Acid and Base Imbalances fall into two primary category types: Respiratory and Metabolic. Understanding the underlying physiology of what can cause each type is essential to being able to confidently answer NCLEX questions related to Arterial Blood Gases (ABGs), fluids and electrolytes, or any illness that can cause an acid or base imbalance.
Metabolic Acidosis and Metabolic Alkalosis
The kidneys have the same goal as the lungs do (and if you missed the Respiratory Imbalances article, find it here). The kidneys just use a different mechanism to accomplish that goal. The goal is to either retain or get rid of H+, depending on what the body needs at the moment. The kidneys can either reabsorb H+ as the blood is being filtered through, or they can excrete the H+ into the urine (usually through the form of ammonia, which is NH3). Just like with Respiratory causes, the Ph will go the opposite direction of H+: if the kidneys cause H+ to decrease, then Ph increases (more basic); if the kidneys cause H+ to increase, then pH decreases (more acidic). You can learn my two easy rules that will help you always remember how the pH moves by checking out “Simplify the pH Scale.”
Common causes of Metabolic imbalances are going to be related to the GI system. For example, vomiting causes patients to lose stomach acid, which is primarily made of of hydrochloric acid (HCl; did you notice there’s a Hydrogen atom there?). Less acid makes the body becomes more basic, so extreme cases of vomiting can lead to metabolic alkalosis.
On the other end of the body (literally), the intestines are a much more alkaline environment. So severe diarrhea can cause the body to lose base (resulting in a relative “increase” in H+, since there is now less base to neutralize them). More free H+ leads to metabolic acidosis.
Of course, there are conditions other than vomiting and diarrhea that can lead to Metabolic Imbalances, such as the overuse of antacids or laxatives, ketoacidosis (as seen in Type I Diabetes), and certain medications. But you should notice that even these causes have something to do with the GI system. That is why these are considered metabolic causes.
It’s also important to note that Metabolic changes typically occur more slowly than Respiratory changes. That’s just the nature of each system. After all, the lungs can respond much more quickly to an imbalance simply by increasing or decreasing the breaths per minute…but they can also cause an imbalance much more quickly! Just think about a patient who begins to hyperventilate, and is feeling dizzy and weak within minutes.
The kidneys take a little bit longer to make a dramatic effect simply because they have to wait for the blood to circulate through before they can take action to retain or remove H+. Incidentally, did you think about how kidney disease could play a role in metabolic imbalances?
I hope you’ve found a better understanding of Metabolic imbalances. Leave your questions and comments below!