Physiology of Acid-base balance-2Dr. Eman El Eter

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The Henderson–Hasselbalch equation can be applied to relate the pH of blood to constituents of the bicarbonate buffering system:where:pKa H2CO3 is the cologarithm of the acid dissociation constant of carbonic acid. It is equal to 6.1.[HCO3

−] is the concentration of bicarbonate in the blood[H2CO3] is the concentration of carbonic acid in the blood

equation:[3]

where:[H2CO3] is the concentration of carbonic acid in the bloodkH CO2 is the solubility  constant of carbon dioxide in blood. kH

CO2 is approximately 0.0307 mmol/(L-torr)pCO2 is the partial pressure of carbon dioxide in the blood.

where:pH is the acidity in the blood[HCO3

−] is the concentration of bicarbonate in the bloodpCO2 is the partial pressure of carbon dioxide in the arterial blood

Referring to the HendersonHasselbalch equation, acidosis occurs when the ratio of HCO3– to CO2 in the extracellular fluid decreases, thereby decreasing pH. -If this ratio decreases because of a fall in HCO3–, the acidosis is referred to as metabolic acidosis. -If the pH falls because of an increase in PCO2, the acidosis is

referred to as respiratory acidosis.The opposite occurs in alkalosis

Acid-base disturbances

Rates of correction

• Buffers function almost instantaneously• Respiratory mechanisms take several minutes to hours• Renal mechanisms may take several hours to days

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Acid-base disorders

Respiratory Acidosis

• Carbonic acid excess caused by blood levels of CO2 above 45 mm Hg.

• Causes of respiratory acidosis:

• Depression of respiratory center in brain by drugs or head trauma

• Paralysis of respiratory or chest muscles• Emphysema/COPD.• Pulmonary edema. 8

Respiratory acidosis, cont…..

• Characteristics:-Reduction in blood and ECF pH, - Increase in PaCO2, which is the initial cause of the acidosis.

• Compensation: 1. Buffers2. Renal: An increase in plasma HCO3- caused by the addition of new bicarbonate to the extracellular fluid. The rise in HCO3–counteracts the increase in PaCO2, thereby returning the plasma pH toward normal.

Respiratory Alkalosis

• Conditions that stimulate respiratory center and wash out CO2 (Hyperventilation):

• Oxygen deficiency at high altitudes.

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Respiratory alkalosis, cont…..

Characteristics:-Increase in pH.- Decrease in PaCO2.Compensation:

• Renal: increased renal excretion of HCO3- in an attempt to reduce plasma HCO3- concentration toward normal.

• .

Metabolic Acidosis• Bicarbonate deficit:

blood concentrations of bicarbonate drops below 22mEq/L

• Causes:• Loss of bicarbonate e.g. severe diarrhea.• Hypoaldosteronism. • Accumulation of acids e.g:.

Diabetic ketosis .Failure of kidneys to excrete H+

Drug toxicity e.g. salicylates.12

Metabolic acidosis, cont……..

• Characteristics of metabolic acidosis:- A decrease in pH & a rise in extracellular fluid H+ concentration.- The primary abnormality is a decrease in plasma HCO3–

• Compensations :1. Respiratory: increased ventilation rate reduces PaCO2 ,2. Renal : adds new bicarbonate to the extracellular fluid and excretes more H+..

Metabolic Alkalosis

• Bicarbonate excess - concentration in blood is greater than 27 mEq/L

• Causes:• Excess vomiting = loss of stomach acid• Excessive use of alkaline drugs• Certain diuretics• Endocrine disorders: Hyperaldosteronism.• Severe dehydration

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Metabolic Alkalosis, cont……

• Characteristics:Increase in plasma pH and a decrease in H+ concentration.The primary cause of metabolic alkalosis is a rise in the extracellular fluid HCO3– concentration.

• Compensation:Respiratory: decrease in ventilation, which raises PaCO2.Renal: increased renal HCO3- excretion.

Example:• A patient is in intensive care because he suffered a severe

myocardial infarction 3 days ago. The lab reports the following values from an arterial blood sample:

• pH =7.21, PCO2= 42, HCO3- = 12:• List the condition: acidosis or alkalosis, metabolic or

respiratory, compensated or uncompensated?• Answer:• Metabolic acidosis, uncompensated

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pH PaCO2 PaHCO3

Com/un

Resp/metab

Acid/alkalosis

7.21 32 147.5 26 217.36 54 32

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Characteristics of primary acid-base disturbances