Mechanism for compensatory RESPIRATORY acidosis caused by metabolic alkalosis?

[shredder666]

mechanism for compensatory RESPIRATORY acidosis caused by metabolic alkalosis?

I don't get it... someone please explain :(

 

[fedaykin]

It's mostly from slowed respiratory rate. Chemoreceptors lining the blood vessels, especially the carotid bodies detect blood CO2 levels. The decreased partial pressure of CO2 will cause these bodies to send less stimulatory signals to the respiratory center in the medulla which leads to decreased respiration rate and retention of more CO2 (which becomes carbonic acid in the blood). **edit, the carotid bodies detect H+ concentration, not CO2 partial pressure directly.

Take this with a grain of salt as I'm only an undergrad who's read Guyton & Halls Physiology text a few times.

 

[SW VandeCarr]

It's mostly from slowed respiratory rate. Chemoreceptors lining the blood vessels, especially the carotid bodies detect blood CO2 levels. The decreased partial pressure of CO2 will cause these bodies to send less stimulatory signals to the respiratory center in the medulla which leads to decreased respiration rate and retention of more CO2 (which becomes carbonic acid in the blood). **edit, the carotid bodies detect H+ concentration, not CO2 partial pressure directly.

Take this with a grain of salt as I'm only an undergrad who's read Guyton & Halls Physiology text a few times.

This is essentially correct in terms of compensation for metabolic alkalosis. The kidneys also pitch in by excreting more bicarbonate when they are not the primary cause of metabolic alkalosis (via loss of H ions). BTW, the frequently used descriptions metabolic alkalosis with respiratory acidosis, or metabolic acidosis with respiratory alkalosis are somewhat misleading. The patient is either acidotic or alkalotic if not normal. It's better IMO to say metabolic alkalosis with partial respiratory compensation, etc.

Causes of metabolic alkalosis are due to anything that causes loss of extracellular H ions (such as excessive vomiting or low extracellular potassium causing a shift of H ions into the intracellular compartment in exchange for potassium) or excess bicarbonate. One of the unfortunate causes of this is excess bicarbonate administration in the treatment of acute keto or lactic acidosis in diabetics.

 

[mtc1973]

Metabolic alkalosis results in a depletion of blood H ions right - leaving blood pH higher than normal. Enzymes don't like the pH being off in the slightest - it affects their function - it affects the function of all proteins and all processes. Therefore there is a rapid way to get more H ions into the blood and correct pH. Slow breathing down - by reducing ventilation the partial pressure of CO2 builds up in blood - Co2 and water form bicarbonate and importantly for blood pH - a H ion. So now blood pH comes back down to normal pH. People generally consider the kidney when we talk of acid base control - but actually kidney takes hours to days to change function. the lungs by changinf ventilation can rapidly change blood ph. Slow ventilation - increase CO2 and acidify blood. Increase ventilation - blow off CO2 and reduced the H ion concentration.

All linked to

CO2 + H2O - H+ and HCO3-

by changing CO2 we drive the reaction forward or back. And hence compensate for any change in blood pH.