Acid-Base Balance:- Part 4 – Arterial Blood gases (Blood Gases), Acid-Base balance Mechanism

9. pH calculation formula:
   1. Where pH = 7.4
   2. pKa = 6.1
   3. Base (bicarbonate) = 24
   4. Acid = dissolved PaCO₂ = 0.03 x PaCO₂ = 0.03 x 40 = 1.2
   5. pH = 7.4 = 6.1 + log(24/1.2) = 6.1 + 1.3 = 7.4
10. The body makes every effort to maintain the validity of the above equation.
11. Our body will maintain pH 7.4, and pK is constant to alter the bicarbonate and paCO_{2
    (H2CO3)}.
12. The pH is dependent upon the total concentration of:
    1. CO₂.
    2. HCO^–₃.
    3. Dissolved CO₂.
    4. H⁺ ions.
    5. All these are inter-related.
13. The body normally maintains the arterial pH between 7.35 to 7.45. This takes place through the buffer system of bicarbonate.
    

    acid-base balance: Role of H⁺ ions for acidemia and alkalemia

Buffer pairs	Buffer system	pK value	Chemical reaction
HCO3¯ / H2CO3	Bicarbonate/carbonic acid	6.1	H+ + HCO3 >< H2O + CO 2
HPO4- /H2PO4-	Phosphate	6.8	H2PO4 ↔ H+  + HPO4-
Hb / HHb	Hemoglobin	7.3	HHb ↔H+ + Hb
Pr- /HPr	Protein in blood	6.7	HPr ↔ H+ + Pr-

Common acid/base disorders  examples are:

1. Lactic acidosis and diabetic ketoacidosis.
   1. 1. Intermediate organic acids are lactic acid and β-hydroxybutyric acid.
      2. The above acids are metabolized to CO₂ and water.
   1. These may accumulate and cause acidemia.
2. Body acids are formed as end products of cellular metabolism. The average person generates acid 50 to 100 meq/day from the metabolism of protein, carbohydrates, and fats and the stool’s loss.
3. To neutralize this acid formation, the body needs to maintain the pH in the range; in that case, an equal amount of acid needs to be neutralized or excreted.
4. Now our various buffer systems come into play like:
5. Retention or excretion of H⁺ ions.
6. Respiratory system.
7. Renal system.
8. These systems are interrelated and work together.

Blood arterial gases

pH

1. The pH of a solution is the negative logarithm of the hydrogen ion activity.
   1. 1. 1. pH = -logaH⁺.
2. The acid-base status of the body is assessed by:
   1. 1. 1. pH.
         2. pCO2.
3. While blood passing through the lung, O2 moves to blood, and CO₂ goes into the lung.
   

   Role of the lungs in Acid-base balance

4. As the blood hydrogen concentration increases, the pH decreases, and if hydrogen ions decrease, the pH increases.
   1. The decrease of one unit of pH represents a 10 times increase in H⁺ activity.
5. The average pH of the blood of 7.40 is equal to the H⁺ ions concentration of 40 nmol/L.
   1. The lungs and the kidneys regulate the pH of the plasma.
      

      Acid-base metabolism control

6. The acids found in the blood are carbonic acid (H₂CO₃), dietary acids, keto acid, and lactic acid.
7. pH indicates acidity and alkalinity.
8. Respiratory or metabolic alkalosis, the pH will be high.
9. Respiratory acidosis or metabolic acidosis will decrease the pH value.
10. pH alkaline when it is >7.4.
11. pH acidic when it is <7.35.
12. Acidemia = pH <7.35
13. Alkalemia = pH >7.45

pCO2 (Partial pressure of the carbon dioxide CO2)

1. pCO₂ measures the partial pressure of CO₂ gas in the blood (arterial blood, plasma, or serum) measured in mm Hg.
   1. This is proportional to the amount of dissolved CO2 or the concentration of the CO2.
   2. pCO2 is a measurement of ventilation capability.
   3. The units are the same as pO2.
2. pCO₂ in the blood is 10% in the plasma and 90% carried by the red blood cells.
3. With respiration, CO₂ is breathed out, and pCO₂ level drops will depend upon the breathing rate.
4. The faster and more deeply one breaths, the more CO₂ is blown off, and the pCO2 level drop.
   

   Acid-base balance: Role of breathing and pCO2

5. pCO2 is referred to as the respiratory component in acid-base determination because the lungs control this value.
6. As the CO₂ level increases, the pH level will decrease.
7. The  pCO₂ level in blood and CSF is a major stimulant to the breathing center in the brain.
8. As the pCO2 level rises, breathing is stimulated.
9. When the brain can not cope with increased pCO₂ and cannot blow off excess CO_2, the brain is depressed, ventilation decreases, and the patient goes into a coma.

10. In metabolic acidosis, the lungs try to compensate by more blowing of CO₂ to raise pH.
11. In metabolic alkalosis, the lungs try to compensate by retaining the CO₂ to lower pH.

HCO₃ or CO₂ content

1. Method to measure CO2 contents:
2. The total CO2 contents are determined from the heparinized arterial or venous blood drawn anaerobically.
   1. This can be done in a vacuum tube or a syringe which is quickly capped.
   2. The blood is centrifuged and plasma is separated.
   3. Next, plasma is analyzed for CO2 by converting HCO3^– and H2CO3 to the gas form.
3. Most of the CO₂ contents are HCO₃¯ in the blood because the dissolved amount of the CO2 and H2CO3 contents are very small.
   1. The HCO3^– ions can be measured directly as HCO3^– or indirectly by CO 2 contents.
4. Total CO₂ =  HCO₃¯ + Dissolved CO₂.
5. The most important buffer system of the plasma is HCO₃¯ / H2CO₃.
6. It is also present in the RBC but at a lower concentration.
7. The ratio of base: acid = 20: 1 in plasma.
8. The kidney regulates HCO3¯ ions, and it is the measure of the metabolic (Renal) component of the acid-base balance.
9. CO2 contents should not be confused with pCO₂.
10. CO2 contents are indirectly measured by HCO3¯.
    1. HCO₃¯  : Dissolved CO₂ =  25 : 1
    2. Any change in the above equation leads to a change in the pH.
    3. As the HCO3¯ level increase and the pH also increases.
11. The HCO3¯ level:
    1. Metabolic alkalosis, the HCO₃^– level is elevated.
       1. Metabolic acidosis, the HCO₃^– level is decreased.
    2. Respiratory acidosis, kidneys attempt to compensate for increased reabsorption of HCO3¯.
       1. In respiratory alkalosis, kidneys excrete an increased amount of HCO3¯ to lower the pH.

Clinical conditions	pH	Bicarbonate (HCO3) level
Metabolic acidosis	decreased	decreased
Metabolic alkalosis	increased	increased
Respiratory alkalosis	increased	decreased
Respiratory acidosis	decreased	increased

pO₂

1. Oxygen in the blood carried in two forms:
   1. Dissolved in plasma = <2%.
   2. Combined with hemoglobin = 98%.
   3. This partial pressure of the oxygen gas determines the force it exerts in attempting to diffuse through the pulmonary membrane.
   4. The pO2 reflects the amount of oxygen passing from the pulmonary alveoli to the blood.
2. pO₂ is the measure of the pressure of O₂ present in the plasma.
   1. pO₂ is the indirect measure of O₂ contents of arterial blood.
3. The pO₂ level is decreased in:
   1. Pneumonia.
   2. Shock lung.
   3. Congestive heart failure.
   4. Congenital heart diseases.
   5. Patient under-ventilated.

O₂ saturation

1. O2 saturation indicates % of hemoglobin saturated with oxygen. OR:
   1. This measurement is the ratio between the actual O₂ content of the hemoglobin and the potential maximum carrying capacity of the hemoglobin.
   2. O₂%  saturation is the percentage indicating the relationship between O₂  and hemoglobin.
   3. This is not the O₂ content.
2. The combined measurement of:
   1. O₂ saturation.
   2. pO_2.
   3. Hemoglobin.
   4. This indicates the amount of O₂ available to the tissues for oxygenation.
3. When hemoglobin 92 to 100% carries O2, then perfusion or oxygen supply to the tissue is normal.
4. With the decrease of the pO₂ level, the saturation of hemoglobin also decreases.
5. When the O2 saturation is 70% or low, the tissues cannot get adequate oxygen.
6. Precaution:
   1. Please avoid smoking or exposure to close second-hand smoke or CO (carbon monoxide). In such cases, the COHb level is increased.
   2. Avoid the use of paint or varnish.

O₂ content

1. The actual amount of O₂  in the blood is termed the O₂ content.
   1. Normally all O₂ is bound to hemoglobin.
2. About 98% of all O2 delivered to the tissue is transported in combination with the hemoglobin.
3. The following formula calculates O2 contents:
   • • O₂ content = O₂ saturation x Hb x 1.34 + pO2 × 0.003

Normal

Source 1

pH

• Adult / child = 7.35 to 7.45
• Newborn = 7.32 to 7.49
• 2 months  to 2 years = 7.34 to 7.46
• pH venous blood = 7.31 to 7.41
  

  Body fluids	pH
  Arterial blood	7.38 to 7.42
  Venous blood	7.37
  Cerebrospinal fluid	7.32
  Pancreatic fluid	7.8 to 8.0
  gastric juice	1.0 to 3.0
  Urine	5 .0 to 6.0

pCO₂

• Adult /child = 35 to 45 mm Hg
• Child <2 years = 26 to 41 mm Hg
  • Venous blood = 40 to 50 mm Hg

HCO₃

• Adult / child = 21 to 28 mEq/L
• Newborn / infants =16 to 24 mEq/L

pO₂

• Adult / child = 80 to 100 mm Hg
• Newborn = 60 to 70 mm Hg
  • Venous blood = 40 to 50 mm Hg

O₂ saturation

• Adult / child = 95 to 100%
• Old people = 95%
• Newborn = 40 to 90%

O₂ content

• Arterial blood = 15 to 22 vol%
• Venous blood = 11 to 16 vol%

Source 2

Chemicals	Arterial	Venous
pH	7.35 to 7.45	7.31 to 7.41
pCO2	35 to 45 mm Hg	40 to 50 mm Hg
pO2	75 to 100 mm Hg	40 to 50 mm Hg
O content	15 to 22 %	11 to 16 %
HCO3			21 to 28 meq/L
Anion Gap			3 to 10

Source 3

Normal Values of Analytes 

	Blood	Venous	Arterial
pH		7.32 to 7.43	7.35 to 7.45
Bicarbonate (HCO3)	22 to 26 mmol/L
Albumin	3.5 to 5.0 G/dL
pCO2	Male = 35 to 48 mm HgFemale = 32 to 45 mm Hg
Anion Gap	3 to 10
Oxygen saturation O2	94 to 98% (decrease with age)
O2 content		11 to 16%	15 to 22%
pO2	80 to 108 mm Hg (depends on altitude)

Source 4

Arterial blood gases ordered in routine:

Biochemical parameter	Adult	Pediatric group
pH	7.35 to 7.45	7.32 to 7.42
pCO2	35 to 45 mm Hg	30 to 40 mm Hg
pO2	>80  mm Hg	80 to 100 mm Hg
O2 saturation	>94%
CO2 content	45 to 51 vol% (19.3 to 22.4 mmol/L)
O2 content	15 to 22 vol% (6.6 to 9.7 mmol/L)
Base Excess	>2 meq/L (>2 mmol/L)
Base deficit	< – 2 meq/L  (< – 2 mmol/L)
HCO3	22 to 26 meq/L (22 to 26 mmol/L)

Interpretations and role of arterial gases in the acid-base balance:

Normal picture =  pH normal,  PCO₂ normal, HCO₃ normal.

• • Acidemia means arterial blood pH <7.4.
    • Acidosis means a systemic increase in H⁺ ions.
  • Alkalemia means arterial blood pH >7.4.
    • Alkalosis means a systemic decrease in H⁺ ions.

Respiratory acidosis

1. pH and HCO₃^– go in the opposite direction.
2. pH lower, pCO₂ high, HCO₃^– high.
3. Seen in respiratory depression due to any cause.
   1. Hypoventilation.
   2. Excessive retention of CO₂.
      

      Acid-base balance: Respiratory acidosis and compensatory mechanism

Metabolic acidosis

1. pH and HCO₃^– go in the same direction.
2. pH low, pCO₂ low,  HCO₃^– low.
3. Seen in diabetes, shock, renal failure, and an intestinal fistula.
   

   Metabolic acidosis changes and findings

    

   

   Acid-base balance: Metabolic acidosis and compensatory mechanism

Respiratory alkalosis

1. pH and HCO₃^– go in the opposite direction.
2. pH high.
3. pCO₂ low.
4. HCO₃^– is normal or slightly decreased.
   1. Seen in hyperventilation.
   2. Excessive loss of CO2.
      

      Acid-base base: Respiratory alkalosis and compensatory mechanism

Metabolic Alkalosis

1. pH and HCO₃^– go in the same direction.
2. HCO3 is >30 meq/L.
3. pH high,  pCO₂ high,  HCO₃^– high.
4. Urine pH >7.0 (Unless there is severe hypokalemia).
5. Serum K is usually low.
6. Seen in sodium bicarbonate overdose, prolonged vomiting, and nasogastric drainage.
   

   Acid-base balance: Metabolic alkalosis and compensatory mechanism

Urine Interpretation of the various parameters:

• pH = < than 7.4 = acidosis.
• pH = > than 7.4 = alkalosis.

pCO₂

1. pCO2 is high = Acidosis then it is respiratory acidosis.
2. pCO2 is low = Acidosis then it is metabolic acidosis.
3. pCO2 is low = Alkalosis; then it is respiratory alkalosis.
4. pCO2 is high = Alkalosis; it is metabolic alkalosis.

HCO₃^–

1. HCO₃^– is high in respiratory acidosis.
2. HCO₃^–  low in metabolic acidosis.
3. HCO₃^–  is low in respiratory alkalosis.
4. HCO₃^–  is high in metabolic alkalosis.

Calculation of Anion gap = Na (140 ) + K (4) — Cl (110 ) + HCO3(24) = 10 meq/L

• • Normal anion gap =   10 to 12 meq/L   = < 12

Table showing the values of pH, HCO3, pCO2, and etiology:

Clinical condition	pH	HCO3	pCO2	Etiology
Metabolic acidosis	<7.4	low	low	Diabetic ketoacidosis. lactic acidosis.
Metabolic alkalosis	>7.4	high	high	Vomiting
Respiratory acidosis	<7.4	high	high	COPD, weakness of respiratory muscles
Respiratory alkalosis	>7.4	low	low	Anxiety and pain

Critical values

Biochemical parameter	Less than	More than
pH	7.25	7.55
pCO2	20 mm Hg	60 mm Hg
HCO3	15 meq/L	40 meq/L
pO2	40 mm Hg
O2 saturation	75% or lower
Base Excess	± 3meq/L

Summary of the acid-base system:

1. Acidosis leads to coma and death due to depression of the CNS.
2. Alkalosis leads to irritability, tetany, and possible death due to the stimulation of the CNS.
3. The acidosis state is more threatening than alkalosis.

Summary of the parameters needed for the acid-base balance:

Lab test	Importance
pH	This will tell: Increased pH value indicates alkalosis Decreased value of pH indicates acidosis
pCO2	This is the partial pressure of CO2, and it will tell: The respiration modulates this pCO2 This is the index of ventilation
pO2	This is the partial pressure of the O2 in the arterial blood and tell: Low values indicate hypoxia pO2 is the indirect measure of O2 contents of arterial blood.

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