Evolution Icon Evolution
Intelligent Design Icon Intelligent Design
Medicine Icon Medicine

Understanding Respiration: Why Real Numbers Really Matter


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 our body is made up of cells that consist of atoms and molecules, it must follow the laws of nature. These laws demand that for the body to survive in the world it must have enough energy. The body obtains this energy from cellular respiration, which breaks down glucose in the presence of oxygen (O2).

the-designed-body4.jpgWe can only live for a few minutes without O2, but the amount of O2 our body needs is related to what we’re doing. The more active we are the more O2 our body needs. The respiratory center in the brainstem monitors the level of O2, carbon dioxide (CO2), and hydrogen ion (H+), while also receiving data on muscular activity. It analyzes this information and sends out orders to the lungs telling them to breathe in and out at a rate and depth that allows the body to keep doing what it needs to do to survive. Here’s how that works.

The lungs consist of large airways called bronchi and smaller ones called bronchioles. They are lined with mucous membrane that warms and humidifies the air as it moves down into the hundreds of millions of alveoli, deep inside the lung. Each alveolus is surrounded by hundreds of small blood vessels called capillaries. It’s in the capillaries that O2 enters the bloodstream and CO2 comes out.

The main force that makes the lungs breathe in air is the dome-shaped sheet of muscle, called the diaphragm, which points upwards and separates the chest from the abdomen. The laws of nature determine how air is forced into the lungs by the action of the diaphragm. These laws state that the pressure inside a chamber, with a given amount of air or water, is inversely proportional to the size of the chamber. If the space inside the chamber decreases, the pressure inside increases, and if the space inside the chamber increases, the pressure inside decreases.

Think of how your heart works. The heart fills up with blood and then, when it contracts, making the space inside the chamber decrease, the increased pressure within the chamber propels the blood out into the arteries. The heart pumps blood by positive pressure.

The lungs work in exactly the opposite way. When the diaphragm contracts, causing it to flatten, this increases the volume of the chest cavity, decreasing the air pressure inside the lungs. When the air pressure inside the lung drops below the air pressure outside the body, this forces air into the lungs by negative pressure. If you want to get a feel for what this vacuum effect is like, hold your nose and keep your mouth shut, then try to breathe in as hard as you can. The tugging sensation in your head, neck, and upper chest area is due to your diaphragm contracting and being frustrated at not being able to expand the lungs.

At rest, the body uses about 250 milliliters per minute (mL/min) of O2. Think of it like how much fuel a car uses when it’s idling. It needs a minimum amount of energy just to get the pistons in the engine going while the oil lubricates the moving parts and the anti-freeze circulates to keep everything cool. So too with the body. It needs a minimum amount of energy just to maintain adequate brain, heart, lung, kidney, and liver function, to name a few. The tidal volume is the amount of air moving in and out with each breath. For most people, at rest, their respiratory center sets the tidal volume at 500 mL/breath and the respiratory rate at about 12 breaths/minute. Let’s see if that’s good enough to get the required 250 mL/min of O2 at rest.

We first need to realize that 150 mL of the tidal volume (500 mL) is in the airways that aren’t involved in gas exchange. So, at rest, the amount of air the alveoli actually receive with each breath would be about 350mL (500 – 150). A respiratory rate of 12 breaths/min would result in the alveoli receiving 4,200 mL/min (350 mL x 12) of air. However, since O2 only represents 21 percent of inspired air, this means that the amount of O2 the alveoli receive would be about 880 mL/min (4,200 x 0.21). Finally, it’s important to realize that the alveoli do not take all of the O2 out of the air and put it into the blood. In fact, only about 30 percent of the O2 crosses into the circulation. Thus the amount of O2 the blood receives would be about 264 mL/min (880 x 0.3). This matches almost perfectly the amount the body needs at rest (250 mL/min). The respiratory center seems to know what it’s doing.

Any increase in O2 use by the body, above 250 mL/min, is generally due to digestion or physical activity. To stay active our body needs to use more O2 to give our muscles the energy they require to overcome forces such as inertia, gravity, friction, and wind resistance.

But our lungs and heart require more O2 as well to provide the energy they need to bring in more O2 and send it to the muscles so they can keep doing what the body wants them to do. Maximum activity requires at least 3,500 mL/min of O2. For our ancient ancestors, being able to bring this much O2 into the body would have been the difference between eating or being eaten. Needless to say, the respiratory center must know what’s required and the respiratory system must be able to deliver it, otherwise we wouldn’t be here. Let’s see what that takes.

If the body is very active and in need of increased O2, then the accessory muscles of respiration will swing into action. These muscles, located in the neck, the shoulders, the upper back, and the abdomen, assist the diaphragm and the muscles between the ribs, to expand and contract the chest cavity as much as possible. Try breathing heavily and notice which muscles and what parts of the body you’re using.

With physically fit people, the volume of air they can send to their alveoli often exceeds 100 liters/min. That would be an average tidal volume of 2,500 mL per breath at 40 breaths/min. But, since inspired air only contains 21 percent O2, it would mean that the amount of O2 delivered to the alveoli would be about 21,000 mL/min (100,000 x 0.21). And since 30 percent of the O2 in the air that is breathed in crosses into the circulation, it follows that the amount of O2 sent to the blood would be just over 6,000 mL/min (21,000 x 0.3). This would have been more than satisfactory for our ancient ancestors to have the energy to prey upon others and avoid being preyed upon themselves. So it would appear that not only does the respiratory center know what it’s doing, but the respiratory system itself has the capacity for life.

Clearly, the respiratory system must have given our ancestors the right numbers to maintain a functional ability to survive. However, just as a defective car can have problems with performance, or not work at all, so too, various defects in lung function can lead to the loss of control and result in debility and death.

That’s what we’ll look at next time. Meanwhile, think about the different parts of your respiratory system, how your diaphragm brings air into your lungs by negative pressure, and how your respiratory center inherently knows what your blood levels of O2, CO2, and H+ ion should be to keep you alive. Then honestly ask yourself if what evolutionary biologists say about how you came into being makes any sense at all?

Image by Korea.net / Korean Culture and Information Service [CC BY-SA 2.0], via Wikimedia Commons.

Howard Glicksman

Dr. Howard Glicksman is a general practitioner with more than forty years of medical experience in office and hospital settings, who now serves as a hospice physician seeing terminally ill patients in their homes. He received his MD from the University of Toronto and is the author of “The Designed Body” series for Evolution News. Glicksman further develops the arguments from this series in a book co-authored with systems engineer Steve Laufmann, Your Designed Body (2022).



Continuing SeriesHealth & WellnessScienceThe Designed Body