Print version of Knowledge bytes used in this lesson.

Close the window to return to the lesson after printing.

Acid base

Daily filtered load of bicarbonate is 4500 mEq

Proximal tubular absorption of bicarbonate is increased by

Acid anion is filtered in glomerulus as sodium salt

Bicarbonate equivalent to H+ secretion is generated and absorbed in distal nephron

Normal pH is 7.38-7.42

pH values compatible with life 6.8-7.8

Acid production

Blood brain barrier

Single acid base disturbance: the change in concentration of one anion is balanced by a reciprocal change in one other anion.

Mixed acid base disturbance: the anion patterns are more complex

Normal Values

pH = 7.38 - 7.42

[H+]  = 40 nM/L for a pH of 7.4

PaCO2  = 40 mm Hg

[HCO3] = 24 meq/L

Acid base definitions

Acid base disorder is considered present when there is abnormality in HCO3 or PaCO2 or pH.

Acidosis and alkalosis refer to in-vivo derangement's and not to any change in pH.

Acidemia (pH < 7.38) and Alkalemia  (pH >7.42) refer to derangement's of blood pH.

Kidney and Respiratory system play a key roles in maintaining the acid base status.

Primary Acid base disorders

Metabolic acidosis loss of [HCO3]    0r addition of  [H+]
Metabolic alkalosis

loss of  [H+] or addition of  [HCO3]

Respiratory acidosis increase in pCO2
Respiratory alkalosis  decrease in pCO2

Anion and Cations



Chloride Sodium
Bicarbonate(Total CO2) Potassium
Proteins Calcium
Organic acids Magnesium

Electrochemical balance means that the total anions are the same as total Cations. For practical purposes anion gap is calculated using only Sodium, Chlorides and Total CO2.((140-(104+24)) = 12.

Compensatory measures


Bicarbonate is in both ICF and ECF  and participates in buffering capacity



Buffer systems

Bicarbonate buffer system

Calculated bicarbonate:

Total CO2:

Primary changes in bicarbonate leads to metabolic acid base disorder.

Renal (metabolic) compensatory effort: Renal regulation of  [H+] and [HCO3]

Major functions 

  1. Reclamation of filtered bicarbonate
  2. Generation of new bicarbonate in the distal tubule to replenish body buffer stores
  3. Excretion of Acid

Excretion of Acid 

Excretion of  free [H] 

Excretion of  acid stimulated by 

Excretion of  acid inhibited by 

Bicarbonate change for PaCO2 changes



Metabolic compensation 

Compensation is almost complete

Respiratory compensation  for Metabolic acid base disturbance

 Ventilation and pCO2 relationship 

Interrelationship between pH and pCO2

pH pCO2  
7.10 79 70
7.20 63 60
7.3 50 50
7.5 32 30

This formula is usable because in the range of pH values we usually deal with, there is nearly linear relationship between pH and pCO2.

We can expect an almost leniar relationship between pH and acute CO2 changes


Acidosis increases respiratory drive, alveolar ventilation and gets rid of Carbonic acid.

Respiratory system can never completely compensate for a metabolic defect.

Respiratory compensation attempts to maintain pH in a reasonable range.

Compensation is never complete. If the pH is normal there is probably a superimposed second acid base disturbance. Estimation of expected PaCO2 for a given acidic pH also enables us to determine whether respiratory compensation is appropriate.

Anion gap (AG)

The bicarbonate gap

Etiology for acid base disorders

Metabolic acidosis


Metabolic alkalosis

Metabolic alkalosis

In severe alkalosis

Respiratory Acidosis


Respiratory Alkalosis

Respiratory alkalosis

Recquired lab values/information 

Suspect Mixed acid base disorders

Approach to interpreting Acid-base disturbance 

  1. Is the patient Acidemic or Alkalemic
  2. Identify the primary Acidbase disorder by evaluating HCO3 and PaCO2

    What is the [HCO3]

    What is the PaCO2

  3. What is the anion gap (to determine etiology of Metabolic acidosis)
  4. What is the bicarbonate gap (to evaluate mixed disorders)
  5. Is the degree of compensation what you expect (appropriate)?