems:
1. Bicarbonate buffer system.
2. Phosphate buffer system.
3. Protein buffer system.
B) Physiological buffer systems:
1. Respiratory mechanism.
2. Renal (Kidney excretion) mechanism.
• Body response to acid-base imbalance is called compensation.
• It may be complete if brought back within normal limits, while
If the range is still outside normal it will be partial.

8. Bicarbonate buffer
• It is the main extracellular buffer and plays an important
role in maintaining the pH homeostasis of the blood.
• Carbonic acid (H2CO3, weak acid) dissociates reversibly and
releases bicarbonate ions (HCO3
–, , weak base) and protons
(H+) as follows:
1. Response to an increase in pH (H+ proton donor)
H2CO3 -> HCO3– + H+
2. Response to a decrease in pH (H+ proton acceptor)
H2CO3 <- HCO3– + H+
3. Under normal conditions, the ratio between the HCO3–
and H2CO3 in the blood is 20:1 (HCO3– : H2CO3)
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10. Phosphate Buffer
• It is the major intracellular buffer.
• It consists of dihydrogen phosphate ions (H2PO4
-) as
hydrogen-ion donors (acid) and hydrogen phosphate ions
(HPO4
2-) as hydrogen-ion acceptors (base).
• These two ions are in equilibrium with each other as
indicated by the chemical equation below:
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11. Protein buffer
• It includes hemoglobin, work in blood, and intracellular fluid
(ISF) (i.e. plasma proteins).
• Carboxyl group gives up H+
• Amino Group accepts H+
• Side chains that can buffer H+ are present on 27 amino acids.
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13. Respiratory buffer mechanism
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• Respiratory system does not respond as fast as the chemical
buffer systems.
• Powerful, but only works with volatile acids.
• Doesn’t affect fixed acids like lactic acid.
• CO2 produced by the tissue cells enters the red blood cells and
is converted to HCO3
– ions as follows:
CO2 + H2O↔ H2CO3 ↔ H+ + HCO3
-
• Body pH can be adjusted by changing the rate and depth of
breathing.

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15. Renal buffer mechanism
• It can eliminate large amounts of acid and also excrete base.
• Can conserve and produce bicarb ions.
• Most effective regulator of pH
• If kidneys fail, pH balance fails.
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17. Rate of PH correction
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18. Metabolic acidosis
• It occurs due to a decrease blood bicarbonate level, either due to
increased H production or depletion of bicarbonate.
• Usually, associated with hyperkalemia due to sodium
reabsorption with excess H excretion.
• Causes of increase in H ions production:
1- Diabetic ketoacidosis.
2- Lactic acidosis.
3- Poisoning
• Causes of bicarbonate depletion:
1- Loss of bicarbonate through GIT as diarrhea.
2- Renal tubular acidosis.
• Compensatory mechanism: the respiratory mechanism through
hyperventilation
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20. Metabolic alkalosis
• It occurs due to an increased bicarbonate level.
• It is less serious than metabolic acidosis because bicarbonate
production is slower than H production.
• Usually, associated with hypokalemia due to increased sodium
reabsorption with potassium excretion in the renal tubule.
• Causes:
1. GIT loss of H through vomiting or use of antacids.
2. Renal loss of H as in diuretic therapy.
3. Mineralocorticoid excess.
4. Cashing`s syndrome.
Compensatory mechanism: the respiratory mechanism through
hypoventilation
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22. Respiratory acidosis
• It occurs due to the accumulation of PCO2 in the blood.
• Causes:
1. Chronic lung diseases.
2. Bronchial obstruction.
3. CNS depression.
• Compensatory mechanism: increase bicarbonate
reabsorption in the kidney.
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24. Respiratory alkalosis
• It occurs due to a decrease in PCO2 in the blood.
• Causes:
1. Hypoxia.
2. CNS disorders.
3. Pregnancy.
• Compensatory mechanism: increase bicarbonate excretion
through the kidney.
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27. Estimation of blood PH and gases
• Measurement of blood gases
requires the collection of
arterial blood in a specific
syringe containing heparin.
• Put in ice immediately and no
need for fasting.
• Blood gases analyzer used for
estimation of partial pressure
of oxygen (PO2), partial
pressure of carbon dioxide
(PCO2), and PH.
• Then both the anion gap and
bicarbonate were calculated by
the machine.
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28. END OF LECTURE
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