Acid-Base and Blood Gases

Acid

A substance that donates a proton (H⁺) in a chemical reaction

Base

A substance that accepts a proton (H⁺) in a chemical reaction

Buffer

A combination of a weak acid and its conjugate base that resists changes in pH when acid or base is added

Primary buffer system of blood

Carbon dioxide–carbonic acid–bicarbonate (CO₂/H₂CO₃/HCO₃⁻) system responsible for ~60% of buffering capacity

Secondary buffer systems

Phosphate, proteins, and hemoglobin buffer systems that assist in pH regulation

pH

The negative logarithm of hydrogen ion concentration, expressed as pH = −log[H⁺]

Normal blood pH range

7

35–7

45

Dissociation constant (K)

A value describing the strength of an acid or base based on its degree of ionization

Carbonic acid (H₂CO₃)

Formed from CO₂ and water and represents the respiratory acid component

Bicarbonate (HCO₃⁻)

The conjugate base of carbonic acid and the primary metabolic buffer

Henderson–Hasselbalch equation

H₂CO₃ = pCO₂ × 0

1 + log([HCO₃⁻]/[H₂CO₃])

pK value in blood

6.1

1, the dissociation constant for carbonic acid at physiologic conditions

Relationship of pH to acid-base ratio

pH is proportional to the ratio of bicarbonate (base) to carbonic acid (acid)

Blood gas analyzer measures

pH, pCO₂, and pO₂ directly and calculates HCO₃⁻

Calculation of carbonic acid

H₂CO₃ = pCO₂ × 0.3

03

Ion selective electrode (ISE)

Measures pH by detecting voltage changes related to hydrogen ion activity

Severinghaus electrode

Measures pCO₂ using a pH electrode with a CO₂-permeable membrane

Clark electrode

Measures pO₂ amperometrically by detecting electron flow during oxygen reduction

Arterial blood gas use

Required for accurate assessment of oxygenation and acid-base status

Venous blood gas use

Acceptable for acid-base balance but not oxygenation assessment

Heparinized blood gas sample

Prevents clotting and preserves analyte integrity

Blood gas sample stability

Stable for 1 hour on ice or 15 minutes at room temperature

Effect of air exposure on blood gas

Decreased pCO₂, increased pH, and pO₂ shifts toward room air (~140 mmHg)

Delayed blood gas processing

Decreased pH and decreased pO₂ due to cellular metabolism

Base excess

Difference between titratable base and titratable acid at pH 7

40, pCO₂ 40 mmHg, and 37°C

Positive base excess

Indicates a deficit of non-carbonic acid (metabolic alkalosis)

Negative base excess

Indicates an excess of non-carbonic acid (metabolic acidosis)

Purpose of base excess

Used to assess metabolic components and degree of compensation

Respiratory compensation

Changes in ventilation to regulate CO₂ and therefore pH

Increased respiration effect

Decreased pCO₂ and increased pH

Decreased respiration effect

Increased pCO₂ and decreased pH

Role of kidneys in acid-base balance

Long-term regulation by excreting or conserving acids and bases

Renal excreted acids

Hydrogen ions, ammonium, phosphoric acid, and carbonic acid

Red blood cell role in acid transport

Transports CO₂ and buffers hydrogen ions via hemoglobin

Carbonic anhydrase function

Catalyzes the conversion of CO₂ and water to carbonic acid inside erythrocytes

Carbaminohemoglobin

Hemoglobin bound covalently to CO₂ in red blood cells

Hemoglobin buffering

Hemoglobin binds hydrogen ions, especially in its deoxygenated state

Isohydric transport

Maintenance of constant intracellular pH despite hydrogen ion transport

Chloride shift

Exchange of bicarbonate out of RBCs for chloride ions to maintain electrical neutrality

Effect of chloride shift on plasma

pH of plasma becomes lower relative to erythrocytes

Acidosis

A condition characterized by decreased blood pH (<7

35)

Alkalosis

A condition characterized by increased blood pH (>7

45)

Respiratory acid-base disorder

Primary change in pCO₂ causing pH alteration

Metabolic acid-base disorder

Primary change in bicarbonate concentration causing pH alteration

Kintner nomogram

Graphical tool used to determine type and compensation status of acid-base disorders

ΔpCO₂ calculation

Patient pCO₂ minus ideal pCO₂ (45 mmHg)

Primary disorder determination on nomogram

Scale with the greater deviation from zero determines disorder type