Prior to watching this video, I hadn’t given too much thought as to how the cardiac system maps onto a control system. However, it is clear that the cardiac system is well modeled by a standard closed-loop control system. The natural pacemaker of the heart-- the sinoatrial node-- is essentially an effector that is controlled by a network involving the spinal cord, brain stem, and hypothalamus. Based on input to this network regarding blood oxygen levels, arousing or relaxing stimuli, neurotransmitters are released into the bloodstream. Neurotransmitters such as epinephrine and norepinephrine stimulate the sympathetic nervous system which is implicated in the “fight or flight” response, and consequently increase heart rate. On the other hand, acetylcholine stimulates the parasympathetic nervous system and lowers the heart rate. These neurotransmitters work by binding receptors in the sinoatrial node, thereby inducing depolarization or hyperpolarization to regulate the firing of action potentials, and thus, the rhythm of the heart. 

When the sinoatrial node does not function as it should, the heart beats sporadically and is often stressed by the lack of proper control of the rhythm. What a pacemaker seeks to do is replace in part the natural signaling pathways in the body that act as the control system for the heartbeat and manually stimulate the heart with electrical signaling. Basically, a pacemaker is an artificial control system that replaces an “improperly tuned” or uncontrollable natural control system. Cool! 

It’s interesting to reflect on my biology class and MCAT preparation and note how much of biology is controls systems. In the context of these life sciences courses, we always call this control “regulation,” but in essence, it’s all about controllers, disturbances to the system, and response time, just like in Controls class. For example, enzyme-inhibition feedback loops, by which a product allosterically inhibits the enzyme involved in its synthesis, is very common in the metabolic processes of glycolysis and the citric acid cycle-- this is a feedback inhibition loop! Similarly, the entirety of the nervous system is a control system-- probably the most complicated one an engineer could ever encounter! After all, we still aren’t sure exactly how the ultimate controller-- the brain-- works. It’s so much more complicated than determining the transfer function that described the controller. 

Where engineering control theory really takes off in the world of medicine and biology is when things aren’t working as they should be. For example, in diabetes when signaling for insulin release is impaired, or for individuals who have arrhythmia due to a misfiring sinoatrial node. In these instances, engineers have the chance to implement man-made attempts to replicate nature’s control systems-- we’ve come up with glucose pumps, artificial pancreases, and pacemakers. All of these innovations are truly marvels of engineering, but it’s pretty wild to consider just how excellent an “engineer” nature is. 

Come to think of it, we could call Intelligent Design “Intelligent Engineering” and it might be more appropriate, given the extent of engineering (way beyond just control theory-- what about that materials science! ;)) that goes into life as we know it.

References

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4404375/

https://sciencing.com/body-regulate-heart-rate-19639.html

https://www.youtube.com/watch?v=xYtJK9JdAN0