What buffers are used during exercise? (Homeostasis)

During exercise blood pH is reduced.
What buffers are used by the body to maintain blood pH levels?
Please and thank you.

(I think its the carbonic acid buffer. Is this correct?)

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Reply 1

iWoof

OP

16

Anyone?

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Reply 2

loukas2993

3

You are partly correct. When the is exercised, it's heart rate, systolic blood pressure, and cardiac output all increase. Similarly, the body's metabolic activity increases, producing CO₂ and H⁺ in the muscles. With strenuous exercise, the body's metabolic rate will eventually exceed the oxygen supply leading to the utilisation of anaerobic processes, which produce lactic acid (which enters the bloodstream and lowers its pH).

The body maintain's its blood pH of ~7.4 through multi-organ involvement (including the kidneys and lungs) and the carbonic-acid-bicarbonate buffer system. The equilibrium reaction associated with this buffer is:

Screen shot 2015-01-26 at 12.41.21.png

In relation to this equilibrium, extracellular buffers (such as bicarbonate) may reversibly bind to H⁺ and effectively buffer small reductions in pH. Similarly, when blood pH drops too low (acidaemia), the body will compensate by increasing breathing rate thereby expelling CO₂ and shifting the above equilibrium to the left, so that fewer H⁺ ions are free, raising the pH back to normal.

However, when the blood pH is outside the optimum buffering range, the addition of protons to the blood due to strenuous exercise may be too great for the carbonic-acid-bicarbonate buffer alone to effectively control the pH of the blood. With this in mind, the lungs removes excess CO₂ (raising the pH through appropriate shifts int he equilibrium above), and the kidneys remove excess HCO₃^- (helping to lower the pH, if necessary).

Hope this helps.

Reply 3

Decorpi12

5

Original post by loukas2993

You are partly correct. When the is exercised, it's heart rate, systolic blood pressure, and cardiac output all increase. Similarly, the body's metabolic activity increases, producing CO₂ and H⁺ in the muscles. With strenuous exercise, the body's metabolic rate will eventually exceed the oxygen supply leading to the utilisation of anaerobic processes, which produce lactic acid (which enters the bloodstream and lowers its pH).

The body maintain's its blood pH of ~7.4 through multi-organ involvement (including the kidneys and lungs) and the carbonic-acid-bicarbonate buffer system. The equilibrium reaction associated with this buffer is:

In relation to this equilibrium, extracellular buffers (such as bicarbonate) may reversibly bind to H⁺ and effectively buffer small reductions in pH. Similarly, when blood pH drops too low (acidaemia), the body will compensate by increasing breathing rate thereby expelling CO₂ and shifting the above equilibrium to the left, so that fewer H⁺ ions are free, raising the pH back to normal.

However, when the blood pH is outside the optimum buffering range, the addition of protons to the blood due to strenuous exercise may be too great for the carbonic-acid-bicarbonate buffer alone to effectively control the pH of the blood. With this in mind, the lungs removes excess CO₂ (raising the pH through appropriate shifts int he equilibrium above), and the kidneys remove excess HCO₃^- (helping to lower the pH, if necessary).

Hope this helps.

This.
But the last sentence doesn't make much sense to me. Why would the kidney want to further lower the pH in the case of strenuous exercise? It shouldn't be getting rid of the bicarbonate it needs for buffering, right? Unless you mean that the body's reached a state of alkalosis due to the compensation and the compensation has to stop?

Reply 4

seaholme

19

The kidneys retain HCO3 to lower the pH in a compensatory mechanism, you are correct. Equally they can lose HCO3 in alkalotic states.

In exercise the kidneys don't do a great deal though, it's almost exclusively the lungs. You have to be acidotic for a much longer period of time to start retaining HCO3- and trying to compensate metabolically rather than just through the respiratory mechanism. Just doing some exercise and then returning to normal isn't worth the kidneys responding to, that mechanism is far too slow whereas the lungs are very immediate controls of body pH.

The kidneys only start acting in more chronic acidotic states e.g. DKA, or COPD where your lungs are shot by smoking so you're retaining CO2 and unable to blow it off efficiently enough - then HCO3- will come up to try and keep your body at an acceptable pH. Medics can tell whether your acidosis is acute or not simply via looking at HCO3- and whether the kidneys have started trying to compensate or not.

(edited 9 years ago)

Reply 5

Decorpi12

5

Thank you ^
Just a "technical correction" if you want, in cases of acidaemia, you want to raise the pH, not lower it.

(edited 9 years ago)

Reply 6

iWoof

OP

16

Thank you! That is very very helpful.
However is there a difference between carbonic acid buffer and the mechanism 'exhalation of carbon dioxide'?

Reply 7

seaholme

19

Original post by iWoof

Thank you! That is very very helpful.
However is there a difference between carbonic acid buffer and the mechanism 'exhalation of carbon dioxide'?

My bad, that is what I meant to put.

And exhalation of CO2 just means getting rid of CO2. If you look at the equilibrium equation posted above, then imagine you're getting rid of CO2 from that. If you remove CO2 from the end then it'll push the reaction towards that side in order to balance things out and achieve equilibrium.

The carbonic acid is the step in the middle (H2CO3) and allows for a small amount of buffer, but larger changes and greater level of acidosis require removal of CO2 from the other end. They're two separate things in equilibrium with each other.

Reply 8

Decorpi12

5

Original post by iWoof

Thank you! That is very very helpful.
However is there a difference between carbonic acid buffer and the mechanism 'exhalation of carbon dioxide'?

They are two different mechanisms of homeostasis.
1-"Carbonic acid" buffer takes place in the extracellular compartment. When protons increase (like in the case of exercise), the HCO3- in the ECF combines with the protons to serve as a buffer.
2-"Exhalation of carbon dioxide" is a respiratory form of compensation. When the HCO3- levels decrease due to the first buffering mechanism (HCO3- combined with protons, check the equation given in the first reply), the pH decreases which is detected by chemoreceptors and this causes hyperventilation. This is because you want to keep the pH constant and according to the Hendersson-Hasselbalch equation pH=[HCO3-]/CO2. HCO3- decreased due to the buffering mechanism, so CO2 levels must decrease to maintain a constant value. (Both numerator and denominator decrease). Hence, you hyperventilate. Both those mechanisms happen immediately to return you back to your original state of homeostasis.

(edited 9 years ago)

Reply 9

iWoof

OP

16

Thank you guys you have all been so helpful!
However I just need help on one small thing.

What is happening in then following equation?
H+ + HCO3 ---> H2CO3 ---> H2O + CO2

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Reply 10

loukas2993

3

Original post by Decorpi12

This.
But the last sentence doesn't make much sense to me. Why would the kidney want to further lower the pH in the case of strenuous exercise? It shouldn't be getting rid of the bicarbonate it needs for buffering, right? Unless you mean that the body's reached a state of alkalosis due to the compensation and the compensation has to stop?

That is what I meant, sorry DeCorpi12, I can get carried away sometimes and often forget to fully explain myself haha!

Reply 11

loukas2993

3

Original post by iWoof

Thank you guys you have all been so helpful!
However I just need help on one small thing.

What is happening in then following equation?
H+ + HCO3 ---> H2CO3 ---> H2O + CO2

Posted from TSR Mobile

In your proposed equation; bicarbonate is reacting with an hydrogen ion to produce carbonic acid, which then dissociates into water and carbon dioxide. You have essentially quoted the acid/base metabolism equilibirum. :smile:

Btw, the arrows you have included should be reversible.

Reply 12

iWoof

OP

16

Original post by loukas2993

In your proposed equation; bicarbonate is reacting with an hydrogen ion to produce carbonic acid, which then dissociates into water and carbon dioxide. You have essentially quoted the acid/base metabolism equilibirum. :smile:

Btw, the arrows you have included should be reversible.