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Respiratory vs metabolic acidosis/alkalosis

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Respiratory vs metabolic acidosis/alkalosis

Postby shorty7805 » Wed Apr 13, 2011 10:24 am

Hi all,

Just wondering if anyone can help me with understanding respiratory and metabolic acidosis/alkalosis. I understand the acidosis/alkalisos part, just not how to determine whether it is respiratory or metabolic. The types of questions I have been asked in quiz's are:

Blood analysis indicates a low pH, and the patient is breathing rapidly. which of the following is most likely?
respiratory acidosis, metabolic acidosis, metabolic alkalosis or respiratory alkalosis (answer, metabolic acidosis)

AND

A patient is breathing slowly and blood pH analysis indicates an abnormally high value. What is the likely diagnosis?
respiratory acidosis, metabolic acidosis, metabolic alkalosis or respiratory alkalosis (answer, metabolic alkalosis)

Thanks!
shorty7805
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Postby CarlosG » Thu Jul 28, 2011 5:03 am

This is an old question, but I will answer it for the sake of people doing searches.

Respiratory acidosis/alkalosis is normally a result of a deviation from normal rates of CO2 exchange; that is to say, the body is not keeping up with the demand, or exceeding it. The two textbook causes are hypoventilation (acidosis), and hyperventilation (alkalosis).

As a result of aerobic respiration, your body produces CO2, which when mixed with water, creates H2CO3, which disassociates into HCO3- and H+, a reversible reaction

When you are not exchanging enough CO2 (hypoventilation), you increase CO2 concentration. This will lower the pH because of the above mechanism. If you are exchanging too much CO2(hyperventilation), you are driving the equilibrium to the left via Le Chatelier's (sp?) principle and increasing the pH.

What you are witnessing in the questions you posed is the body's natural reaction to deviations from homeostasis. Breathing fast (1st question) would counter a low pH by getting rid of excess CO2. Breathing slow (2nd question) would lower pH by keeping more CO2 in circulation, countering his high pH.

Metabolic acidosis/alkalosis can be attributed to a myriad of causes, from excessive vomiting, diabetes, diarrhea, lactate build up, massive cell death, etc etc.

The mechanism is different in many cases, but the end result is a deviation in pH, which overcomes the body's buffer systems. The big difference is that metabolic acidosis and alkalosis are not due to respiratory issues.
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