K7 - Acid Base Balance I: Renal Mechanisms involved in Regulation of [H+] Plasma Flashcards
What is the pH of arterial blood?
7.45
What is the pH of venous blood?
7.35
What is the average pH of blood?
7.4
What do small changes in pH reflect?
large changes in [H+]
An increase in [H+] reduces what?
pH
what is acidosis?
increase in H ion secretion which is coupled with a decrease in K ion resulting in K ion retention which has a knock on effect of the membrane potential
what can acidosis lead to?
depression of the CNS – result in a state of coma
what can alkalosis lead to?
over excitability of the peripheral NS and later the CNS
What does [H+] exert a marked influence on?
enzyme activity
What does changes in [H+] influence?
K+ levels in the body
H+ is continually added from what 3 sources?
- Carbonic acid formation
- Inorganic acids produced during breakdown of nutrients
- Organic acids resulting from metabolism
What disease is a result from the build up of H+ ions in body fluids
Diabetes mellitus
what does a buffer system consist of ?
a pair of substances –
one can yield free H+ as the [H+] decreases
the other can bind free H+ when [H+] increases
What is the most important physiological buffer system
CO2- HCO3 buffer
What is the formula for carbonic acid?
H2CO3
What is the formula for bicarbonate?
HCO3-
what is the equation for the CO2- HCO3 buffer?
CO2 + H2O >< H2CO3 >< H+ + HCO3-
How is H2CO3 formed?
from CO2 and water, catalysed by carbonic anhydrase (CA)
What is the CO2- HCO3 buffer in Henderson-Hasselbalch notation?
pH = 6.1 + log x [HCO3-]/PCO2 x 0.03
where 0.03 is solubility coefficient
Why is the buffer system so important?
[HCO3-] is controlled by the kidneys and PCO2 is controlled by the lungs.
Interplay of these two systems ensures precise control
what sets normal plasma pH at 7.4?
pH = 6.1 + log [24 mmol/l]/ 40mmHg x 0.03
what is the role of the kidney in control of [HCO3]p?
- Variable reabsorption of filtered HCO3-
2. Kidneys can add “new” HCO3- to the blood
what are the two roles of the kidney n control of [HCO3]p dependent on?
upon H+ secretion into the tubule
How much HCO3- is filtered per day? (working)
Rate of HCO3- filtration
= GFR x [HCO3-]plasma
= 125 ml/min x 24 mmol/l
= 180 l/day x 24 mmol/l
= 4320 mmol/day
how is bicarbonate re absorbed in the proximal tubule?
no membrane transporter
- infiltrate bicarbonate reacts with H+ secreted from epithelial cells forming carbonic acid
- this is broken down to CO2 and H2O
- H2O taken into epithelial cells and combined with CO2 which has been absorbed from interstitial fluid making carbonic acid
- this is broken down to bicarbonate and H+
- bicarbonate leaves the cells in symport with Na+ into interstitial fluid
- H+ enter the filtrate in antiport with sodium and used to from more carbonic acid etc
What does secreted H+ combine with when [HCO3-]tubular fluid is low?
phosphate
what is the action, location and importance carbonic anhydrase?
action - catalyses the formation of carbonic acid from carbon dioxide and water
location - luminal membrane of epithelial cells lining proximal tubule
What is “titratable acid”?
The amount of H+ excreted as H2PO4-
How can “titratable acid” be measured?
Measure amount of strong base (NaOH) added to titrate the urine pH back to 7.4
What is the maximum amount of titratable acid that can be excreted?
40mmol/day
How does ammonia act as a tubular buffer?
- Tubular cells produce and from NH3 from glutamine
- this is a gas it diffuses through apical membrane
- binds with the H ion to form ammonium ion which is excreted through same process as bicarbonate
Acid excreted as NH4+ & 2. “new” HCO3- generated
What does measurement of titratable acid not measure?
NH4+
What three things does H+ secretion by the tubule do?
- Drives bicarbonate ion reabsorption
- Forms acid phosphate which removes H and adds HCO3 into blood
- Forms ammonium and new bicarbonate ions are added to the blood
what is the role of tirateable acid in acid excretion?
leads to addition of new bicarbonate ions to the body
What is the total H+ secretion into the tubule?
HCO3- reabsorption = 4300 mmol/day
TA excretion = 20 mmol/day
NH4+ excretion = 40 mmol/day
TOTAL H+ SECRETION = 4360 mmol/day
What is the total H+ excretion into the tubule?
TA excretion = 20 mmol/day
NH4+ excretion = 40 mmol/day
TOTAL H+ EXCRETION = 60 mmol/day
What is the vast majority of H+ secretion used for?
HCO3- reabsorption to prevent generation of acidosis
What does excretion of TA and NH4+ do?
Simultaneously rids the body of acid load and regenerates buffer stores