acid base question

[obiwan]

in brs physio, it says that in respiratory alkalosis, there is decreased CO2 which causes decrease in both H+ and HCO3.... it would seem like they would cancel out each other and there wouldn't be any acid base problem...

its sad that after 2 years of medical school, i can't seem to think through something as simple as acids and bases

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[Mellifera]

The decrease in pCO2 is the initial disturbance. The decrease in HCO3 is the compensation -- respiratory alkalosis due to high altitude is actually one of the few situations where you can see an almost full compensation. The best way I heard this explained was to forget Henderson-Hasselbach and use this equation:

pH ~ HCO3 / pCO2

So in respiratory alkalosis, you have an increased pH. Looking at that equation, you know that pCO2 in the denominator must be decreased to cause an increase in pH. HCO3 will then be excreted because the arrow for the compensation must face the same direction as the initial disturbance. By decreasing HCO3, you compensate for the decreased pCO2. I hope that makes a little bit of sense. I really struggled with acid-base until I heard it explained that way (Goljan).

 

[Instatewaiter]

sorry if this is a bit simple but trying to break it down for you:

respiratory alkalosis= low CO2 (ie low acid)

To get you back up to normal pH your body (kidneys) decrease their reabsorption of HCO3 (less HCO3 or less base). So you see initially decrease in CO2 because of the respiratory alkalosis and then the kidenys compensate by decreasing HCO3.

The reason the low CO2 and low HCO3 dont cancel each other out is that the compensation is not complete. So you dont decrease HCO3 enough to lower your pH to nl (it does lower some)

Note that this compensation doesnt happen immediately but takes a few days so you sometimes wont see this compensation if the respiratory alkalosis (or resp acidosis for that matter) has just begun.

So, similarly, if you have an IV drug user that ODed on heroin he may have high CO2 (resp acidosis) because of repsiratory depression from his opiate but the HCO3 may be normal b/c his kidneys havent had enough time to start their compensation (which in this case the kidneys would try to increase HCO3 to compensate for the increase in CO2).

 

[hdu]

Then let's try to use the information you had before these two years:

Look at it this way:

pH is a function of [H+]

(By definition, pH = -log [H+] )

A basic equation in the participation of the bicarbonate buffer in the regulation of pH is:

CO2 + H2O <-----------> CO3H2 <------------> CO3H- + H+

The CO2 in the aqueous phase (blood in capillaries) diffuse to the gas phase (lung air space). The CO2 in the air space of the lungs is in equilibrium with the Bicarbonate buffer in the blood plasma passing through the lung capillaries.

Recall from Chemistry: "When a change is imposed on a system at equilibrium, the position of the equilibrium shifts in a direction that tends to reduce the effect of that change." (Le Chatelier principle)

It means that when a reactant or product is added to a system at equilibrium, the system shifts away from the added component. On the other hand, if a reactant or product is removed, the system shifts towards the removed component.

In the case of Hyperventilation there is an excessive CO2 exhalation and the reaction will shift to the left. It provokes a decrease in the concentration of H+ and consequently and increase in the pH (Once again, by definition, pH = -log [H+] )

The principle in having a person to breath from and in a bag during a panic attack (a cause of Respiratory Alkalosis) is to try to restore the equilibrium by increasing the CO2 in the lungs, so the equilibrium of the reactions will shift to the right again.

Respiratory alkalosis can be caused by hyperventilation in head injuries or drug induced toxicity: since the hyperventilation decrease CO2 in the lungs, and consequently in blood, the reaction shifts to the left consuming H+ and so, increasing pH.

In the case of Respiratory acidosis, the opposite occurs: Respiratory acidosis can be caused by a chronic lung disease or by a depression of the respiratory rate as a consequence of a CNS disturbance. These conditions can produce an increase of CO2 in lungs, as it increases, the reaction shift to the right, producing an increase in H+ and so, a decrease in pH.

I hope it helps. I suggest to memorize this equation and try to apply it.

CO2 + H2O <-----------> CO3H2 <------------> CO3H- + H+

My blog:

http://www.biochemistryquestions.wordpress.com

Note: I do not mean that the other explanations are not valid. I just wanted to give a different approach