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Friday, November 9, 2012

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 = -logpKa where Ka 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, CO32- 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.

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