Acid Base Disorder

role of lung in acid – base balance

• ACIDS are proton (H+) donors

• BASES are proton (H+) acceptors

• Ionic substances dissociate in solutions

• STRONG ACIDS dissociate completely

• WEAK ACIDS dissociate incompletely

• pH is a measure of H+ in solution

• Normal blood pH necessary for normal enzyme activity

• 7.35 - 7.45

• Below 7.35 = acidosis

• Above 7.45 = alkalosis

• Changes in pH due to by-products or end products of metabolism.

• [H+]= 35- 40meq/L or nmol/L’

• Pco 2 [approximately 1.1 kPa (8 mmHg) higher in venous blood] and

• Po2 [approximately 7.5 kPa (56 mmHg) lower in venous blood].

PRODUCTION OF ACID

• Volatile acid• Carbonic acid

Non-volatile acid• phosphoric acid (nucleoproteins) • lactic acid (anaerobic glycolysis)• sulphuric acid (S-AA)• ketones and fatty acids (fat metabolism

Animal protein Acidic

PRODUCTION OF BASE

• Normally negligible.

• HCO3 by metabolism of lactate and citrate.

• NH3 AA metabolism urea base

• Veg diet salts of organic acid (sodium lactate) + utilize H+.

• Alkalizing effect on the body.

• Urine excretion neutral or slight alkaline.

REGULATION OF BLOOD pH

• Chemical Buffer SystemsIntracellular Extracellular

Bicarbonate buffer system – Na HCO3/ H2CO3Phosphate buffer system – Na2 HPO4/ NaH2PO4Protein buffer system – Na+ Protein/ ProteinRBC buffer – KHb/ HHb

• Respiratory System

• Renal Mechanisms

CHEMICAL BUFFER SYSTEMS

• Molecules that resist pH change.• Bind or release H+ to control pH.• Limited in ability to control pH.• Weak acids and its conjugate salt.

• Bicarbonate buffer system is most important ECF.

• Phosphate & protein buffer systems are important ICF.

BICARBONATE BUFFER SYSTEM

• (40-50%) Pka – 6.1Carbonic acid Hydrogen ions + Bicarbonate ions

H2CO3 H+ + HCO3

PHOSPHATE BUFFER SYSTEM

• (Pka – 6.8)Dihydrogen phosphate monohydrogen phosphate

H2PO4-3 HPO4

-2

PROTEIN BUFFER SYSTEM

Amino Acid

NH3-C-COOH

R

Some can accept H+ Some can donate H+

Weak base Weak acid

RESPIRATORY MECHANISM FOR pH REGULATION

• Lungs excrete 20,000 – 40,000 mEq H+/day.• By regulating conc. of carbonic acid.• About 300 L CO2/ day endanger, acid base

equilibrium.• All CO2 eliminated in the expired air.• Respiratory centers in medulla sensitive to pH

change. pH hyperventilation blow off CO2

H2CO3.

Hb AS A BUFFER

• Tissue level Hb binds to H+ & help to transport CO2 as HCO3

-

• Lungs Hb combines with O2 H+ removal

2H + 2HCO3- 2H2CO3 2H2O + 2CO2 (Exhaled)

Transport of CO2 through the mediation of Hb

2CO2 + 2H2O

2H2CO3

2H+ + 2HCO3-

4O2

Hb2H+

C.A

2CO2 + 2H2O

Carbonic Anhydrase

2H2CO3

2HCO3- 2H+

4O2

HbO2

CO2

Henderson-Hasselbach Equation

• Demonstrates interrelationship between – Carbonic acid– Bicarbonate– pH

pH = pK + log [HCO3-] /[H2CO3]

Kassirer-Bleich equation

• [H+] = 24 x PCO2/ [HCO3-]

• Can be used to calculate any component of buffer system provided other 2 components are known

Fundamental acid base disorders

• Respiratory Disorder – first affect pCO2

• Metabolic disorder – first affect HCO3-

DISORDERS OF ACID BASE BALANCE

Acidosis blood pH

• Respiratory acidosis - CO2

• Metabolic acidosis - HCO3

Alkalosis blood pH

• Respiratory alkalosis CO2

• Metabolic alkalosis HCO3

RESPIRATORY ACIDOSIS

• Low pH due to high pCO2

pH = 6.1 + log [HCO-3]

[PCO2] x 0.3

RESPIRATORY ACIDOSIS

Causes: in alveolar ventilation [Emphysema, pneumonia]• COPD like Asthma• Overdose of Respiratory depressant drugs• Trauma to medulla oblongata• Cystic fibrosis

Compensation: - Kidney retains bicarbonate

RESPIRATORY ALKALOSIS

• High pH due to low pCO2

pH = 6.1 + log [HCO3-]

[PCO2 ] x 0.3

RESPIRATORY ALKALOSIS

Caused by• Hyperventilation• Hysterical over breathing incracranial pressure• Brain stem lesions stimulate respiratory centre• High altitude, Hypoxia – stimulate Resp. centre• Anxiety• Drugs – Salicylate overdos

Compensation – Kidney secrete bicarbonate

ACID-BASE DISORDER H+ PH HCO3 PCO2 BODY COMPENSATION

Metabolic acidosis

↑ ↓ ↓↓  ↓ Increased breathing rate (hyperventilation) to increase CO2

elimination

Metabolic alkalosis

↓ ↑ ↑↑  ↑ Slowed breathing (hypoventilation) to decrease CO2 elimination

Respiratory acidosis

↑ ↓ ↑  ↑↑ Kidney increases production of HCO3-

and excretion of H+ (acid)

Respiratory alkalosis

↓ ↑ ↓  ↓↓ Decreased production of HCO3- and

excretion of H+

↑ increased level; ↑↑ greatly increased level; ↓ decreased level; ↓↓ greatly decreased level

Results seen:

Electrolytes refers to a group of four tests: • Na+ (sodium), • K+ (potassium), • Cl- (chloride) and • bicarbonate (total CO2 content).An anion gap can be calculated from the electrolytes and

provides a clue to the cause of the acid/base imbalance.• Depending on the suspected cause, a number of other

tests may be ordered: CMP (comprehensive metabolic panel), ketones in blood and ketones in urine, lactate, salicylates, ethylene glycol, and methanol, to name a few.

Anion Gap

• AG= [Na+] – ([HCO3] + [Cl-])

• Normal anion gap is 7 +/- 4

• Is the unmeasured anion concentration

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