Physiology - Acid/Base Balance Flashcards
What is normal ECF pH?
7.4
What are the sources of H ions within the body?
Respiratory acid - CO2 + H2O
Metabolic acid - inorganic and organic acids
What is the normal level of bicarbonate in the body? What is it used for?
What is normal PCO2?
~24mM bicarb, used to buffer metabolic acidosis
5.3kPa CO2 (4.8-5.9)
Apart from the bicarb/CO2 buffer system, what other buffers exist in the ECF and ICF?
ECF:
- Plasma proteins (Pr- + H+ <> HPr) - minor
- Phosphate (dibasic + H+ <> monobasic) - minimal
- Ammonia buffering system
ICF:
- Proteins (Pr- + H+ <> HPr)
- Phosphates
- Haemoglobin in erythrocytes
Bone
- Calcium carbonate - important in prolonged metabolic acidosis as causes bone wasting
What effect does H+ have across the membrane in acidotic/alkalotic conditions regarding electrolytes?
Buffering causes changes in plasma electrolytes
- H+ movement accompanied by Cl- or exchanged for K+
In acidosis - H+ moves into cells, K moves out > hyperkalaemia
How does the kidney regulate HCO3- balance?
Reabsorbing filtered bicarb
Generating new bicarb
Both processes depend on active H secretion from tubule cells into the lumen
What is the mechanism for reabsorption of bicarb? Where does it mostly occur?
Active H secretion from tubule cells
Coupled to passive Na reabsorption
Filtered HCO3- joins H to form H2CO3
- in the presence of carbonic anhydrase, this then goes to CO2 + H2O
CO2 is freely permeable, enters cell
Becomes H2CO3 via carbonic anhydrase again
- dissociates into H+ and HCO3-
H goes back into cycle, Bicarb moves into capillary with Na
Bulk of reabsorption occurs in proximal tubule (>90%)
- no net effect on H+
What buffers exist for the urine? Where does most urine buffering occur?
Mostly dibasic phosphate
Also uric acid and creatinine
Generates new bicarb and secretes H
Occurs mostly in distal tubule (as this is where phosphate most concentrated)
What is the mechanism of urine buffering?
Na + phosphate exist in lumen
Na+ reabsorbed, coupled with H+ secretion
H+ joins monobasic phospate, is excreted
New bicarb generated from CO2 from blood
- enters tubule cell, combines with H2O to form H2CO3, which then dissociates to H+ which is secreted, and bicarb which passes into the capillary with Na
What is the role of ammonium in acid/base balance?
Only used for acid loads
Works as NH3 is permeable, NH4+ is not (lipophobic)
NH3 produced by AA deamination (mainly glutamine by renal glutaminase) within tubule cells
NH3 moves into lumen, combines with H+ to form NH4+, which then combines with Cl (from NaCl) to form NH4Cl
- source of H is CO2, corresponding bicarb passes into capillary with Na
This is all mainly in distal tubule
In proximal tubule, there is NH4/Na exchanger on luminal membrane - net effect is same
How does the ammonium buffer system depend on renal glutaminase?
Renal glutaminase needed to produce the ammonia
Its activity is pH dependent
- when pH falls, activity increases > more NH4 excreted
This is the main adaptive response of kidney to adapt to acid load, but takes 4-5 days to reach maximum effect
What is the body’s response to respiratory acidosis?
Increases bicarb via retention and generation
Decrease H via secretion (and the buffers)
Renal glutaminase increased to increase NH3/4 buffering
Only restoration of normal breathing can correct disturbance
- in chronic, blood gases are never normalised
What is the body’s response to respiratory alkalosis?
Bicarb decreases
- less CO2 means less H available for secretion, therefore less bicarb reabsorption
What is the body’s response to metabolic acidosis?
Stimulate ventilation (inc depth) to decrease PCO2
Decreased bicarb available as it buffers the H+ (or increased loss led to acidosis in the first place)
Kidney restores bicarb and secretes H
- H+ secretion technically decreased as source would be CO2 (which has been decreased in compensation), but what matters is proportion to bicarb
What is the body’s response to metabolic alkalosis?
PCO2 will increase (so reduced respiration)
Increased bicarb excretion
