Chemistry
Chemistry in the clinical lab covers a broad range of subjects from toxicology and endocrinology, to liver and heart health, blood gases, electrolytes, and diabetes. The key to understanding chemistry is understanding basic concepts we encounter frequently but sometimes don’t fully understand. Let’s get started.
pH:
The normal reference range for blood pH in the human body is 7.35-7.45. It is a highly regulated narrow range, and your body will compensate to keep the pH in the 7.35 -7.45 range. When the pH is lower than 7.35, this is referred to as acidosis (an acidic environment), and when it is above 7.45 it is referred to as alkalosis (a basic environment). pH is determined by the hydrogen ion (H+) concentration in the blood.
Bicarbonate buffer system:
One of the big keys to understanding acid-base reactions in the blood is to understand the bicarbonate buffer system! This equation is of utmost importance!
CO₂ + H₂O <-> H₂CO₃ <-> H+ + HCO₃-
CO₂ = carbon dioxide
H₂O = water
H₂CO₃ = carbonic acid
H+ = hydrogen ion
HCO₃- = bicarbonate
The body’s buffer system wants to keep the pH around 7.35-7.45. This buffer system will compensate in either direction depending on if the pH is too low (acidic) or too high (basic).
Henderson-Hasselbalch equation:
pH = 6.1 + log(HCO₃-/(0.03 x PCO₂))
This equation can help illustrate the important relationship between bicarbonate and carbon dioxide. The only inputs into the equation are bicarbonate and carbon dioxide. The equation can be broken down into a simple bicarbonate to carbon dioxide ratio, and this ratio determines the pH.
HCO₃- compensation comes from the kidneys (renal) and the compensation is referred to as metabolic. CO₂ compensation comes from breathing (lungs) and the compensation is referred to as respiratory.
When the pH system gets out of whack there are four conditions you need to be aware of: metabolic acidosis, metabolic alkalosis, respiratory acidosis, and respiratory alkalosis.
Metabolic acidosis:
↓pH ↓ HCO₃-
When you see metabolic your first thought should be HCO₃-. When you look at it as HCO₃- acidosis, it’s easy to see that will mean a decrease in HCO₃- because HCO₃- is a base. When HCO₃- is decreased, it tips the scale in the image above to the right (acidic) because there is less (think a teeter-totter).
Causes of metabolic acidosis:
1. Acid production exceeds elimination (bicarbonate can’t keep up)
2. Renal failure (acid accumulates and since your kidney isn’t functioning properly HCO₃- isn’t either)
3. Increased excretion of bicarbonate (too much bicarbonate is being lost resulting in increased acid)
Compensation:
Hyperventilation will lower PCO₂ (you inhale O₂ and exhale CO₂) which normalizes the HCO₃-/CO₂ ratio which raises the pH.
Metabolic alkalosis:
↑pH ↑HCO₃-
Same thing here, HCO₃- alkalosis means there is too much base (HCO₃-). Now we’re going the opposite way on the scale raising the pH.
Causes of metabolic alkalosis:
1. Acid loss
2. Bicarbonate increase
Compensation:
A decreased respiratory rate (less CO₂ lost) or renal excretion of HCO₃- will raise the acid in the body.
Respiratory acidosis:
↓pH ↑PCO₂
When you see respiratory, your first thought needs to be CO₂. Since CO₂ is an acid, too much acid from breathing (aka respiratory acidosis) must mean an increase in CO₂ and a decreased pH.
Causes of respiratory acidosis:
1. Chronic lung obstruction
2. Acute airway obstruction
3. Circulatory failure (insufficient delivery of blood to lungs)
4. Impaired function of respiratory center (head trauma, anesthesia)
5. Insufficient O₂ from mechanical ventilation
Compensation:
Increased renal excretion of H+ ions, and renal absorption of HCO₃- (both of these have a slow onset, the bicarbonate system cannot react as quickly as the carbon dioxide system).
Respiratory alkalosis:
↑pH ↓PCO₂
In respiratory alkalosis CO₂ is lost faster than it is produced.
Causes of respiratory alkalosis:
1. Hyperventilation
2. High altitude
Compensation:
H+ ions from tissue and RBCs consume a small amount of HCO₃-. Renal compensation by increased excretion of HCO₃-.
Study tip
The best thing about acid-base conditions is you can think your way through if you are aware of a few simple rules.
Rules:
1. Acidosis and alkalosis determine the pH, acidosis down, alkalosis up
2. Metabolic means you figure out what bicarbonate is doing first
a. For example, in metabolic acidosis the pH is becoming more acidic which means the bicarbonate which is basic must be dropping.
3. Respiratory means you figure out what carbon dioxide is doing first
a. For example in respiratory alkalosis the pH is becoming more basic which means the PCO₂ which is acidic must be dropping.
4. After you have determined what bicarbonate or carbon dioxide are doing, figure out what either bicarbonate or carbon dioxide would do to compensate and bring the system back to equilibrium.
a. For example in metabolic alkalosis, the pH is becoming more basic which means the bicarbonate is increasing. What would carbon dioxide do to “compensate” for the increasing basic environment? It will increase since it’s acidic to lower the base.
| pH | HCO₃- | PCO₂ | |
| Metabolic acidosis | ↓ | ↓ | ↓ |
| Metabolic alkalosis | ↑ | ↑ | ↑ |
| Respiratory acidosis | ↓ | ↑ | ↑ |
| Respiratory alkalosis | ↑ | ↓ | ↓ |
red arrows = compensation
If however you still find yourself unsure, there are a few memory tricks that can be used:
1. In metabolic disorders the HCO₃- and PCO₂ are in the same direction as the pH.
2. In respiratory disorders, the HCO₃- and PCO₂ are in the opposite direction as the pH.
If you’re time crunched, these tricks can be helpful.