acid-base balance clinical application

RENAL MODULE

SOLA AOUN BAHOUS

Acid-Base Balance: Clinical Application

Learning Objectives

Discuss renal contribution to acid-base maintenance and compensation

Calculate the amount of filtered bicarbonate given the GFR and plasma concentration

Calculate the amount of secreted H+ given GFR, plasma bicarbonate concentration, titratable acidity and urine bicarbonate concentration

Calculate titratable acidity from urinary and plasma parameters

Outline

Quantitation of renal acid-base compensation

Exercises

Relationship between the arterial pH and [H+] in the physiologic range

pH [H+], nanomol/L

7.80 16

7.70 20

7.60 26

7.50 32

7.40 40

7.30 50

7.20 63

7.10 80

7.00 100

6.90 125

6.80 160


How much bicarb is excreted?

How much new bicarb is added to the plasma through phosphate buffer?

How much new bicarb is contributed by glutamine?


How much bicarb is excreted? Ubicarb x V

How much new bicarb is added to the plasma through phosphate? We titrate the urine with NaOH to a pH of 7.4 (plasma pH). The number of milliequivalents of OH-

required to reach this pH must equal the number of milliequivalents of H+ that were added to the tubular fluid. This value is the titratable acid.

How much new bicarb is contributed by glutamine?

UNH4+ x V

wiam

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mainly this happens in the distal tubule coz bil proximal bikoon we r still returning the hco3-

HCO3- loss = negative

contribution

HCO3- reabsorption

prevents further loss

H+ secretion and binding

to phosphate yields

titratable acidity

H+ excretion in the form of

ammonium contributes to

correction of acidosis


Titratable acid excreted + NH4+ excreted – HCO-

3

excreted = contribution of the kidney to acid-base regulation

Total = net HCO-3 gain or loss

Negative value loss

Positive value gain


• The glutamine-NH4+ mechanism for new HCO3-

generation becomes the preeminent renal process for opposing the acidosis mainly the 1st few days of an acidosis

• Many inputs exist that affect tubular H+ secretion: PCO2, pH, volume, ionic composition of the plasma, renal sympathetic nerves and many hormones


Renal compensation for respiratory acidosis and alkalosis

Respiratory acidosis CO2 retention and plasma pH

Respiratory alkalosis CO2 loss and plasma pH


By HCO-3 the acidosis will be compensated

Low pH and high PCO2 stimulate the kidneys to excrete H+ and NH4

+, consequently, blood pH returns toward normal

In respiratory alkalosis the opposite occurs


Renal compensation for metabolic acidosis and alkalosis:

• Metabolic disturbance acid-base disturbance not caused by a primary disturbance in PCO2

• Metabolic acidosis = addition to the body (by ingestion, infusion or production) of increased amounts of any acid other than carbonic acid, or alternatively, the loss from the body of HCO-

3 (as in diarrhea)

• The pH and HCO-3


The lungs will try to compensate for metabolic acidosis by hyperventilation to decrease PCO2

The kidneys, to compensate, must reabsorb all the HCO-

3 and add new bicarb to the blood through increased formation and excretion of NH4

+ and titratable acid


The kidneys may be a cause of metabolic acidosis or alkalosis or may fail to compensate for a metabolic disturbance


Factors causing the kidneys to generate or maintain a metabolic alkalosis:

1- extracellular volume contraction:

EC volume contraction stimulates Na+ reabsorption and H+ secretion by stimulating aldosterone secretion, Ang II formation and SNS activity


Aldosterone stimulates luminal H+-ATPase

Renal nerves and Ang II stimulate Na+/H+ antiporter in the proximal tubule

Net result = all the filtered bicarb is reabsorbed and not excreted


2- Chloride depletion:

Chloride depletion stimulates H+ secretion and/or inhibits HCO-

3 secretion


3- Aldosterone excess and K+ depletion:

Excessive diuretic use

ECV contraction

aldosterone secretion

K+ depletion

Creation or maintenance of metabolic alkalosis

Outline


Exercises

Exercise 1

1. The daily H+ load is excreted in the urine as titratableacidity and NH4

+. Would H+ retention leading to metabolic acidosis occur if there were:

a. A marked reduction in titratable acid excretion, as a result of a decrease in the plasma phosphate concentration?

b. A marked reduction in NH4+ formation?

wiam

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Exercise 2

2. Two patients with a normal GFR of 180 L/day are studied, one with normal acid-base balance and one with metabolic acidosis. The following laboratory data were obtained:

Parameter Patient 1 Patient 2

Plasma [HCO3-] 24 meq/L 6 meq/L

Titratable acidity 30 meq/day 75 meq/day

NH4+ excretion 50 meq/day 140 meq/day

Urine pH 5.5 5.0

Exercise 2

Assuming that all the filtered bicarbonate is reabsorbed, calculate:

Net acid excretion in any form

Total H+ secretion

wiam

Callout

30+50 la patient A

wiam

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24*gfr + titratable acid only

Exercises

3. Is NH4+ excretion included in the measurement of

titratable acidity?

4. A patient with persistent vomiting develops metabolic alkalosis as a result of the loss of HCl in gastric juice. Why isn’t the condition corrected spontaneously by excretion of the excess bicarbonate in the urine?

wiam

Callout

no bas l hpo4

wiam

Callout

volume is low so we have reabsorbtion of na secretion of h secretion of k reabsorbtion of bicarb

Exercises 5 and 6

5. The following results were obtained on a patient under normal physiological conditions: plasma HCO-

3 = 22 mmol/L, GFR = 7.5 L/hour, urinary HCO-

3 = 8 mmol/d, titratable acid = 24 mmol/d.

What is the amount of HCO-3 filtered?

a.24 mmol/d

b.180 mmol/d

c.3960 mmol/d

d.4328 mmol/d

e.4340 mmol/d

wiam

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3960

wiam

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Exercises 5 and 6

6. What is the total amount of hydrogen ion secreted?

a. 8 mmol/d

b. 24 mmol/d

c. 3312 mmol/d

d. 3976 mmol/d

e. 4339 mmol/d

wiam

Oval

wiam

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gfr thingy - excereted + l titratable

Exercise 7

7. Hypoventilation is associated with which acid-base disturbance?a. Increased plasma pH, increased plasma HCO-

3 , alkaline urine

b. Increased plasma pH, decreased plasma HCO-3 ,

alkaline urinec. Decreased plasma pH, increased plasma HCO-

3 , acidic urine

d. Decreased plasma pH, decreased plasma HCO-3 , acidic

urinee. Decreased plasma pH, decreased plasma HCO-

3 , alkaline urine

wiam

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Exercise 8

8. Choose the correct statement describing the reabsorption or the addition of new bicarbonate to the blood.

a. When secreted H+ binds to HCO-3 in the tubular

lumen, addition of new HCO-3 occurs

b. When secreted H+ binds to non HCO-3 buffers in the

tubular lumen, addition of new HCO-3 occurs

c. When NH4+ is produced and excreted, reabsorption of filtered HCO-

3 occurs

d. When H+ binds to phosphate in the tubular lumen, reabsorption of HCO-

3 occurs

wiam

Oval

acid-base balance clinical application

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