I love the action movie Top Gun. While capturing the immense delight of flying high-performance aircraft, the movie also highlights the magnitude of physiological stressors that military aviators encounter. When men and women first took to the skies, it was soon discovered that pilots would pass out when trying to recover from a steep dive. And with the advent of high-velocity jets, the physiological challenges pilots face became more pronounced.
During certain high-dynamic maneuvers, the increased positive gravity (positive g's) impedes the flow of blood up the neck and to the brain, resulting in fainting (syncope) and probable death. Physicians and engineers combined efforts in the early 1940s to come up with a solution—and the g-suit was invented.
The g-suit is a garment that encloses an aviator's legs and abdomen and is fitted with a system of internal air bladders. A hose connected to a pneumatic source is used to inflate the bladders, causing them to compress upon the legs and abdomen. The resulting constriction impedes arterial blood flow down the body. In other words, the constriction causes a "traffic jam” in the lower arterial system. This, in turn, artificially elevates the blood pressure in the upper arterial system, which helps to avoid syncope.
Of course, the g-suit system must be activated and deactivated selectively. For such, there is an accelerometer in the aircraft that senses the onset and magnitude of the g-forces and activates the pneumatic pump to counter the effects of blood pooling in the lower extremities. The sensor also detects the decrease and termination of the increased g-force and adjusts the pneumatic pump, accordingly, to allow blood to flow back into the pilot's lower extremities.
Additionally, pilots are trained to perform certain acts that help to maintain the blood in the thorax and above by increasing the pressure in the abdomen. Typically, most humans can withstand 3–5 g's, as anyone who has survived a great roller coaster ride can attest. Both the g-suit and the g-straining maneuver provide an increase in resistance.
A Biological G-Suit
Now we come to the crux of the issue. If you've ever picked up a water balloon by the knotted end, then you know that all the water just flops to the bottom. Humans are like water balloons. When we stand up, the arterial blood heading to the brain has to move uphill against gravity, and so suffers impedance. Simultaneously, the venous blood in the lower extremities trying to return to the heart is also impeded, causing a decreased supply of blood to the heart and, consequently, a decreased cardiac output. Together, these two phenomena may cause a decrease in blood supply to the brain, making an individual feel light-headed and weak (pre-syncope) or susceptible to losing consciousness (syncope). Obviously, the average healthy person rarely faints upon standing up. Why is that? Because we have a g-suit system built into our bodies!
Like the accelerometer in the aircraft that detects changing g-forces, humans have baroreceptors in the sinuses of the carotid arteries that detect changes in blood (hydraulic) pressure. If these receptors detect a sudden drop in pressure, they emit a signal to the brainstem via the glossopharengeal nerve. After several connections and pathways, some excitatory and some inhibitory, activity of the sympathetic branch of the autonomic nervous system increases and constricts the arteries (vasoconstriction) in the bilateral lower extremities.
And like the air bladders in a pilot's g-suit, this constriction impedes the flow of arterial blood traveling down the body and forces the blood in the upper arterial system higher into the neck and head, thus avoiding syncope.1 Sound familiar? Additionally, the heart is signaled to beat faster, thereby increasing cardiac output.
A Result of Evolution—or Design?
Such a complex integrated function presents a challenge to Darwinian evolution, which requires generating such a system in a piecemeal fashion. However, without the detectors (baroreceptors), the regulators (command and control), the integrators (neural communication systems), the actuators (the muscularized arteries and adjustable heart rate), and the necessity (maintaining consciousness/perfusion to the brain) all at the same time and in the same organism—and all of it working at a velocity suitable to the condition/stimulus (defined performance specifications)—then the function cannot work. To draw from biochemist Michael Behe, the human body's g-suit function is "irreducibly complex.” But the evolutionary mystery only gets deeper.
Another enormous difficulty for the evolutionary paradigm is the fact that humans demonstrate a physiologic capacity (to tolerate up to 3–5 g's) that is useful to us now, but which theoretically evolved without an environmental pressure to enable or "guide” natural selection. How then can strictly naturalistic processes account for the human organism evolving or adapting such a function to begin with? We do not experience high dynamic states except as an artifact of the modern Western industrial revolution. To have evolved this capacity seems impossible.
The reflective reader may ask, "Isn't this function found in other mammals?" Yes, it is. But though some might argue that this "homologue" supports Darwinian evolution, it still suffers from the same problem of irreducible complexity. Moreover, we can argue that the correspondence between the human and the animal capacity to tolerate certain levels of g-forces demonstrates common design—a divine Engineer's reuse of functional systems and designs—rather than common descent.
A simple solution to this conundrum posits design by an intentional Creator. After all, we know that the g-suit created for aviators required thought, planning, and design by human agents. Both the very existence of humanity's built-in g-suit function and the function's irreducible complexity make much better sense from a creation model perspective.
*Article updated May 28, 2014
Dr. Eddy M. del Rio
Dr. Eddy M. del Rio received his MD from Saint Louis University in 2004, and currently serves as a practicing physician for the Veterans Health Administration in the greater Springfield, MO region.