PHYSIOLOGICAL BASES OF ACID BASE

1. MAGDI SASI
ROLE OF KIDNEY IN ACID BASE
BALANCE

3. Objectives
 Describe the role of kidney in regulation of
acid base balance
 Discuss the factors affecting renal handling
of H+ and bicarbonate
 Compare differences between proximal
and distal nephron H+ ion secretion
 Discuss the physiological basis of renal
tubular acidosis

21. PCT
1. For each H+ ,Na & HCO3- are reabsorbed.
2. Low gradient as H+ secretion stops if H+ in lumen 3-4folds
as in PCT
3. High capacity as 100% NaHCO3 Ra & 95% H+ secreted.
4. HCO3 Ra --active process--- need H+ & enzyme
5. Occur in thick loop and part of DCT
6. H+ is dependent on Na+ reabsorption while in DCT & CD,
H+ secretion is either dependent or independent of Na+
7. No Tm for HCO3 reabsorption but renal threshold 26-
28meq/L

27. DCT & CT
 In the intercalated cells of DCT & collecting duct, H+ secretion
mechanisms is either dependent on Na+ ( formation of acid
phosphate) or independent of Na+ (formation of ammonia).
 Acid phosphate:
Tubular cell---CO2 enters from blood to the cell—HCO3+ H+
H+ is secreted in exchange with Na+( from filtered NaHPO4) by
secondary active transport
In the lumen H+ + NaHPO4(diphasic,alkaline) ---
Na+NaH2PO4(monophasic phosphate ,acid)
HCO3 formed inside the cell moves passively to interstitial fluid where
bind with actively pumped Na+ to form NaHCO3(alkaline tide) in
blood.

29. Phosphate Buffer System
 This system is an effective buffer in urine and
intracellular fluid (ICF)
 Works much like the bicarbonate system
 System involves:
 Sodium dihydrogen phosphate (NaH2PO4
-)
OH- + H2PO4
-  H2O + HPO4
2-
 Sodium Monohydrogen phosphate (Na2HPO4
2-)
 H+ + HPO4
2-  H2PO4
-

32. The net result:
 NaHCO3 added to blood.
 H+ excreted in form of acid phosphate in the urine
 Characterized by:
1. Low capacity by which 5% of H+ excreted
2. High gradient as H+ can be excreted against high
concentration –1000folds PH4.4
3. It associated with CL- excretion

36.  Ammonia traps H+ & excreted in urine as ammonium
salts ((if H+ is not removed ,the PH in urine
decreased(till4.5) –no further H+ secretion whatever
the high H+ gradient.
 This is LIFE SAVING ADAPTATION MECHANISM.
 1 H+ secreted = 1 NaHCO3 reabsorbed—alkaline tide.

40. Factors affecting H+ secretion:
 Plasma content of CO2
 K+ secretion
 CL secretion
 Carbonic anhydrase
 Aldosterone hormone