Acid-Base Balance Flashcards
Why is is important for H+/pH be kept constant?
What can this disturb?
What is the normal pH value?
why do we use pH? What does a low pH mean in terms of H+?
It is very important for normal body functions that [H+]/pH of body fluids is kept constant.
This is because enzymes function at a particular pH within a narrow range and as enzymes have such a huge number of functions in the body an abnormal pH can disturb may body systems e.g. blood clotting, cardiac function, drug metabolism.
• The normal [H+]/pH of body fluids is 7.35-7.45.
• Normal plasma [H+] is 0.00000004mol/L (40nM). This would be difficult to communicate so we use pH which is the inverse log of [H+]. –log10[H+].
Because it is inverse correlation, a low pH means there is a high concentration of hydrogen ions, it is acidic.
Source of this acid in the body?
How can we control H2CO3?
How does the meabolism of proteins differ?
Why acids are produced and why do they need to be removed?
We get acid from the metabolism of carbs and fats, which produces CO2, this reacts in the well-known equation
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-
H2CO3 is carbonic acid, it is a volatile acid and so it is usually not really a problem, because we can reduce the amount of carbonic acid by blowing off CO2, which shifts some of those protons to the left.
Another source of acid is the metabolism of proteins and this generates non-volatile (fixed) acids.
So, proteins that contain Sulphur containing amino acids (cysteine, methionine) will produce H2SO4.
- Lysine, arginine, histidine will produce HCl
These non-volatile acids need to be removed otherwise we will build up H+
Control of pH
We learnt about control of pH through experiments about dog and water
• 156ml of HCl were infused IV into a dog and the same amount of acid was added to 11.4L of water (equivalent to total body water of the dog), the pH changes in both were compared.
- It was found the pH of the dog’s arterial plasma decreased gradually from 7.44 to 7.14 (a state of severe acidosis, but one still comparable to survival). However, the pH of the distilled water dropped rapidly to a final level of 1.84, that would be fatal if it occurred in vivo (in living thing).
- So, it is the presence of buffers which are controlling the pH in vivo.
Overview of pH control
What 3 systems compensate for a disturbance in pH?
What is the base buffer?
What contols the acids?
What system has a dual role? How can it have dual roles?
A disturbance in [H+]/pH is compensated for by:
- ICF and ECF buffering systems
- The respiratory system
- The kidney.
The first line of defence against pH changes consists of the intracellular and extracellular buffer systems. All buffer systems participate according to their pK and their quantity. Of particular importance is the bicarbonate buffer system which is the major EC buffer.
The second mechanism is the respiratory system which regulates plasma PCO2, by controlling excretion or retention of metabolically produced CO2, the acid component of the bicarb buffer system.
The third mechanism is the kidney which plays a dual role, it regulates excretion or retention of HCO3- and also regulates the regeneration of HCO3-.
This is how the kidney helps regulate pH by dealing with H+ through bicarbonate.
Blood Buffering Systems
what are the three blood buffering systems?
How do they work? (equations)
Bicarbonate buffer system
H+ + HCO3- H2CO3 CO2 + H2O
[H+] = K1 [CO2] / [HCO3-]
The phosphate system
H+ + HPO42- H2PO4-
[H+] = K2 [H2PO4-] / [HPO42-]
The protein buffers (inc. Hb)
H+ + Pr- HPr
H+ = K3 [HPr] / [Pr-]
The equilibrium reactions shown (with acid on top and anion on bottom) tell us that by altering the concentrations of what is in the numerator/denominator will alter the [H+] and hence pH.
K is the equilibrium constant of the reaction.
pH Buffering
How do we measure the effectiveness of a buffer? What value do we look at?
When is the phosphate system effective? Bi-carbonate system?
BIochemistry wise which is the better buffer?
Why is this different in reality?
To measure the effectiveness of a buffer we look at the pK, when the pH is equal to the pK is means the concentration of acid equals the concentration of the base. A buffer is means to be effective 1pH under and above its pK.
The phosphate system is very effective over the pH range of 5.8-7.8 and the bicarbonate buffer system is most effective over 5.1-7.1. We know that physiologically the range of survival of for us is around 6.8-7.8.
If we take a line up from the x-axis for this range so that it intersects each curve at two points, we can see the effectiveness of each buffer.
So, if we take the line up and see on the phosphate graph, we can see that for 100mmol of phosphate buffer will buffer this amount [H+] (the height on y-axis) over that range of pH we’re looking at.
- I.e. The phosphate buffer can maintain this range of pH compatible with life despite adding this large amount of [H+]
If we do the same for bicarbonate, we see that 100mmol of bicarbonate will only buffer a small amount of [H+] over this range compatible with life.
- I.e. It can only cope with a smaller amount of [H+] to keep pH within range, if we add just a bit more, pH will start drop below 6.8
This obviously tells us at the range of pH we’re looking at, compatible with life, the phosphate buffer is much better than the bicarbonate buffer. This is from the biochemical perspective
However, despite this we know that physiologically the bicarbonate buffer system is the most important buffer in the body. The reason is because both the CO2 and HCO3- can be regulated independently and relatively quickly in the body.
Bicarbonate Buffer System
HOw do we switch H+ equation to pH?
whta is plasma co2 proportional to?
how do we convert pco2 to co2?
what is the concentration ratio of bicarbonate to carbon dioxide?
How can you calculate pH clinically?
We have the equation to calculate [H+] but as this is tedious we want to work out the pH, so we do the inverse log of it instead, this means we have to log K, so it becomes pK and also switch around the fraction.
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 to [CO2] we multiply PCO2 by 0.03.
This gives us a ratio of 24:1.2. Commonly measured and spoken about clinically is 20:1.
measure pH with arterial blood gases (ABG)
Importance of Buffers Summary
What is pK?
When is buffer most effective?
What is unique about the co2-hco3 buffer?
(2 systems)
role of renal and resp systems? (why must excess base/acid be eliminated quickly?)
pK is the equilibrium constant of the reaction. Buffer solutions resist changes in pH when [base] = [acid], the buffer is most effective 1pH either side of the pK.
At 6.1 the pK of CO2-HCO3- buffer is not close to the desired plasma pH of 7.4.
However the unique thing about this buffer is that alveolar ventilation controls PCO2 (acid form) and the kidneys control [HCO3-]ECF (salt form).
Each do this INDEPENDENTLY, we can control it very quickly, we can add/replenish each.
Very importantly, buffers are in limited supply, as buffer capacity is used there is less available to control pH! 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.
Renal Control of Acid-Base Levels
How does kidney control both acid and base levels?
what is the primary renal mechanism behind this? (hco3- and H+)
How can we release H+ in the urine?
So these H+ we are generating need to be eliminated, kidneys control acid-base levels by excretion of acidic or basic urine.
The primary renal mechanisms involved in this are:
- We can reabsorb and secrete HCO3- to alter the ECF levels of bicarb (if HCO3- is excreted into urine it removes the base from the blood, similarly to replenish bicarb that was used in buffering fixed acids, an equivalent amount of H+ are excreted into urine)
- Kidney can also form new HCO3-
- Kidney can secrete [H+] into the tubular fluid
- Remember that [H+] does have to be buffered in the urine, so there are buffer systems within the tubule that react with secreted [H+]:
- > NH3 – NH4+, HPO42- - H2PO4-, HCO3- - H2CO3
Kidneys & Buffering System
What are the 3 main buffers?
Role of each buffer
The main buffers as we spoke of previously are bicarbonate, phosphate and proteins.
o Proteins are not filtered so remain in the plasma while the bicarbonate and phosphate are freely filtered through.
o The phosphate can be reabsorbed from the tubule, if needed back in blood. But they are also present in the tubular fluid to bind any [H+] present in the tubular fluid.
o The bicarbonate can also be reabsorbed in order to replenish its levels in the blood, if it is being used up. However, the kidney also has the ability to make new bicarbonate.
Even if small amounts of bicarb were excreted, our normal body stores of this buffer would quickly be exhausted, which is why reabsorption is so important.
Ammonia also contributes to buffering. Overall, we produce an acidic urine.
Renal Control of [H+] and [HCO3-]
How do we get carbonic acid? what enzyme is needed?
How is H+ excreted into urine? 2 ways
What happens to bi-carbonate?
What inhibits carbonic anhydrase? What is a consequence of this?
In the tubular cells, water combines with CO2 to form carbonic acid and this is via the enzyme, carbonic anhydrase.
Carbonic acid dissociates into its two ions, bicarbonate and H+.
Depending on the part of the kidney you’re in, the H+ will be kicked out into the lumen via an antiport that brings in a Na+ at the same time, or in another part of the kidney you’ll have it kicked out via a H+ ATPase pump (common in intercalated cells)
The bicarbonate goes back into blood along with Na+ which moves down its gradient.
Carbonic anhydrase can be inhibited by acetazolamide and other thiazide diuretics, a consequence of this is there will not be formation of H+ or bicarbonate, hence you will not get acidification of the urine, you will instead be at risk of becoming acidotic.
Renal Control of [H+] and [HCO3-]
What happens in pct?
What happens to HCO3 in PCT?
Is there H+ ATPase in PCT?
Comparison of HCO3 levels in plasma vs GF?
Why must HCO3 must be reabsorbed?
What happens in the lumen to filtered hco3?
Where is carbonic anhydrase situated in lumen?
What happens to co2?
What happens to Bicarb and H+ in cell?
Why is it the formation of new bicarb that accounts for reabsorption?
Why isn’t urine pH very low?
what is the net result?
In the PCT, the majority of HCO3- is ‘reabsorbed’. The PCT also has a great capacity to secrete H+
(note in PCT we do not have the H+ ATPase)
Bicarbonate ions are freely filtered by the glomerulus, hence the concentration of bicarbonate in the tubular fluid is equivalent to that of plasma.
If bicarbonate were not reabsorbed the buffering capacity of the blood would rapidly be depleted.
The process of reabsorption of bicarbonate mainly occurs in the PCT, filtered bicarb combined with secreted H+ forming carbonic acid in the tubular lumen. Carbonic acid then dissociates into carbon dioxide and water, this reaction is catalysed by carbonic anhydrase present on the luminal brush border of the PCT cells only.
The CO2 readily crosses the tubular cell down its conc. gradient. Once inside, CO2 recombines with water and again under carbonic anhydrase forms carbonic acid.
Carbonic acid dissociates into H+ and bicarbonate. Bicarb passes back into blood with Na+ and H+ passes back into the tubular fluid in exchange for Na+.
In this way, virtually all filtered bicarb is ‘reabsorbed’ in a healthy individual. Technically the bicarb cannot be absorbed at the luminal membrane it is really the formation of a new bicarb from within the tubule that accounts for ‘reabsorption’.
One bicarb ion disappears from the tubular fluid and another one is added to the blood.
The H+ appear in urine as water and hence the urine pH is not changed that dramatically. The net result is the reabsorption of bicarb, a slight fall in tubular pH (from 7.4 – 7, there is not much acidification) and no change in PCO2 of tubular fluid.
What happens in DCT? Cell involved?
what pump is more important here?
What is the pH at this point?
IF bicarb levels are low, what does H+ react with?
Will this lower ph? what is the point of reacting wth other buffer?
Remember that Ang II can also stimulate the Na+/HCO3- symporter, which can (in cases of high Ang II) lead to alkalosis.
In the intercalated cells of the late distal tubule and collecting duct the H+ ATPase pump is much more important.
The H+-ATPase pump becomes more important in the later part of the nephron in allowing H+ to be secreted against a substantial [H+] gradient.
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).
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.
Other Buffers – Phosphate
why is phopshate a poor buffer in ECF? Why does this change in kidneys? Why is phosphate an effective buffer in kidneys? Why is the ATP-H+ effective here? what happens to H+? What happens to bicarb?
One of the very important buffers as mentioned earlier is phosphate. Phosphate ions are poor buffers in ECF due to their low concentration. But when filtered at the glomerulus the filtered load of phosphate exceeds the Tm, so excess phosphate gets concentrated.
The further secretion of H+ into the lumen is buffered by HPO42-. This is because it is a very effective buffer due to the pK being 6.8 (close to pH of filtrate). It also is lipid-insoluble due to its negative charge even after accepting a proton so can’t carry it back into blood.
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.
In this part of the tubule, the bicarbonate enters back into the blood through an antiporter system with chloride.
Other Buffers – Phosphate
why is phopshate a poor buffer in ECF? Why does this change in kidneys? Why is phosphate an effective buffer in kidneys? Why is the ATP-H+ effective here? what happens to H+? What happens to bicarb?
One of the very important buffers as mentioned earlier is phosphate. Phosphate ions are poor buffers in ECF due to their low concentration. But when filtered at the glomerulus the filtered load of phosphate exceeds the Tm, so excess phosphate gets concentrated.
The further secretion of H+ into the lumen is buffered by HPO42-. This is because it is a very effective buffer due to the pK being 6.8 (close to pH of filtrate). It also is lipid-insoluble due to its negative charge even after accepting a proton so can’t carry it back into blood.
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.
In this part of the tubule, the bicarbonate enters back into the blood through an antiporter system with chloride.
The ammonium is secreted as ammonium salts, these levels can vary.