S2: Acid Base Balance Flashcards
Where is acid in body coming from?
Acid is from the metabolism of carbohydrates and fats, which produces CO2 which reacts with water to produce carbonic acid.
CO2+H2O H2CO3 H+ + HCO3-
Another source of acid is the metabolism of proteins and this generates non volatile (fixed) acids. Proteins containing sulphor in AA will produce H2SO4.
Other will produce HCl
What type of acid is more dangerous?
Non volatile acids need to be removed or there will be a build up of H+
What type of acid is carbonic anhydrase?
It is a volatile acid so it isn’t usually a problem. Removing CO2 will remove the amount of carbonic acid which shifts some protons to the left of the equation.
How do living organisms control PH?
Presence of buffers in ICF and ECF, the respiratory system and the kidney
What is the first line of defence against PH changes?
Intracellular and Extracellular buffer systems. All buffer systems participate according to their pK and their quantity.
eg. bicarbonate buffer system which is the major EC buffer
What is the second line of defence against PH changes?
The respiratory system which regulates plasma PCO2, by controlling excretion or retention of metabolically produced CO2 which is the acid component of the bicarbonate buffer system
What is the third line of defence against PH changes?
The kidney plays a dual role, it regulates excretion or retention of HCO3- and also regulates the regeneration of HCO3-.
What are the 3 blood buffering systems?
- Bicarbonate buffer system
H+ + HCO3- H2CO3 CO2 + H2O
- The Phosphate System
H+ + HPO42- H2PO4-
- The Protein Buffers (including Hb)
H+ + Pr- HPr
How do you measure the effectiveness of a buffer?
- Look at pK
- When pH is equal to the pK it means that the concentration of acid equals the concentration of the base
- A buffer is effective 1pH above and under its pK
- Buffer solutions resist changes in pH when [base]=[acid]
What is the range of pH needed for survival?
6.8-7.8
Compare the amount of pH the phosphate and bicarbonate system can buffer
On the graph, with the range of pH we’re looking at (compatible with life) the phosphate buffer is much better than the bicarbonate buffer as it can cope with a larger amount of H+ while keeping pH in range 6.8-7.8. This is from the biochemical perspective
What is the Henderson-Hasselbalch Equation?
It describes the derivation of pH as a measure of acidity (using pKa, the neagitive log of acid dissociation constant)
The equation is useful for estimating the pH of a buffer solution and finding the equilibrium pH in acid-base reactions.
How would you calculate the pH of the bicarbonate system using the henderson-hasselbalch equation?
[H+] = K1 [CO2] /[HCO3-]
Doing inverse log gives us..
pH= pK + log [HCO3-]/[CO2]
pH = 6.1 + log 24/1.3
where pK is the equilibrium constant
The equation allows us to focus on the [HCO3-]:[CO2] ratio, this is the most important thing in determining pH.
Plasma [CO2] is proportional to partial pressure of CO2 (pCO2) in plasma.
So to convert PCO2 (mmHg) to [CO2] mmol/L we multiply PCO2 by 0.03.
This gives us a ratio of 24:1.2. Commonly measured and spoken about clinically is 20:1.
The pK of CO2-HCO3- is not close to the desired plasma pH of 7.4
How is this still a good buffer?
The unique thing about this buffer is that alveolar ventilation controls pCO2 (acid form) and the kidneys control [HCO3-] in ECF.
Each do this independently and this can be controlled very quickly so CO2/HCO3- can be added/replenished quickly
Why does excess acid/base need to be eliminated if we have buffers?
Buffers are in limited supply.
So the excess acid/base must eventually be eliminated otherwise it will start to cause a change in pH, this is the role of the renal and respiratory systems.
How do kidneys control acid-base levels?
Excretion of acidic or basic urine
What are the primary renal mechanisms involved in renal control of acid-base levels?
- HCO3- can be reabsorbed and secreted to alter the ECF levels of bicarbonate
- Kidneys can also form new HCO3-
- Kidneys can secrete [H+] into tubular fluid
- There are buffer systems in tubular fluid that react with secreted [H+] e.g. NH3 – NH4+, HPO42- - H2PO4-, HCO3- - H2CO3
Explain Kidney and Buffering System
The main buffers: Bicarbonate, phosphate and proteins
- Phosphate can be reabsorbed from tubule back into blood if needed. They are also present in the tubular fluid to bind to any [H+] present in the tubule
- Bicarbonate can be reabsorbed in order to replenish its levels in the blood. Kidney also produces bicarbonate - this is released into plasma at a controlled rate.
- Proteins are not filtered into tubule and remain in plasma.
- Kidney also produces Ammonia which also contributes to buffering
What type of urine do we produce overall?
Acidic Urine
Explain renal control of [H+] and {HCO3-}
Carbonic acid dissociates to from H+ and HCO3-
Depending on the part of the kidney:
- H+ will be exchanged out into the lumen (tubular fluid) via and antiport that brings Na+ in at the same time
- In another part e.g. intercalated cells, there is an H+ ATPase that kicks H+ out into tubular fluid
The bicarbonate goes back into the blood along with Na+ in an symport
What can inhibit carbonic anhydrase?
What is the consequence of this?
- Acetazolamide and other thiazide diuretics can inhibit CA
The consequence of this is that there will not be formation of H+ or bicarbonate hence there will not be acidification of urine and there is an risk of becoming acidotic.
Where in the tubule is most of the bicarbonate reabsorbed and H+ secreted?
PCT
Is H+ ATPase found in the PCT?
No
Explain what happens to bicarbonate and H+ at PCT
- Filtered bicarbonate combines with secreted carbonic acid in the tubular lumen.
- Carbonic acid then dissociates into carbon dioxide and water- this is catalysed by carbonic anhydrase present on the luminal brush border (epithelium) of the PCT cells only
- CO2 readily crosses the tubular cell down its concentration gradient. Once inside the cell, it recombines with H2O forming carbonic acid. This then dissociates into H+ and bicarbonate.
- Bicarbonate passes back into plasma with Na+ symporter
- H+ passes back into tubular fluid with Na+ antiporter
H+ appear in urine as water. Net result is re absorption of bicarbonate and a slight fall in tubular pH and no change of pCO2 of tubular fluid
What would happen if bicarbonate were not reabsorbed?
The buffering capacity of the blood would be rapidly depleted
This is because bicarbonate is freely filtered at the glomerulus hence the concentration of bicarbonate in the tubular fluid is equal to that of plasma
What stimulates Na+/HCO2- symporter?
What does this lead to to?
Ang II can also stimulate the Na+/HCO3- symporter, which can (in cases of high Ang II) lead to alkalosis.
Where is the H+ ATPase pump?
What is the significance of it?
It is found in intercalated cells of the late distal tubule and collecting duct
Here we have a lot of H+ being pumped in and the pH drops quite significantly (a lot of acidification, from 7-4.6 by end of collecting duct).
What does H+ do in distal parts of the nephron?
In the distal part of the nephron, the bicarb levels are low as most has been reabsorbed, so H+ needs to react with other buffers (remember, these will not lower the pH, they are weak acids themselves, instead they help reform the HA by accepting protons, keeping H+/pH around about constant) in the tubule.
Explain the role of phosphate as a buffer in the kidney
Phosphate ions are poor buffers in ECF due to low concentrations
When filtered at the glomerulus the filtered load of phosphate exceeds the Tm so excess phosphate gets concentrated
Further secretion of H+ into the lumen is buffered by HPO4 2-. This is effective as pK of buffer is 6.8 which is close to that of filtrate. Phosphate is also lipid insoluble due to negative charge so even after accepting proton it can’t carry it back into blood so phosphoric acid is excreted in urine
On a cellular level, we get the same reaction of generating carbonic acid. The H+ ATPase pump is effective here (note it is aldosterone sensitive).
The H+ is pumped through and binds to the mono-hydrogen phosphate and it accepts it very readily. Because the phosphate retains a –ve charge it essentially traps the proton in the urine.
Explain the role of ammonia as a buffer in the kidney
The tubular epithelium produces NH3 from metabolism from glutamine with the enzyme glutaminase
- Glutamine is broken down into 2NH3 or alpha-ketogluterate
- a-ketogluterate is broken down to 2 carbonic acid molecules which dissociate into 2H+ and 2 bicarbonate molecules (de novo synthesis of bicarb)
- The H+ bind to ammonia producing ammonium which is sent to urine with Na+ antiport
- Bicarbonate is taken back up into the blood through Na+ symporter
Ammonium is secreted as ammonium salts
What are the 3 stages of urine buffering?
- Reabsorption of bicarbonate
- Formation of phosphoric acid
- Ammonia secretion which creates new bicarbonate
What are disturbances in acid -base balance?
Disturbances in acid-base balance are described as acidosis (plasma pH < 7.4) or alkalosis (plasma pH > 7.4). The cause can either be respiratory (lungs to blame, blown off to much acid, or is retaining it) or metabolic (body to blame, has added/removed H+)
Importantly, if we do not correct the underlying cause of the acidosis/alkalosis, perfect compensation is not possible.
Explain the role of the respiratory system in adjusting CO2
Respiration is regulated primarily by [H+] in the CSFin chemosensitive areas of medulla (central chemoreceptors)
Charged ions like H+ cannot cross BBB but CO2 can so it diffuses through and undergoes reaction with water to produce H+
The central chemoreceptors measure pH of CSF and therefore monitor pH balance indirectly
Increase in plasma pCO2, plasma pH will drop detected by peripheral chemoreceptors in arch of aorta and carotid bodies and CSF pH will also drop
As a result, to prevent further acidosis the central chemoreceptors will signal to increase ventilation to blow off this excess CO2 and normalise pH
How does erythrocyte haemoglobin link respiratory and renal mechanisms?
Hb in RBCs acts as an buffer and it is important in signalling changes in pH to lungs and kidney.
- High concentraction of carbonic acid in RBC
- H+buffered by Hb
- HCO3- exchanged for Cl- (chloride shift)
Hence RBC affects HCO3- and pCO2 levels in ECF and therefor pH
What is metabolic acidosis?
Metabolic acidosis is characterised by low pH as a result of increased ECF [H+] or low [HCO3-]
What can cause metabolic acidosis?
- Severe sepsis where lots of lactic acid is produced
- Uncontrolled diabetes where there is an overproduction of 3-OH butyruc acid and keto acids
- In diarrhoea there may be substantial loss of HCO3- from GI tract
Describe the integrated renal and pulmonary compensation for metabolic acidosis
Increased H+
ICF/ECF Buffering: HCO3- will be used up trying to buffer this system, this will work to a certain extent but there is not infinite amounts of bicarbonate so [H+] remains high
Lungs: The high [H+] will stimulate peripheral chemoreceptors to increase ventilation to get rid of CO2. this will therefore decrease pCO2 and from the henderson-Hasselbach eq. we know this will result in an increase in pH
KIdneys: Come into play after a day or two. They will increase H+ secretion which will bind to bicarbonate or phosphate in urine.
As levels of H+ get high they will need something else to bind to so there is increased NH3+ secretion. Consequence of releasing ammonia is increased bicarbonate reabsorption
This will all help compensate for decreased pH. However, you do have to try remove the causative factor, e.g. diabetes, otherwise the system will get overloaded.
What is metabolic alkalosis?
Metabolic alkalosis is characterised by high pH caused by high ECF [HCO3-] or low [H+]
What can cause metabolic alkalosis?
- Excessive use of diuretic (thiazide) use, leading to chronic loss of Na+, Cl- and K+ and there is increased H+ secretion into tubule
- Excess vomiting can lead to loss of H+ from GI tract
- Ingestion of alkaline antaacids
- Hypokaemia
In alkalosis getting alkaline urine with bicarbonate in it. Normally would always get acidic urine as HCO3 destroyed.
How does thiazide diuretics cause metabolic alkalosis?
Thiazide diuretics are the most commonly used diuretic and work by inhibiting sodium-chloride transporter in the distal tubule, this will increase luminal [Na+]. This then signals increased Na+ reabsorption via an aldosterone sensitive pump (ENaC) in the distal end of distal tubule in exchange for increased secretion of K+ and H+ which are lost in urine.
This can lead to hypokalaemia as well as metabolic alkalosis.
In a situation of volume reduction and reduction in BP (which would be occurring due to diuretics) you will also get activation of RAAS. Which would also exacerbate sodium reabsorption and loss of potassium and hydrogen ions.
When are a-intercalated cells active?
Severe Hypokalaemia
H+/K+ ATPase
Describe the integrated renal and pulmonary compensation for metabolic alkalosis
Increased HCO3 -
ICF + ECF Buffering: H+ is used up trying to reduce bicarbonate to reform carbonic acid but bicarbonate remains high. A fall in [H+] then stimulates peripheral chemoreceptors
Lungs: Ventilation will be reduced so then less CO2 is expelled. pCO2 in plasma rises so more H+ is generated and bicarbonate level rises. However, pH starts to fall to normal as ration of HCO3-:CO2 falls
Kidneys: The kidneys then correct this disturbance over the next few days, a rise in pH in tubule cells will reduce H+ secretion and HCO3- reabsorption/formation, allowing plasma [H+] to rise and correct the plasma [HCO3-], this will remove the ventilation inhibition.
Describe respiratory acidosis
what is is, causes, compensatory mechanism
caused by hypoventilation due to actions of drugs (anaesthetics/barbiturates), chronic emphysema, bronchitis. These conditions impair the removal of CO2 from the lungs, hence it builds up in plasma. Because CO2 enters into cells very rapidly and they contain CA get rapid rise in H+. Rapid ↑[H+] quickly buffered by proteins in plasma (within hours) there is ↑[HCO3-]. Limited by buffering capacity of blood. Within days kidney compensates by stimulating H+ secretion & increasing HCO3- reabsorption.
Describe respiratory alkalosis
what is is, causes, compensatory mechanism
Less CO2 enters cells and less HCO3 diffuses out into plasma so [HCO3] is reduced. Within days kidney compensates by reducing H+ secretion & decreasing HCO3- reabsorption
What is Siggard-Anderson In-Vivo Nomogram?
These diagram represent the behaviour of the whole body during acid-base disturbances and the compensatory mechanisms in play and the body’s responses to therapeutic intervention
