ACID-BASE BALANCE AND ACID-BASE STATUS:

Acids and bases are the product of metabolism. They are transported and handled in the lungs and kidney. They are responsible for maintaining the arterial pH of 7.35-7.45 and venous pH of 7.32-7.38 which is accomplished by buffering capacity of blood and respiratory and renal regulatory mechanism.

Our body is the net producer of acids. E.g.

1.      Anaerobic metabolism of glucose produces lactate and hydrogen ions occurring especially in skeletal muscle and RBC. But gluconeogenesis uses this hydrogen ion and regenerates bicarbonate. Also lactate in muscle can be oxidized utilizing produced hydrogen ions.

2.      Lipolysis and Ketogenesis also produces hydrogen ions. But re-esterification of fatty acid to TG and oxidation of ketone bodies utilizes these hydrogen ions.

3.      Amino acid metabolism both produces and consumes hydrogen ions. During formation of urea (e.g. from alanine, lysine, aspartate, methionine, cysteine, etc.) there is generation hydrogen ions, but are utilized during the metabolism of carbon skeleton.

4.      CO₂ produced by oxidative metabolism can be hydrated to weak carbonic acid that partly dissociated to produce hydrogen ion and bicarbonate.

 Acid base status is assessed by total CO₂, plasma pH and pCO₂ and sometimes lactate and NH₃ (to determine etiology).

Acid-base parameters:

pH and pK:

pH = -log aH⁺ where [H⁺] of aH⁺ given in nmol/L. pK = -logpK_a where K_a is the ionization constant of weak acid. pK is the pH at which an acid is half dissociated existing as equal proportion of acid and conjugate base. So lower is the pK strong is the acid and higher is the pK strong is the base or weak is the acid

Blood pH is 7.4 so

-7.4 = log aH⁺

[H⁺] = antilog (-7.4)

[H⁺] = 4 x 10^-8 mol/L = 40 nmol/L.

pH of plasma is maintained by, (i) pCO₂ which is regulated by lungs and represents the acid component of the carbonic acid/bicarbonate buffer system, and (ii) the concentration of titrable base (base excess or deficit) which is regulated by kidney.

Bicarbonate and dissolved CO₂
Bicarbonate is the second largest fraction (behind Cl^-) of plasma anion (about 27 mmol/L). It includes bicarbonate, HCO₃^- ions, carbonate, CO₃^2- ion and carbamino compound (CO₂ bound to amino group of proteins)
 In laboratory the analyte measured include HCO₃^- and dCO₂ (includes undissociated H₂CO₃ and dissolved CO₂, cdCO₂). Therefore cdCO₂ = solubility coefficient of CO₂ at 37⁰C in blood (α = 0.0306 mmol/L/mm Hg) multiplied by measured pCO₂ in mm Hg. This cdCO₂ concentration can be used in Handerson-Hasselbalch equation to calculate total bicarbonate concentration.