ACID-BASE BALANCE
A. Importance of H+ (pH = log(1/[H+])
1. Because the rate of most (if not all) enzyme catalyzed reactions
in the body are sensitive to pH, small changes in pH can cause large and generally
disadvantageous changes in body function.
2. Membrane excitability is pH dependent
B. Plasma pH Range
| |
pH |
H+ |
| Normal |
7.40 |
40 x
10-6 meq/L |
| Normal Range |
7.37-7.43 |
43-37 x 10-6
meq/L |
| Vital Limits |
6.8-7.8 |
159-16 x 10-6
meq/L |
Note 1: Although the site at which pH is the most important
is intracellular, the only place that pH can be easily measured is extracellularly,
particularly in the plasma. Generally, plasma pH is an adequate index of body
acid-base status.
Note 2: The site at which plasma pH is generally measured is
in systemic arterial blood because systemic arterial blood is less affected
by metabolic CO2 production (systemic venous blood
is more acid).
C. Abnormal Plasma pH
1. Acidosis: plasma pH below the normal range (more acid)
2. Alkalosis: plasma pH above the normal range (more alkaline)
Note: Sometimes a distinction is made between the pathophysiological
conditions of Acidosis & Alkalosis and the description of blood pH as
Acidemia & Alkalemia
D. Effects of Abnormal pH
1. CNS dysfunction
acidosis => depressed excitability
alkalosis => increased excitability (partly due to Ca2+
binding)
2. Electrolyte disturbances, particularly K+,
Na+, Ca2+
3. Cardiac arrhythmias
E. Renal Regulation of H+
1. response to acidosis
a. renal tubule secretion of H+
and reabsorption of HCO3-
(source: tubular cell CO2); H+ exchanges for
Na+ (proximal tubule) or K+
(distal tubule/col. ducts)
b. renal tubule synthesis of NH3
c. excretion of acid in the forms of
1) free H+ ion (small amount);
excretion of acid urine
2) bound to filtered fixed buffers (e.g. phosphate), termed "titratable"
acid
3) combine with filtered bicarbonate (low HCO3-
excretion)
4) combine with NH3 and excreted as NH4+
2. response to alkalosis
a. inhibit tubule secretion of H+
b. inhibit tubule synthesis of NH3
c. excretion of an alkaline urine with high bicarbonate
concentration, low NH4+,
and low titratable acid
3. relatively slow, order of 1-2 days or up to a week for full
effect
4. very powerful; can return the pH all the way to normal (given
sufficient time)
