Acid-Base Homeostasis Respiration Flashcards
Define pKa
the pH at which 50% is ionised and 50% is unionised in the reaction
Where does the equilibrium lie in this equation and how is the equilbrium affected by changes in H+ concentration?
H2CO3 ⇔ H+ + HCO3-
Equilbrium lies to the right
Ratio of H2CO3:HCO3- = (1:20)
H+ rises = left
H+ falls = right
What is the pKa for carbonic acid/bicarbonate?
6.1
What is normal pH?
7.4
What is the main buffering agent in the plasma?
bicarbonate
What can the absolute levels of bicarbonate be changed by?
breathing?
Indicate relative to the equation what section causes respiratory or metabolic acid-base disturbances in the body
CO2 + H2O ⇔ H2CO3 ⇔ H+ + HCO3-
CO2 + H2O = respiratory
H+ + HCO3- = metabolic
What pH indicates acidosis?
< 7.35
What pH indicates alkalosis?
> 7.45
What does each line show?
AB = plasma pH changes as CO2 changes (respiratory)
CD = plasma pH changes when non-volatile acid is added/removed (metabolic)
What are the 4 major causes of acid-base disturbances?
- increased CO2
- decreased CO2
- increased non-voltaile acid/decrease base
- decreased non-volatile acid/increase base
What type of disturbance occurs when the primary change is to CO2?
Respiratory
What type of disturbance occurs when the primary change is to bicarbonates?
metabolic
What can acidosis be caused by?
- Rise in PCO2
- Fall in HCO3-
What can alkalsis be caused by?
- Fall in PCO2
- Rise in HCO3-
What are the 2 main methods used by the body to compensate for acid-base disturbances?
- respiratory system alters ventilation
- kidneys alter excretion of bicarbonate
What causes respiratory acidosis?
Increase in PCO2 caused by:
- Hypoventilation (less CO2 blown away)
- ventilation:perfusion mismatch
What biochemical changes occurs in respiratory acidosis?
- increase in PCO2 causes an increase in H+; lowering the pH
- Plasma HCO3- levels increase to compensate for increased H+
- Renal compensation - increased HCO3- reabsoprtion and increase HCO3- production raises the pH towards normal
What are the main causes of respiratory acidosis?
- COPD
- blocked airway - foreign body or tumour
- lung collapse
- injury to chest wall
- drugs reduceding respiratory drive e.g. morphine, barbiturayes, general anaesthetics
What causes respiratory alkalosis?
Results from a decrease in PCO2 generally causedby alveolar hyperventilation (more CO2 being blown off)
What are the biochemical changes in respiratory alkalosis?
- hyperventilation causes a decrease in H+ and a rise in pH
- Renal compensation - reduced HCO3- reabsoption and reduced HCO3- produtcion
- Plasma levels of HCO3- fall, compensating for lower H+ moving the pH back to normal
What are the major causes of respiratory alkalosis?
- increased ventilation, from hypoxic drive in pneumonia
- diffuse interstitial lung diseases
- high altitude
- mechanical ventilation
- hyperventilation:
- brain damage
- infection driving fever
When does renal compensation occur?
When someone cannot change their respiratory rate
e.g. COPD
Draw a diagram indicating the changes in pH and plasma HCO3- during respiratory alkalosis and acidosis
Describe the changes in this graph
- Normal to A:
- lower pH and increase in bicarbonate
- no resp. compensation
- kidneys secrete less bicarbonate to buffer extra H+
- kidneys secret more acid
- PCO2 has not changed during compensation
- Normal to C:
- Increased pH and decrease in bicarbonaye
- no respiratory compensation
- Kidneys secrete less H+ and more bicarbonate
- PCO2 has not changed during compensation
What causes metabolic acidosis?
excess H+ in the body, which reduces HCO3- levles (shifts equilibirum to the left)
What is PCO2 in metabolic acidosis?
Normal
What is the respiratory compensation in metabolic acidosis? And what other mechanisms occur?
- lower pH is detected by peripheral chemoreceptors, causing an increase in ventilation which lowers PCO2
- Bicarbonate equation is driven further to the left lowering H+ and HCO3- concentration further
What causes metabolic acidosis?
- loss of HCO3- (diarrhoea)
- Exogenous acid overloading (aspiring overdose)
- endogenous acid production (ketogenesis)
- failure to secrete H+ (renal failure)
What causes metabolic alkalosis?
an increase in HCO3- concentration or a fall in H+
What is the compensation for metabolic alkalosis?
- increase pH detected by peripheral chemoreceptors - decreases ventilation which raises PCO2
- Increase in H+ moves pH back towards normal
- Respiratory compensation cannot fully correct the imbalance
What are the causes of metabolic alkalosis?
- vomitting - loss of HCl from stomach
- Ingestion of alkali substances
- potassium depletion (diurectic)
Draw a diagram indicating the compensations in metabolic acidosis and alkalosis
Describe this diagram
- Normal to A
- fall in pH due to addition of H+ or loss of base
- PCO2 = initally norml
- Low pH stimulates respiration
- Shift from A –> B
- PCO2 falls
- Renal compensation not shown
- Normal to C
- rise in pH due to loss of H+ or addition of base
- PCO2 = initally normal
- Raised pH depresses respiration
- Shift from C –> D
- PCO2 rises
- Renal compensation not shown
