The document discusses acidification of urine and the kidney's role in maintaining acid-base balance. 1) The kidneys excrete acidic or alkaline urine to maintain blood pH within a narrow range of 6.8-7.8. When blood pH changes, the kidneys compensate by regulating urine pH. 2) The kidneys secrete hydrogen ions into the tubular fluid in exchange for sodium and bicarbonate ions to be reabsorbed into the blood. This maintains bicarbonate levels and helps buffer acids produced by metabolism. 3) When acidosis occurs, the body responds through intracellular and extracellular buffering, increased ventilation, and enhanced renal acid secretion and bicarbonate re

1. Acidification of Urine
Dr Ambika Jawalkar

2. Acidification of Urine
• Acid = [] H+ conc
• Alkali =  H+ conc.
• Normal body pH [ECF] = 7.4
• Metabolic functions of body are highly sensitive to pH.
• Compatible for life = pH range 6.8 -7.8 [ECF]
• Acid –Base balance is co–ordinated function of lungs &
kidney.
• Lungs –
• Major excretory route of volatile acids e.g. carbonic acid.
• Kidney –
• pH of urine range – 4.5 to 8. [depend on rate of acid
secretion]
• When body pH changes  kidney excretes acidic OR alkaline
urine  maintain Acid base balance of ECF.

3. •Buffers in blood –
•NaHCO3 buffers strong acids e.g.
H2SO4 + 2 NaHCO3 Na2SO4 + 2CO2 + 2H2O
HCl + NaHCO3  NaCl + CO2 +H2O
•Functions of kidney –
The acid salts are excreted by kidney
Replacement of blood buffer [NaHCO3]

4. Buffer system of Kidney
•H+ secretion into tubular fluid is buffered by 3
imp. buffers in kidney.
1. HCO3
- buffer system
2. PO4
- buffer system
3. NH3
- buffer system

5. 1] HCO3
- buffer system
• It is regulated both by Lungs & kidneys.
•  in plasma HCO3
- buffer  metabolic acidosis.
• [] in pCO2  respiratory acidosis
• Note –
• Kidney regulate plasma HCO3
- buffer
• Lungs regulate plasma pCO2 changes.

7. • In PCT –
• For secretion of one H+
ion into tubule there is
gain of one Na+ + one
HCO3
- in plasma.
• H + secretion is in
exchange for Na + by
• 1] secondary active
transport. &
• 2] active transport of H+
ATPase

8. • Note –
• For one H + secretion = one HCO3- gain in plasma
• Normal plasma HCO3- = 26 mEq/L
• Renal plasma threshold for HCO3- is = 28mEq/L
• If more than 28mEq/L = HCO3- appears in urine [alkaline
urine]
• Limiting pH
• for PCT = 6.9 &
• for Distal tubule = 4.5
• H + secretion stops at limiting pH.

9. Factors affecting H+ secretion
[] H +secretion [] H +secretion
1] [] In intracellular pH 1] [] In intracellular pH
2] [] intracellular PCO2 2] [] intracellular PCO2
3] [] in filtered load of HCO3 3] [] in filtered load of HCO3
4] [] in ECF volume 4] [] in ECF volume
5] [] in Aldosterone 5] [] in Aldosterone
6] Hypokalemia 6] Hyperkalemia
7] [] in Carbonic anhydrase 7] [] in CA = CA inhibitors
e.g. acetazolamide [Diamox]

10. PO4 Buffer system
• Primary urinary buffer system.
• Derived from diet.
• Distal nephron –
Alkaline PO4 + H+  Acidic PO4 i.e.
H+ + HPO4
2-  H2PO4
-
• NOTE –
• Titrable acidity = PO4 buffer system. [not measure HCO3
& NH3 buffers]
It is Amount of alkali added to raise pH of urine to 7.4
Substances contributing to titrable acidity = excreted PO4 +
ketoacids + lactic acid + creatinine

11. NH3 Mechanism
• Production in Kidney.-
• In PCT & DCT
• By metabolism of AA Glutamine
• NH3 is lipid soluble  free passage across tubular
membrane.
• NH3
+ + H +  NH4
• Trapping of NH4 = Non ionic diffusion Or Diffusion
trapping
• In acidosis  [] NH4 in urine

12. Compensatory Responses of the body in
Acid –Base disturbances
1. Intracellular & Extracellular
buffering.
2. Changes in ventilation.
3. Renal adjustments

13. Compensatory Responses of the body in Acid –Base disturbances
1] Intracellular & Extracellular buffering.
A.ECF buffers –
 HCO3 buffer
 Plasma protein buffer system
B. Intra cellular buffers –
 HCO3 buffer
 PO4 & histidine group of proteins
 Reduced Hb.

14. Compensatory Responses of the body in Acid –Base disturbances
2] Changes in ventilation.
• Depend on pCO2 level
1.  in pCO2  respiratory acidosis 
stimulation of peripheral & Central chemoreceptors 
[] hyperventilation
2.  in H+  metabolic acidosis 
 stim of Central chemoreceptors 
[] hyperventilation.
E.g. Diabetic ketoacidosis.  Kussmaul breathing.

15. Compensatory Responses of the body in Acid –Base disturbances
3] Renal adjustments
•In acidosis  due to  NH4 secretion
• H+ secretion   production of HCO3 
 buffering capacity of blood