Dr. Saidunnisa Professor Of Biochemistry Acid-Base regulation

Learning Objectives At the end of the session student shall be able to: Explain the bicarbonate and phosphate mechanism which operates in PCT and study the action of C.A. Explain the ammonia mechanism which operates in DCT with neat diagram in regulating the acid base balance. Explain the role of respiration in pH regulation. Define and explain Isohydric transport of CO 2. Explain chloride shift.

Renal Regulation The renal mechanism tries to provide a permanent solution to the acid base disturbances. This in contrast to the temporary buffering system and a short term respiratory mechanism.

Important function of the kidney is to regulate the pH of the ECF. Kidney excretes urine pH approx.6 with a pH lower than that of ECF pH approx.7.4. This is called acidification of urine.

Major Renal Mechanisms 1.Excretion of H + ions and generation of HCO Reabsorption of HCO 3 - or (reclamation of HCO 3 - ). 3.Excretion of titratable acid. 4.Excretion of ammonium ions.

Excretion of H + ions and generation of HCO This process occurs in PCT. 2.Carbonic anhydrase catalyses the production of H 2 CO 3 from CO 2 and H 2 O in the Proximal renal tubular cells. [CO 2 + H 2 O H 2 CO 3 ] 3.H 2 CO 3 than dissociates to H + and HCO 3 -. [H 2 CO 3 H + + HCO 3 - ] 4.The H + ions are secreted into the tubular lumen in exchange for Na +.

Excretion of H + ions and generation of HCO The Na + in association with HCO 3 - is reabsorbed into the blood. 6. There is net excretion of H + ions and net generation of HCO 3 - which adds upto the alkali reserve of the body.

Reabsorption of HCO 3 - or (Reclamation of HCO 3 - ). 1.HCO 3 - filtered by the glomerulus is completely reabsorbed by the PCT so that urine is normally HCO 3 - free. 2.HCO 3 - freely diffuses from the plasma in to the tubular lumen. 3.Here HCO 3 - combines with H + ions secreted by the tubular cells to form H 2 CO 3. [H+ + HCO 3 - H 2 CO 3 ]

4. HCO 3 - is the only buffering anion that can be regenerated by the kidney and returned to body fluids to replenish the base deficit. 5. H 2 CO 3 cleaved by CA of tubular cell membrane to form CO 2 +H 2 0 [H 2 CO 3 CO 2 + H 2 O] 6. As the CO 2 concentration builds up on the lumen it diffuses into the tubular cells along the concentration gradient.

7. In the tubular cell C0 2 again combines with H 2 0 to form H 2 CO 3 which then dissociates into H + and HCO The H + is secreted into the lumen in exchange for Na HC0 3 - is reabsorbed into plasma in association with Na +.

Excretion of titratable acid Titratable acidity is a measure of acid excreted into urine by the kidney. It reflects the H + ions excreted into urine which resulted in a fall of pH from 7.4. The excreted H + ions are actually buffered in the urine by phosphate buffer.

H + ion is secreted into tubular lumen in exchange for Na + ion. This is obtained from the base Na 2 HPO 4. The Na 2 HPO 4 in turn combines with H + to produce the acid NaH 2 PO 4. In which form the major quantity of titratable acid in urine is present.

As the tubular fluid moves down the renal tubules more and more H + ions are added resulting in the acidification of urine, this causes a fall in the pH of urine to as low as 4.5.

Net acid Excretion(NAE) In quantitative terms titratable acidity refers to the number of milliliters of N/10 NaOH required to titrate 1liter of urine to pH 7.4. This is a measure of net acid excretion by the kidney. Major titratable acid present in the urine is NaH 2 PO 4.

Excretion of ammonium ions. This predominantly occurs at the DCT. The glutaminase present in the tubular cells can hydrolyze glutamine to ammonia and glutamic acid. The NH 3 diffuses into the tubular lumen and combines with H + to form NH4 +.

Ammonium ions (NH 4 + ) cannot diffuse back into tubular cells and therefore excreted into urine. NH 4 + is major urine acid. In about 1 / 2 to 2 / 3 rd of body acid load is eliminated in form of NH4+ ions.

For this reason renal regulation via NH4 + excretion is very effective to eliminate large quantities of acids produced in the body. This mechanism becomes predominant particularly in acidosis.

Anion Gap (A - ) The body exists in a state of electro neutrality in which sum of the cations is equal to the anions. A - is the difference between the total concentration of measured cations (sodium and potassium) and measured anions (chloride and bicarbonate). A - = [Na + + K + ] - [Cl - + HCO 3 - ] Normal range: 10-20mmol/ L (mean-16)

Not all the cations and anions are measured. Unmeasured cations: Ca ++ Mg ++. Unmeasured anions: PO4 -- SO4 --, protein, organic acids. An increased anion gap is found when there is an increased unmeasured anions. Causes: 1.Lactic acidosis 2.Aspirin toxicity 3.Methanol toxicity 4.Diabetic ketoacidosis 5.Starvation

An decreased anion gap is found when there is an increased unmeasured cations. (Ca ++ Mg ++.) Causes: 1.Lithium toxicity 2.Multiple Myeloma

Generation of HCO 3 - by RBC Due to lack of aerobic metabolic pathways RBC produce very little CO 2. The plasma CO 2 diffuses into the RBC along the concentration gradient where it combines with H 2 O to form H 2 CO 3. In the RBC, H 2 CO 3 dissociates to produce H+ and HCO 3 -. The H + ions are trapped and buffered by Hb.

Chloride shift As the concentration of HCO 3 - increases in the RBC it diffuses into plasma along with the concentration gradient in exchange for CL - ions to maintain electrical neutrality. This phenomenon referred to as “chloride shift’’ helps to generate HC0 3 -.

RESPIRATORY MECHANISM The rate of respiration is controlled by the respiratory centre located in medulla of the brain and is highly sensitive to the changes in the pH of the blood. In the Lungs: Formation of Hbo 2 from HHb must release H + ions which will react with HCO 3 - to form H 2 CO 3. Because of low CO 2 tension in the lungs, equilibrium then shifts towards the production of CO 2 which is continually eliminated in the expired air.

In the tissues Due to decrease O 2 tension, HbO 2 dissociates delivering O 2 to the cells and HHb is formed. At the same time CO 2 produced as result of metabolism in the cells is hydrated to form H 2 CO 3 which ionizes to form H + and HCO 3 -.

ISOHYDRIC TRANSPORT OF CO 2 H + ions are accepted by Hb to form HHb, very little change in pH occurs because of the arrival of new H + ions. It allows only 0.6 H + ions to be buffered. At the tissue level Hb bind to H + ions and helps to transport CO 2 as HCO 3 - with minimum change in pH. This is referred to as ‘’ISOHYDRIC TRANSPORT OF CO 2 ’’.