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Acid Base Balance
PCC1 / CCNA Sandra Batcheler
Normal Blood Gas Values
Values Arterial Venous Capillary pH 7.35 – 7.45 7.33 – 7.44 7.35 – 7. PCO 2 (kPa) 4.6 – 6.0 5.0 – 6.4 4.6 – 6. PO 2 (kPa) > 10.6 5.3 Variable HCO 3 (mmol/L) 22 – 28 22 – 28 22 – 28 BE +1 / - 2 +1 / - 2 +1 / - 2 Saturations > 95 72 – 75 variable Lactate (mmol/L) 0.5 – 2.2 0.5 – 2.2 0.5 – 2.
Acid Base Balance
Why is acid base balance important? Normal metabolism results in the production of acids. An acid is a hydrogen ion donor A base is a hydrogen ion acceptor The acids produced by metabolism have to be either buffered or excreted to maintain blood pH within normal parameters What is the main acid produced by the body as a result of normal metabolism?
Acid Base Balance
Body systems can only function with a narrow
pH range
Normal pH is between 7.35 – 7.
- as the free H+^ concentration increases, so the
pH decreases, and vice versa
Systemic effects of pH < 7.
Right shift in oxyhaemoglobin dissociation curve Increased PVR Lower threshold for ventricular fibrillation Decreased response to catecholamines Decreased mesenteric blood flow
Systemic effects of pH > 7.
Decreased vascular resistance and tone Left shift in oxyhaemoglobin dissociation curve Increased response to catecholamines
Buffering System In the lungs the process reverses in order to release the CO 2 H+^ bound to the Hb recombines with bicarbonate to form carbonic acid. The Hb is then free to transport oxygen H+^ and HCO 3 -^ H 2 CO 3 (Carbonic acid) Carbonic acid then dissociates into CO 2 and H 2 O H 2 CO 3 CO 2 and H 2 O CO 2 is excreted by the lungs Respiratory System Ventilation plays a major role in maintaining pH balance Respiratory system can activate changes in pH within minutes Balance is achieved through conservation or elimination of CO 2 Impact of this system is more efficient than that of the other systems Respiratory System Excessive H+ concentration (from any source) stimulates the respiratory centre in the medulla to increase respiratory rate and clear CO 2 Conversely elevated pH due to an increase in base causes inhibition of the respiratory centre and respiratory rate falls CO 2 retention occurs – allows formation of more carbonic acid which buffers the excess base thus returning pH to normal
Respiratory System The respiratory system is particularly useful at compensating for changes in pH relating to metabolic disorders e.g. DKA through regulation of pCO 2 Sepsis through regulation of pCO 2 But if the changes in pH are related to a respiratory disorder e.g. consolidation / pneumothorax, then the respiratory system will be limited in it’s ability to adjust the pCO 2 and affect the pH Renal System Controls acid base balance through several active transport processes Compensation is a slower process
- 1 to 2 days for respiratory alkalosis
- 3 to 5 days for respiratory acidosis Renal system reacts to changes in pH by regulating the excretion / conservation of Hydrogen and Bicarbonate ions Renal System Low pH stimulates the excretion of Hydrogen ions into the urine As Hydrogen moves into the urine Sodium and Bicarbonate are reabsorbed Sodium Bicarbonate becomes available to buffer excessive Hydrogen ions in the circulation The pH increases The reaction is reversed when pH is > 7.
Respiratory Acidosis Occurs when there is a failure to eliminate CO 2 Can you think of some causes of respiratory acidosis? Respiratory Acidosis
- The pCO 2 rises above 6.0kPa
- The blood pH drops
- The kidneys compensate by excreting hydrogen ions and reabsorbing sodium and bicarbonate This compensation will take days to fully activate It will bring the pH back towards normal but the pCO 2 will remain elevated Metabolic Acidosis Occurs when there is an excess of acid or a lack of bicarbonate – causing the pH to fall Can you think of some causes of metabolic acidosis? What effect will this have on the respiratory system in the medulla?
Metabolic Acidosis
- There is a high lactate or ketone level or the bicarb is below 22mmol/L
- The blood pH increases
- The medulla enforces increased rate and depth of respiration This compensation will take a few minutes to fully activate It will bring the pH back towards normal but the lactate / ketones / bicarb will remain abnormal Metabolic Alkalosis Ill-health will mostly result in an acidosis, but very occasionally children may present alkalotic Metabolic Alkalosis Occurs when there is a loss of acid causing the pH to increase Can you think of some causes of metabolic alkalosis? What effect will this have on the respiratory system in the medulla? Respiratory Alkalosis Occurs when the pCO 2 falls below 4.6kPa Can you think of some causes of respiratory alkalosis? What effect will this have on the renal system?
Scenario C Charlie, aged 6 mths, is admitted with 10% dehydration. He is pale and lethargic with a respiratory rate of 48 ABG in room air shows:- pH 7. pCO 2 3.8 kPa pO 2 11.7 kPa HCO3 17 mmol/L BE - 22 Lactate 5 mmol/L How would you interpret this blood gas?
