renal physiology part 2 - acid base balance 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
why is venous blood more acidic than arterial blood
due to presence of CO2
what is considered acidotic
below 7.35
what is considered alkalotic
above 7.45
what is the equation for pH using log and [H+]
log(1/[H+])
an increase in [H+] does what to the pH
lowers the pH
small change in pH reflect ____ changes in [H+]
large
acidosis can lead to ____ of the CNS
depression
alkalosis can lead to ____ of the peripheral and later the central nervous system
overexcitability
what are some examples of changes in the nervous system due to alkalosis
pins and needles (sensory) muscle spasms (motor) - fatal if respiratory
change of pH of bodily fluids will alter the ______ of proteins causing a knock on effect on ____
secondary structure
enzymes
increased plasma [H+] ______ K+ secretion in renal tubules
decreases
what are the 3 ways in which H+ is added to the bodily fluids
carbonic acid formation
inorganic acids produced during breakdown of nutrients
organic acids resulting from metabolism e.g. lactic acid
what is a strong acid
dissociates completely in solution
what is a weak acid
partially dissociates in solution
what is the first line defence to any change in pH
buffer systems
what does a buffer system consist of
one substance can yield a H+ ion if [H+] decreases
one substance can bind to H+ if [H+] increases
what is the equation of a buffer solution
HA H+ + A-
what is the base in the buffer solution
A-
what is the undissociated acid in the buffer solution
HA
what is the proton in the buffer solution
H+
if H+ is added to a buffer system equilibrium shifts to the ….
why?
left
H+ is mopped up by A- leading to formation of more HA
[HA] rises, [A-] falls
if base B- is added to a buffer system, equilibrium shifts to the….
why?
right
base is tied up by combining with H+ allowing more HA to dissociate
[HA] falls, [A-] rises
at equilibrium what is the equation for K (dissociation constant) of a weak acid
[H+]{A-}/[HA]
what is the henderson hasselbach equation
pH = pK + log[A-]/[HA]
what is the most important physiological buffer system in the body
CO2 - HCO3 buffer
what enzyme converts CO2 + H20 –> H2CO3 (carbonic acid)
carbonic anhydrase
what does carbonic acid dissociate to
H+ + HCO3-
what is the pK for carbonic acid
6.1
what is normal arterial PCO2
40mmHg
what is normal [HCO3-]
24mmol/L
what controls [HCO3-]
kidneys
what controls PCO2
lungs
how can the kidneys control HCO3- (2 ways)
what do both these processes depend on
variable reabsorption of filtered HCO3
and kidneys can add new HCO3 to the blood
- both depend on H+ secretion into the tubule
true/false
the concentration of bicarbonate can be higher in the renal vein than in the renal artery
true
where is bicarbonate reabsorbed
PCT
what is needed in order to reabsorb bicarbonate ions
hydrogen ions
how is bicarbonate reabsorption started
H2O and CO2 in the epithelial cell of the PCT form carbonic acid which then dissociates into HCO3- and H+
how does the bicarbonate get from the epithelial cell into the interstitial fluid
Na+/HCO3- co transporter
where does the H+ ion (the other dissociate) go and how
goes into the filtrate via the Na+/H+ antiporter
what happens to this H+ ion when it goes into the tubular fluid
binds with a bicarbonate already in the filtrate to form carbonic acid
how does this carbonic acid formed get back into the epithelial cell
breaks down to form CO2 and H20 which diffuses across apical membrane into cell
what drives the secretion of H+ through the apical membrane
CO2 partial pressure
why is the reabsorption of HCO3 known as unorthodox reabsorption
the same HCO3- doesn’t cross the epithelium
what does secreted H+ bind to when bicarbonate concentration in tubular fluid is low
phosphate - the next most plentiful buffer
how does excretion of titratable acid (phosphoric acid) and ammonia increase the pH of plasma
H+ has been excreted with a net gain of a new bicarbonate ion
for every hydrogen ion secreted, how many new bicarbonate ions are made and absorbed into blood
one
if 40mmol of titratable acid was passed out in the urine, how much new bicarbonate has been created
40mmol
how do you see how much [H+] has been secreted
titrating H+ excreted as phosphoric acid against a strong base
if a patient is very acidotic what happens
H+ binds to ammonia to form an ammonium ion which is excreted
what is the chemical from the liver that is broken down to give ammonia in acidotic conditions
glutamine
how does ammonia get into the tubular fluid from the epithelial cell
diffusion - it is a gas
what is the enzyme that breaks down glutamine
glutaminase
can H+ excreted as ammonium ions be titrated and therefore measured
no
for every ammonia acid created how many new bicarbonate ions are generated
1
what 3 things does H+ secretion do
drives the reabsorption of bicarbonate ions
forms acid phosphate
makes an ammonium ion
what regenerates buffer stores and rids the body of an acid load
excretion of titratable acid and ammonium ions
what is the range of [HCO3-]
23-27 (close to 25)
what is the range of PCO2
35-45 (close to 40)
what is compensation
restoration of pH irrespective of what happens to the plasma concentration of bicarbonate and PCO2
what is correction
restoration of pH and the PCO2/bicarbonate back to normal
what happens first in acid base disturbance
compensation
what 2 processes are involved in the immediate buffering of a pH change
immediate dilution of the acid or base in the ECF
buffers present in the blood and ECF
what buffers are present in the blood
Hb and HCO3-
what buffers are present in the ECF
HCO3-
deoxygenated blood has a higher/lower affinity for H+
higher
acidosis will ____ the plasma levels of bicarbonate
reduce
what happens when the stores of buffer are depleted (happens quickly)
up to the kidney to replete the stores
using the HH equation, a blood gas analyser can measure the __ and ___ allowing the conc of ____ to be measured and the results are shown on a ____
pH and PCO2
bicarbonate ions
davenport diagram
what causes respiratory acidosis
retention of CO2 in the body - hypoventilation
give 5 conditions that would result in respiratory acidosis
chronic bronchitis chronic emphysema airway restriction chest injury respiratory depression - morphine/GA
CO2 retention drives the equilibrium to the
right
acute retention of CO2 causes levels of H+ to ___ and levels of HCO3- to ____
both rise
what would indicate uncompensated respiratory acidosis
pH < 7.35
PCO2 > 45
true/false
there is virtually no extracellular buffering in respiratory disorders. since the cause is respiratory the kidneys must compensate
true
CO2 retention stimulates what
H+ secretion into the filtrate
are any bicarbonate ions excreted in the urine in acidosis
no - all filtered HCO3- is reabsorbed as H+ is secreted into the urine
what happens as H+ continues to be secreted
TA and NH4+ are generated which means new bicarbonate ions are added to the blood and acid is excreted out in the urine
why does [HCO3-]plasma rise in respiratory acidosis
the condition itself - shifting the equation to the right due to high levels of CO2
and as a result of renal compensation introducing new bicarbonate into the blood
what is involved in correction of respiratory acidosis
lowering the PCO2 by restoration of normal ventilation
what causes respiratory alkalosis
excess removal of CO2 from the body - hyperventilation
give some examples of when respiratory alkalosis might occur
fever
panic/hysterical overbreathing
low inspired PO2 at high altitude leading to hyperventilation
brainstem damage
excessive CO2 removal drives equilibrium to the
left
excess removal of CO2 from the body causes levels of H+ to ____ and levels of HCO3- to ___
both fall
what would indicate uncompensated respiratory alkalosis
pH > 7.45
PCO2 < 35
how does the kidney compensate for respiratory alkalosis
lack of CO2 in the plasma causes a decrease in H+ secretion meaning [H+] goes up
and further lowers the amount of bicarbonate in the plasma
is bicarbonate secreted in the urine in respiratory alkalosis
yes - secretion of H+ into the tubular fluid is insufficient to reabsorb all of the bicarbonate so even though it is lower in level than normal some is excreted and the urine is alkaline
what does correction of respiratory alkalosis require
restoration of normal ventilation
what is the most common of the acid base imbalances
metabolic acidosis
what causes metabolic acidosis
excess H+ from any source other than CO2
what are some examples of causes of metabolic acidosis
ingestion of acids or acid producing food
excessive production of H+ e.g. lactic acid during exercise
excessive loss of base e.g. diarrhoea
what does metabolic acidosis do to the levels of [H+] and [HCO3-] in the body
increases [H+]
decreases [HCO3-]
why is [HCO3-] decreased in metabolic acidosis
either due to loss from the body
or depleted as a result of buffering excess
what indicates uncompensated metabolic acidosis
pH < 7.35
HCO3- low
what is involved in the compensation of metabolic acidosis
respiratory compensation - decrease in plasma pH stimulates peripheral chemoreceptors to increase ventilation and blow off CO2
blowing off CO2 moves the equilibrium to the
left
respiratory compensation by blowing off CO2 causes H+ to ____ and HCO3- to ____
both decrease
what is involved in correction of metabolic acidosis
filtered HCO3 is very low and readily reabsorbed
H+ secretion continues and produces TA and NH4+ generating new HCO3-
kidneys lose H+ and gain HCO3-
correction of metabolic acidosis by the kidneys causes the urine to become ___
acidic - acid load is excreted
why is respiratory compensation essential in metabolic acidosis
acid load cannot be excreted immediately
how is metabolic alkalosis caused
excessive loss of H+ from the body
give some examples of situations that would cause metabolic alkalosis
loss of HCl from stomach e.g. vomiting
ingestion of alkali or alkali producing food e.g. antacids
aldosterone hypersecretion
metabolic alkalosis causes [H+] to ____ and [HCO3-] to ____
H+ fall
HCO3- to rise
how does metabolic alkalosis cause HCO3- to rise
either due to loss of H+ or addition of base
what would indicate uncompensated metabolic alkalosis
pH > 7.45
HCO3- high
how does increased pH from a metabolic source cause respiratory compensation
peripheral chemoreceptors detect increased pH and slow ventilation to retain CO2 (PCO2 rises)
CO2 retention shifts the equilibrium to the
right
CO2 retention causes [H+] to ____ and [HCO3-] to ____
H+ rises
HCO3- also rises
what does renal correction for metabolic alkalosis involve
filtered HCO3- load is so large compared to normal that not all of the filtered HCO3- is reabsorbed so is excreted
no TA or NH4+ is generated, no new bicarbonates are formed
[HCO3-] of blood returns to normal
renal correction of metabolic alkalosis causes urine to be ….
alkaline - bicarbonate is excreted
what is harder to compensate for: respiratory acidosis or metabolic acidosis
respiratory acidosis - renal compensation takes longer than respiratory compensation
