Editor’s note: Physicians have a special place among the thinkers who have elaborated the argument for intelligent design. Perhaps that’s because, more than evolutionary biologists, they are familiar with the challenges of maintaining a functioning complex system, the human body. With that in mind, Evolution News & Views is delighted to present this series, “The Designed Body.” For the complete series, see here. Dr. Glicksman practices palliative medicine for a hospice organization.

Since we live in the material world and our cells are made up of atoms and molecules, our body must follow the laws of nature. These laws demand that our body follow the rules by making sure its cells have enough energy to do what they need to do to keep us alive. Moreover, having enough energy requires our cells to have enough oxygen (O₂). We can’t live more than a few minutes without breathing in air because, in contrast to water, salt, and sugar, our body can’t store oxygen. It must therefore constantly resupply itself.

However, respiration involves much more than just making sure the body has enough O₂ for its energy needs. It also involves getting rid of carbon dioxide (CO₂) and controlling its hydrogen ion (H⁺) content as well, since both of these chemicals can be hazardous to the health of the cell and therefore can cause death.

Recall that our cells use O₂ and a specific set of enzymes and carrier proteins to break the chemical bonds within glucose to get the energy they need through cellular respiration.

The air we breathe in consists of 21 percent O₂, but only 0.04 percent CO₂. On the other hand, the air we breathe out consists of 16 percent O₂, a drop of 25 percent, and 4 percent CO₂, a rise of 100 fold.

This change in composition occurs because our cells use O₂ for their energy needs while producing CO₂. Similarly, most of the H⁺ ions in the body (99 percent) come from cellular respiration. The red blood cells, using an enzyme called carbonic anhydrase, take the CO₂ produced in cellular respiration, and join it to water (H₂O) to form carbonic acid (H₂CO₃). In the blood, carbonic acid breaks up into H⁺ ions and bicarbonate ions (HCO₃^–). So the more CO₂ the body produces through cellular respiration, the more H₂CO₃ the red blood cells produce and the more H⁺ ions are present in the blood.

In general, the body functions best with a blood O₂ level above 70 units, a CO₂ level below 40 units, and an H⁺ ion level between 35 and 45 units. As the O₂ level drops, the CO₂ rises, and the H⁺ ion level falls or rises out of the normal range, the more the body malfunctions and the weaker it becomes. In fact, a blood O₂ level below 30 units, a CO₂ level above 90 units, and an H⁺ ion level below 20 units or above 100 units is usually incompatible with life. So, you can see that keeping control of the body’s O₂, CO_2, and H⁺ ion levels is important for survival. Here’s how the body does it.

As I’ve discussed before, the first thing you need to take control in this context is a sensor to detect what needs to be controlled. The body has chemical sensors (chemoreceptors) that are able to detect O₂, CO₂, and H⁺ ions. Peripheral chemoreceptors for all three of these chemicals are located in the main arteries that send blood directly to the brain. Meanwhile, located in the brain are central chemoreceptors that detect H⁺ ions. The exact nature of these chemoreceptors and how they work is as yet poorly understood. However, just like the gas sensor in the fuel tank of your car, these chemoreceptors are located exactly where they need to be to get the information that is needed to help the body maintain control of the relevant chemicals.

The second thing you need to take control is an integrator that interprets the information from the sensors, compares it to a standard, makes decisions about what needs to be done, and then sends out orders. The data from these chemoreceptors, which detect O₂, CO₂, and H⁺ ions, is sent to the respiratory center in the part of the brainstem called the medulla. The respiratory center analyzes this information and sends out nerve messages to the muscles of respiration. How the respiratory center “knows” what the O₂, CO₂, and H⁺ ion levels should be so the body can live and function properly is a as yet a complete mystery. But, just as the gas sensor in the fuel tank has to be connected to the right gauge on the dashboard and be properly calibrated, so too the data from these chemoreceptors must be sent to the right place and be interpreted correctly.

Nobody really understands how these chemoreceptors came into being simultaneously, positioned in the right place, for life. Because loss of control of any one of these three chemical parameters results in death. Nor, as I mentioned, does anyone understand how the respiratory system inherently knows what the levels of O₂, CO_2, and H⁺ ion should be for us to survive. With such a limited understanding, how is it that evolutionary biologists can be so certain that life came about by chance and the laws of nature alone? Is it plausible that the sensor inside your fuel tank, which is connected to the fuel gauge on your dashboard, arose that away?

Leaving that aside, the third thing you need to take control is an effectorthat receives the orders from the integrator and does something. As noted above, the nerve messages from the respiratory center go to the muscles of respiration and thus make the lungs breathe air in and out. But as you’ll anticipate, when it comes to lung function, real numbers have real consequences. So the next article in this series will describe how the lungs work to allow us to survive in the world, while the post immediately following will describe how lung malfunction results in debility and death.

Image: Fresh Air, by Winslow Homer [Public domain], via Wikimedia Commons.