Acid Base Balance
Acid Base Balance Chapter Summary and Credits
Carter Allen and Jennifer Kong
Acid Base Balance Chapter Summary
Identifying acid imbalance can be a key piece of the puzzle in determining diagnoses and prognoses for a patient. We can determine a lot from our venous blood gases and associated labs. Anion gap will indicate the presence of organic acids, CO2 levels can give us clues about ventilation and perfusion, and our bicarbonate levels can indicate mixed disorders or tell us about kidney compensation.
Diagnosing acid base imbalance is a stepwise and relatively straightforward process, and once we have established a patient meets criteria for a particular imbalance, we can then associate it to an underlying process. Determining the underlying process is essential as treatment of that process is the mainstay of treatment for acid base imbalance.
The body is its own fluid ecosystem, much like an aquarium. Balance is achieved through both top down and bottom up regulation. We often think of those top down control mechanisms: the function of the nervous and endocrine systems, action potentials, various protein receptors and communication networks of signaling molecules. However, it’s important to remember the the body has evolved to take advantage of the laws of physics as part of its regulatory system. By taking advantage of concentration gradients, electrochemical attraction, osmotic forces and equilibria constants, the body regulates its processes.
This decentralization of effect and affect make acid base balance a great example of bottom up metabolic regulation in the human body. Ultimately this system does well to remind us that the organism is a temporary repository of order which utilizes a constant flow of energy to vigilantly maintain equilibrium. The regulation of this state of order runs upstream against the universes tendency to entropy. When the mechanisms that separate us from our environment begin to break down, we find that we must use our minds to assist our bodies in achieving equilibria. If all interventions fail, the organism is no longer separate from the environment, the system will slowly give way to entropy and amalgamate with the environment once again.
Key Takeaways
Acid Base Balance becomes an issue when:
- blood pH goes below 7.35 (acidotic) or above 7.45 (alkalotic) as these abnormal pH will cause impairment in protein structure and function, electrolyte shifts, and impairments to action potentials in electrically excitable tissue.
- The Henderson Hasselbalch equation demonstrates that water + CO2 are in equilibrium with carbonic acid (H2CO3) and that CO2 retention (hence acid) in lungs can lead to a respiratory acidosis. Conversely, a loss of CO2 (hence acid) via ventilation can cause a respiratory alkalosis.
- Metabolic acidosis/alkalosis are caused when the body’s tissues (excluding the lungs) have acquired or retained too much organic acid (acidosis) or base (alkalosis).
- Metabolic alkalosis can also be caused by loss of H+ leading to an abnormal retention of bicarbonate.
- Metabolic acidosis can be further subdivided based on the anion gap, a measurement of the total number of anions available to maintain electroneutrality with the total number of cations which H+ is a contributor. A high anion gap (>12) suggests that the acidosis is caused by an accumulation of organic acid whereas a normal anion gap (<10 or normal) suggests the cause is due to a loss of HCO3–.
- The body will try to compensate for acid base disturbances with its buffers, via ventilation, and fine tuning urine composition for excretion. The goal of compensating respiratory acidosis is to eliminate H+ via urine as well ask trying to promote better ventilation to eliminate CO2. Respiratory alkalosis will see a retention of H+ in urine while the body tries to reduce respiratory rate to retain CO2.
- Compensation for metabolic disturbances are first addressed by ventilation: metabolic acidosis will cause increase rate and depth of breathing to eliminate CO2 (and hence carbonic acid). Metabolic alkalosis will case decreased respiratory rate as CO2 retention will lead to more carbonic acid which will help lower the alkalotic pH.
- Clinical manifestations of pH disturbances are variable due to the cause of the disturbance. Signs of compensation will be noticed as changes to respiratory rate and depth, moreso than changes in urinary composition from renal compensatory mechanisms. However, complications – such as electrolyte disturbances – will manifest in signs of nervous, cardiac, and muscular dysfunction.
- While the body compensates for the pH disturbance, the health care team should work to address the underlying cause to the disturbance.
Credits
Author: Carter Allen & Dr. Jennifer Kong (BCIT & UBC)
