Acid base balance Flashcards
What is the normal pH of arterial blood?
7.45
What is the normal pH of venous blood?
7.35
What can acidosis lead to?
CNS depression
What can alkalosis lead to?
Over excitation of PNS and CNS
What generates H+ in the body?
Metabolism
What is the difference between a strong and weak acid?
Strong acid dissociates fully in solution
What is a buffer system?
Two substances of which one can absorb free H+ and one can donate it
What happens if more H+ is added to a system?
A- mops it up to generate more HA (conjugate acid). Buffering.
What happens if more base is added to a system?
More HA dissociates leading to an increase in H+. Buffering.
How do you calculate the pH of a substance?
pK + log[A-]/{HA]. [A-]=base, [HA]=acid
What is the most important physiological buffer system?
CO2-HCO3 buffer
Describe the CO2-HCO3 buffer system
H2O+CO2 H2CO3 H+ + HCO3-
What is the acid and base in the CO2-HCO3 buffer system?
Acid- H2CO3
Base- HCO3-
What role do the kidneys play in the CO2-HCO3 buffer system?
Variable reabsorption of HCO3
Add new HCO3 to the system
How is HCO3 reabsorbed at the luminal membrane?
Changed back to H2O and CO2 then reabsorbed.
What happens to HCO3 once it’s in the epithelial cells?
Converted from H2O and CO2 back to HCO3
How does HCO3 get out of the epithelial cells?
NaHCO3 cotransporter.
How can the kidneys generate more HCO3?
By allowing free H+ to combine with phosphate and being excreted when HCO3 is low thus allowing excretion of H+ without HCO3 loss.
How much acid is excreted a day combined with phosphate?
40mmol/day
What other molecule can be used as a buffer for H+?
Ammonia
What parameters need to be filled for someone to be near normal acid base balance?
Plasma pH close to 7.4 (7.35-7.45)
HCO3- close to 25mmol/l (23-27)
Arterial pCO2 close to 40mmHg (35-45)
What is the difference between compensation and correction?
Compensation- restore pH to 7.4 at detriment of HCO3 and pCO2
Correction- Restore all three to normal
What are the four disturbances that can affect the acid-base balance?
Resp acidosis
Resp alkalosis
Met acidosis
Met alkalosis
What is immediate buffering?
Buffering of change immediately with HCO3 in the blood etc.
How long can immediate buffering last for?
Not very long
What can cause resp acidosis?
Retention of CO2 in the lungs
What happens in resp acidosis?
Equal increase in H+ and HCO3-
What indicates uncompensated resp acidosis?
pH<7.35 and pCO2>45
How is resp acidosis compensated for?
Excretion of H+ with P in urine and all HCO3 reabsorbed
What causes resp alkalosis?
Excessive removal of CO2
What happens in resp alkalosis?
Both H+ and HCO3 fall
What indicated uncompensated resp alkalosis?
pH>7.4 and pCO2<35
How is resp alkalosis compensated for?
Decreased H+ secretion and increased HCO3 secretion
What causes met acidosis?
Excessive H+ from any source other than resp:
Exercise (lactic acid)
Loss of base (diarrhoea)
What indicated uncompensated met acidosis?
pH<7.35 and HCO3 very low
How is met acidosis compensated for?
Increase ventilation which removes CO2 thus losing H+ and HCO3. HCO3 reabsorbed from urine and H+ secreted to produce new HCO3.
What causes met alkalosis?
Hacessive loss of H+ from the body:
Vomiting
Aldosterone hypersecretion
What indicated met alkalosis?
pH>7.45 and HCO3 very high
How is met alkalosis compensated for?
Ventilation decreased giving rise to both H+ and HCO3. Lots of HCO3 filtered and most not reabsorbed.
How long does extracellular buffering of H+ take to start working in met disturbances?
Immediatly
How long does resp buffering of H+ start to take working in met disturbances?
Minutes
How long does intracellular buffering (proteins etc) of H+ start to take working in met disturbances?
2-4 hours
How long does renal buffering of H+ take to start working in met disturbances?
Days-weeks
How long does extracellular buffering take to start to start working in resp disturbances?
Immediately but not very effective.
How long does renal buffering take to start working in resp disturbances?
Hours to days
