physiology of acid-base balance-2 dr. eman el eter
DESCRIPTION
The Henderson–Hasselbalch equation can be applied to relate the pH of blood to constituents of the bicarbonate buffering system: where: pK a H2CO3 is the cologarithm of the acid dissociation constant of carbonic acid. It is equal to 6.1. [HCO 3 − ] is the concentration of bicarbonate in the blood [H 2 CO 3 ] is the concentration of carbonic acid in the bloodTRANSCRIPT
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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
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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
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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
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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
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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
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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.
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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.
• .
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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
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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+..
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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.
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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