Role of kidney in acid base balance saturday interactive lecture m MAGDI AWAD SASI 2021

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 (NaH
2PO
4
-
)
OH
-
+ H
2PO
4
-
 H
2O + HPO
4
2-

Sodium Monohydrogen phosphate (Na
2HPO
4
2-
)
 H
+
+ HPO
4
2-
 H
2PO
4
-

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