Renal - Regulation of Acid-Base Balance - Quiz 4 Flashcards
Acid-base physiology is all about ____
H+ ion concentrations
Normal ECF H+ concentration
40 nEq/L
Equation for a weak acid
HA [H+] + [A-]
Equation to determine dissociation constant (k)
K = [H+] [A-] / HA
Based on the Henderson Hasselbalch equation, we know the the pH of a solution is related to:
The ratio of the dissociated anion to the undissociated acid
How do solutions of weak acids or bases act as buffers?
They minimize pH changes by donating or accepting electrons (back and forth)
How should pH be in relation to pKa for the most efficient buffer?
pH = pKa
Name 5 buffers in the body. What is the #1 buffer?
- Bicarb (H2CO3 / HCO3) - #1
- Hemoglobin
- Intracellular proteins
- Phosphate (H2PO4- / HPO4–)
- Ammonia (NH3 / NH4)
Hydration of CO2 is catalyses by ______ in the ______
Carbonic anhydrase
Proximal tubule
Is the bicarb buffer effective against resp acid-base disturbances?
No.
It is only effective against metabolic acid-base disturbances.
What is the pKa of bicarb? How does that compare to plasma pH?
6.1
It is rather far from plasma pH of 7.4
What makes bicarb a good buffer?
- it is present in high concentrations in the ECF
2. PaCO2 and HCO3 are closely regulated by the lungs and kidneys
What type of medication is a carbonic anhydrase inhibitor?
A diuretic
Does a carbonic anhydrase inhibitor cause excretion or reabsorption of bicarb?
Excretion
When CA is inhibited, HCO3 can’t get back into the bloodstream from the tubular fluid
What percentage of filtered bicarb is reabsorbed in the proximal tubule?
80-90%
Where is 10-20% of the bicarb reabsorbed?
Distal tubule
What component of the distal tubule allows the formation of a steep gradient for acidifying urine?
H+ pump
it can dump H+ ions into the urine
Which cells in the collecting duct allow the reabsorption and secretion of bicarb?
Intercalated cells
Which buffer traps H+ ions in the urine so that it cannot be reabsorbed?
HPO4– (when combined w H becomes H2PO4-)
What is the pK of phosphate?
6.8
Is the phosphate buffer more effective in acidic or alkaline urine?
acidic
How does the phosphate buffer affect bicarb levels?
It causes bicarb to be generated and enables the kidneys to replenish extracellular bicarb
Where is ammonium (NH4) an important buffer?
tubular fluid
Where is Ammonium found in the nephron?
proximal tubule
thick loop
distal tubule
From what is ammonium synthesized and what is generated in the process of ammonium synthesis?
Glutamine
Bicarb
How is ammonium (NH4) produced in the collecting tubules?
What is generated in this synthesis process?
H combines w NH3 to form NH4 which is excreted.
Bicarb.
What is the renal compensation for alkalosis?
Excreting large ants of bicarb
2 main situations in which metab alk is possible
- Na depletion (via contraction alkalosis)
2. Increased aldosterone activity
Describe contraction alkalosis and with what is it associated?
More Na is reabsorbed in the proximal tubule. To preserve electroneutrality, Cl is reabsorbed with it. As Cl levels decrease in the tubule, HCO3 must be reabsorbed as well.
It can occur w long term diuretic use.
How does increased aldosterone activity cause metabolic alkalosis?
increased aldosterone (mineralcorticoid) activity increases Na reabsorption and H+ secretion in the distal tubule
What is base excess?
The Amt of acid or base that must be added to return blood pH to 7.4 with PaCO2 = 40 mmHg and temp 37*C
What does a positive base excess indicate?
metabolic alkalosis
What does a negative base excess indicate?
metabolic acidosis
How much does K change with a change in pH
0.1 unit decrease in pH = 0.6 mEq/L increase in K
Which direction does the oxygen-hgb dissociation curve shift in acidosis?
Shifts R
Is cardiac contractility altered by acidosis?
Yes, it is decreased
Why might some drugs not be as effective during a code situation?
Acidosis decreases responsiveness to catecholamines
What is the formula to determine anion gap?
Anion gap = Na - (Cl +HCO3)
What is the normal value for the anion gap?
~ 12 mEq/L (range of 7-14)
What is the anion gap a measure of?
unmeasured anions
What causes an acidosis with a high anion gap?
relatively strong non-volatile acids
What is occurring in the case of acidosis w a high anion gap?
H+ consumes the HCO3 and an unmeasured anion accumulates and takes the place of the HCO3
Examples of high anion gap acidosis
Uremia
DKA
Lactic Acidosis
With what is normal anion gap acidosis associated?
Hyperchloremia
Cl takes the place of the bicarb anion
What are 2 causes of normal anion gap acidosis?
- Diarrhea (GI loss of bicarb)
2. Renal loss of HCO3
Tx for metabolic acidosis
- tx the underlying cause
- hypovolemia
- anemia
- cariogenic shock - give NaHCO3 (in certain situations
- Refractory acidosis may require dialysis
Is NaHCO3 a good tx for a pt in resp failure?
No. it will drive the CO2 up even more
How does blood loss (hypovolemia and anemia) lead to acidosis?
Hgb is a buffer in the body so if that buffer is taken away, they become acidotic
True or false: CO2 is a good indicator of acid-base balance. If CO2 is WDL, then there probably isn’t a problem
True
Why are diuretics the main cause of chloride sensitive metab alk?
Na is reabsorbed because of ECF fluid depletion. Cl is reabsorbed to maintain electroneutrality but it runs out so then HCO3 is reabsorbed to take its place and H+ is secreted.
How does hypoK influence H+ secretion?
it augments it.
What acid-base imbalance is caused by loss of gastric fluid from NG suction or vomiting?
alkalosis via loss of HCl
When is spironolactone appropriate to tx alkalosis
if the cause is increased mineralocorticoid activity
What is the body’s defense mechanism for metabolic acidosis?
- ICF and ECF buffers
- Hyperventilation
- Increased renal NAE (net acid excretion)
What is the body’s defense mechanism for metabolic alkalosis?
- ICF and ECF buffers
- hypoventilation
- Decreased Renal NAE (net acid excretion)
What is the body’s defense mechanism for resp acidosis?
- ICF buffers
2. increased renal NAE
What is the body’s defense mechanism for resp alk?
- ICF buffers
2. decreased renal NAE
What exists if compensation of an acid base imbalance is more or less than expected?
a mixed disorder
In acute resp acidosis, what is a normal change in bicarb in response to CO2 increase?
1 mEq/L increase in HCO3 for every 10 mmHg increase in CO2 (from 40 mmHg)
In chronic resp acidosis, what is a normal change in bicarb in response to CO2 increase?
4 mEq/L increase in HCO3 for every 10 mmHg increase in CO2
In metab acidosis, what is a normal change in bicarb in response to CO2 increase?
CO2 decreases 1/2 x the decrease in HCO3 (usually from 24 mEq/L)
In acute respiratory alkalosis, what is a normal change in bicarb in response to CO2 decrease?
2 mEq/L decrease in HCO3 for every 10 mmHg decrease in CO2
In chronic respiratory alkalosis, what is a normal change in bicarb in response to CO2 decrease?
4 mEq/L decrease in bicarb for every 10 mmHg decrease in CO2
In metab alkalosis, what is a normal change in bicarb in response to CO2 decrease?
CO2 increase by 0.7 x the increase in HCO3 (it is very hard for the body to correct and this equation doesn’t always work)
