One of my favorite parts of medical school is getting to learn about the human body’s ability to self-regulate. Life is fragile – don’t get me wrong – but the amount of abuse our bodies can withstand is pretty eye-opening. I thought it would be fun to write about one such example. There are dozens of similar topics I could have chosen, so perhaps I’ll write more posts like this if it’s well-received.
Many of you reading this are probably familiar with the “blood-brain barrier” – a physiological barrier that protects the brain (our most important organ…at least according to the brain) from harmful substances in the bloodstream. More specifically, the barrier seals off surrounding tissue from substances in the blood that are relatively large or polar, while allowing the passage of smaller, nonpolar substances.
Since [H+] ions are naturally prevented from crossing this barrier, it serves as an excellent form of protection against metabolic acidosis or alkalosis. This means that even when a person’s pH levels get thrown out of whack, the brain remains relatively insulated. Such a person might become quite sick until the body’s other homeostatic mechanisms are able to bring pH back into the normal range…but the brain is much less likely to suffer significant or irreversible damage.
So that’s great for metabolic acidosis or alkalosis, when [H+] ions are responsible for the change in pH. In respiratory acidosis or alkalosis, however, the pH changes are being driven by CO2 – a nonpolar molecule that diffuses into the brain with ease.
The blood-brain barrier, it turns out, is utterly ineffective in protecting brain tissue from respiratory acidosis or alkalosis.
This is where things get pretty cool. It turns out that glial cells in the brain are designed to respond to abnormal changes in pH by increasing their production of lactic acid (in the case of respiratory alkalosis) or ammonia (in the case of respiratory acidosis). Since the brain can’t just “seal itself off” from the effects of CO2-induced pH changes, it responds by directly buffering the ECF.