Summary - acid/base balance

Acid and base balance
Blood gases:
 pH, PCO2, PO2 measured in blood
 Measured by electrochemical methods that employ ISE or gas selective electrodes
 Oximetry: measurement of various forms of Hb, including oxyhemoglobin
 Co-oximeters: specialised spectrophotometers of several wavelengths that measure absorbance of various
Hb present in blood
Control of pH:
 Buffer: weak acid of weak base and corresponding salt
o Bicarbonate-carbonic acid
o Phosphate buffer system
 Uses HPO4 2 and H2PO4, 5% of non-bicarbonate buffering capacity
o Protein buffer system
 Albumin, histidine, 95% of non-bicarbonate buffering capacity
o Hb buffer system
 Most important intracellular buffer
Acidemia: arterial blood pH <7.35
Alkalemia: arterial blood >7.45
Hypercapnia: increased blood PCO2
Hypocapnia: decreased blood PCO2
Concentration of dissolved CO2 (cdCO2): undissociated carbonic acid (H2CO3) and CO2 dissolved in blood
Conc of total CO2: bicarbonate, carbamino-bound CO2, carbonic acid, dissolved CO2

Acid-base balance:
 pH of plasma: PCO2, regulated by lungs and conc of bicarbonate, regulated by kidneys
 CO2 is transported as bicarbonate, carbamino compound (bound to serum proteins) and dissolved CO2

1. Bicarbonate-carbonic acid buffer (major)
a. Highest conc buffer in plasma (optimal HCO3- = 24 mmol/L)
b. CO2 is volatile, acidic gas, water soluble
c. Maintain 20:1 ratio HCO3-:H2CO3



d. HCO3- and PCO2 regulated by an equation derived from Henderson-Hasselbach




e. Acid-base theory




2. Phosphate buffer (minor)
a. Major importance as buffer in plasma, conc of 1mmol/L



3. Protein buffers (minor)
a. Due to imidazole groups on histidine with pK of 7.4
b. Albumin, Hb

, 4. Hemoglobin (minor)
a. Transports acid from tissues to lungs
b. In tissue capillaries O2Hb enters an environment of low PO2, high acid and warmth,
i. Promotes release of O2 and binding of H+
c. In lungs, deoxyHb (HHb) encounters an environment of high PO2, low acid and coolness
i. Promotes gain of O2 and release of H+

Regulation of acid-base balance:
 Respiratory mechanisms
o Respiration rate
o pCO2
 Hyperventilation to decrease CO2
 Hypoventilation to increase CO2
 Renal mechanisms
o Excretion
o Regeneration of bicarb
o Na+/K+/H+ exchange
o Excretion of NH4+
o Reclamation/formation of bicarb
 Compensation
o Complete or partial
o If problem is metabolic: respiratory compensation-hyper or hypoventilation
o If problem is respiratory: renal compensation
Metabolic system:
 When H+ conc deviates from normal, kidneys reabsorb or secrete H, bicarbonate and ions to regulate pH
 Kidney is major regulator or bicarbonate:
o Metabolic acidosis occurs if H+ accumulates or bicarb ions lost
o Metabolic alkalosis occurs if loss of H+ or increase of bicarb
 Requires hours-days to affect pH
 Most effective pH regulator
Respiratory system:
 Exhalation and removal of CO2
 H+ accepted by HHb, HHb enters a region of high PO2 in lungs and is oxygenated to O2Hb, promotes loss of
H+
 H+ combines with bicarbonate to produce dissolved CO2, enters alveolar air for ventilatory removal
 Only works with volatile acids
 Doesn’t affect fixed acids like lactic acid
 Elevated PCO2 indicates inadequate ventilation - hypoventilation (resp acidosis)
 Decreased PCO2 indicates excessive ventilation – hyperventilation (resp alkalosis)
 Takes mins-hours

Acid-base imbalances:
 PCO2 represents balance between cellular production and ventilatory removal
of CO2
 PO2 represents lungs’ ability to oxygenate blood from alveolar air
o Decreased arterial PO2 indicates
 Decreased pulmonary ventilation
 Impaired gas exchange
 Altered blood flow within heart or lungs
Body and pH:
 ECF: pH 7.4