Acid and Base

I have a test tomorrow on acid-base balance and renal function. I've been told that teaching something is the best way to learn it. You all get to be my students. Don't you feel lucky? Now pay attention, there will be a question at the end.

pH is determined by how many hydrogen atoms you have floating around in your plasma, which is the part of the blood that is not red and white blood cells. pH can also be measured in the urine, cerebro-spinal fluid, and GI tract for medical analysis.

We are going to focus on blood plasma pH, because that is what is measured in an ABG (aterial blood gas). If the hydrogen atoms are high, the pH is low. If there are not enough free hydrogen atoms, the pH reads high. The normal range for pH in the blood is between 7.35 and 7.45, with 7.4 (obviously, but importantly) being the dividing line.

Two compounds control pH through their respective body mechanisms. Let me explain. Carbon dioxide (CO2) is retained or 'blown off' by the lungs. The measure of CO2 in the ABG tells you if the lungs are functioning like they should, therefore it is the respiratory element of the puzzle. A change in CO2 reflects a problem with respiration; unless it is a compensatory mechanism, which I will discuss later. The normal range for CO2 is 35-45 (just like the decimal points in pH).

Bicarbonate (HCO3) is the other compound we look at in an ABG analysis. It is the metabolic element of an ABG analysis. It shows how well the kidneys are doing their job. It could also be out of normal range as a compensetory act for a respiratory problem. The normal range for HCO3 is 22-26.

These values can give you a plethora of information about a patient's state of health and possible causes for the medical problem.

pH: acid-7.35-normal-7.45-base
CO2- base-35-normal-45-acid
HCO3- acid-22-normal-26-base

Still with me? If so, you're a champ. Ok, onward we go.

If you will notice, there is a much wider range for CO2 than there is for HCO3. Respiratory compensation for acid-base imbalance is much quicker than metabolic and so less exact. It is also not quite as powerful as metabolic (think kidneys) compensation.

There are four main pH imbalances. They are:

1. Respiratory acidosis- pH will be <7.35, CO2 will be >45, and HCO3 will be WNL (within normal limits).
Here is an example.

pH- 7.20
CO2- 47
HCO3- 24

Respiratory acidosis is caused by a retention of CO2. This is an easy one to observe. The person will not be breathing. This can be caused by suffocation, barbituate overdose, trauma, pneumonia, sleep apnea, CHF (congestive heart failure), obesity, pulmonary embolism, and other problem that affect the lungs. You get the point. You don't breath, you get respiratory acidosis. To solve the problem, I suggest breathing again.

2. Respiratory alkalosis- pH will be >7.45, CO2 will be <35, and HCO3 will be WNL.

pH- 7.48
CO2- 31
HCO3- 25

Respiratory alkalosis is caused by hyperventilation. A person flips out over seeing a spider, starts breathing too fast, the lungs 'blow-off' too much CO2 and the blood becomes too alkaline. To solve this problem, kill the spider. Aspirin overdose can also cause respiratory alkalosis.

3. Metabolic acidosis- pH will be <7.35, CO2 will be WNL, and HCO3 will be <22.

The lab values could look like this:

pH- 7.33
CO2- 36
HCO3- 20

Metabolic acidosis (remember it's renal) can be caused by diabetic ketoacidosis (DKA), starvation, dehydration, diarrhea, end-stage renal disease (ESRD), hyperkalemia, or lactic acidosis.

4. Metabolic alkalosis- pH will be >7.45, CO2 will be WNL, and HCO3 will be >26.

pH- 7.48
CO2- 40
HCO3- 29

Metabolic alkalosis can be caused by persistant vomiting, gastric suction (getting your stomach pumped), kidney failure, use of a potassium-wasting duiretic, severe burns, or hypovolemia (blood loss, lack of fluids).

Is your head spinning yet? If not, stay with me and I'll tell you about all that compensation mess I mentioned earlier.

This is fairly simple to understand. If the lungs are causing too much acidity, the kidneys will respond with a more alkaline value. If the metabolic value is too acidic, the lungs will try to 'blow-off' more CO2, causing deep, rapid breathing known as Kussmaul's respirations.

If the lungs are causing the blood to be too alkaline, the bicarbonate value will become more acidic. If the kidneys aren't working right and the bicarbonate value becomes to alkaline, the lungs will compensate with a lower CO2 value.

Fascinating, isn't it?

Now for the question...

Do you like spiders?