Acid Base Homeostasis Flashcards
Define an Acid and Base
Acid - Defined as any chemical substance that can donate a proton. Base - Any chemical that can accept a proton
How can you calculate the pH of plasma?
What is the pKa?
Defined as the pH at which 50% is ionised and 50% is unionised in the reaction
What is the acid-base equation for physiological mechanisms
What happens if there is a rise or fall in hydrogen ions?
Rise - Equation is driven to the left. Falls - Equation is driven to the right
What is the effects of changing the pH on this equation
Increasing the pH over the pKa will mean the equation is shifted to the right. If the pH is decreased below the pKa then the equation will be shifted to the left.
What are some of the causes of Acid-Base disturbances
- Increased CO2, decreased CO2, increased non-volatile acid/decreased base and increased base/decreased non-volatile acid
How can you differentiate between respiratory and metabolic disorders
Resp - When the primary change is to CO2 levels. Metabolic - Change to bicarbonate levels
Acidosis can be caused by what?
Rise in PCO2 or a fall in HCO3
Alkalosis can be caused by what?
Fall in PCO2 or a rise in HCO3
What changes in ions is respiratory acidosis caused by?
Increased in PCO2 via: - Hypoventilation (less CO2 blown off), - Ventilation-perfusion mismatch, - Reduced lung diffusion capacity
What is the compensation method for respiratory acidosis?
Renal compensation - Increased HCO3- reabsorption and increased HCO3- production, raising the pH towards normal.
What are some of the clinical causes of respiratory acidosis?
- COPD, - Blocked airway, - Lung collapse, - Injury to chest wall - Drugs that reduce respiratory drive, eg, morphine
What changes in ions is respiratory alkalosis caused by?
Decrease in PCO2 which is generally caused by alveolar hyperventilation, this will cause a decrease in H+ concentration and thus rise in pH
Describe the renal compensation method for respiratory alkalosis
Reduced HCO3- reabsorption and reduced HCO3- production this causes a fall in plasma HCO3- levels which compensates for the lower H+ conc so pH moves towards normal
What are some of the clinical causes of respiratory alkalosis?
- Increased ventilation from hypoxic drive in pneumonia, diffuse interstitial lung disease, high altitude and mechanical ventilation. - Hyperventilation caused by brainstem damage, infection during fever
What changes in ions is metabolic acidosis caused by?
Excess H+ in the body which reduces HCO3 levels in the body. Ventilation is unaffected so PCO2 initially is normal.
Describe the respiratory compensation method for metabolic acidosis
- Low pH is detected by peripheral chemoreceptors which causes an increase in ventilation which lowers PCO2. Bicarbonate equation is driven further to the left. The decrease in H+ conc moves pH towards normal. Resp compensation cannot fully correct the pH so excess H+ needs to be removed or HCO3- restored.
What are the clinical causes of metabolic acidosis?
- Loss of HCO3- from the gut in diarrhoea. - Exogenous acid overloading (aspirin OD) - Endogenous acid production (ketogenesis). - Failure to secrete H+ (renal failure)
What changes in ions is metabolic alkalosis caused by?
An increase in HCO3- conc or a fall in H+. This drives the reaction to the right.
Describe the respiratory compensation method for metabolic alkalosis
An increase in pH detected by peripheral chemoreceptors causes decreased ventilation which raises PCO2. This driven the equation to the right which increases H+ moving the pH towards normal. However this alone cannot correct the balance.
What are some clinical causes of metabolic alkalosis?
- Vomiting - Loss of HCl from stomach,
- Ingestion of alkali substances,
- Potassium depletion (diuretics)
What is the acid-base nomogram?
A graph used to analyse arterial blood gases. If the point lies outside of designated areas then this implies a mixed disturbances
