Solving the Numbers Problem - Alveoli
Now that we have air in our lungs, that’s pretty much the whole story, right? Wrong! We’re just beginning the real adventure – the journey those oxygen molecules take as they travel to each of our cells. In this article we’ll begin to understand the huge number of molecules involved in each breath, and the marvelous design features of the lungs which allow us to handle all of them
The 5% that is keeping us alive!
A normal breath brings in around ½ liter or 500 mL of air into our lungs (the volume of a typical small water bottle). Within that volume of air are the precious oxygen molecules which will make their way from your lungs into your bloodstream. If you look at the table here you will see that we don’t use all of the oxygen we breathe in. Notice how the air we breathe in is 21% oxygen, while the air we breathe out is 16% oxygen. What happened to the 5% that is missing? Those are the oxygen molecules that actually make it into our bloodstream. This is the 5% that is keeping us alive. The other gas concentration which changes quite a bit from inhaled to exhaled air is carbon dioxide, which we’ll discuss in a later article.
Crew Aboard the ISS
Since 2000 astronauts have been living on the International Space Station which orbits 250 miles above the Earth. Our oxygen needs don’t change when in space, so the ISS is equipped with solar panels whose electricity is used to split water (H2O) to make oxygen. According to NASA, the daily oxygen requirements for an adult is 0.84 kg, or 840 grams. (1) For someone breathing 15 times per minute (every 4 seconds) the amount of oxygen this represents is 0.0389 grams per breath, which is slightly higher than the figure mentioned in an earlier article, but we’ll go with what NASA says. (2) Using some calculations from the field of chemistry we can calculate just how many oxygen molecules have to enter our lungs in each breath in order to keep us alive (2)
Are you ready for this? On average we need a whopping 732,000,000,000,000,000,000 (7.32 x 10^20) molecules of oxygen in each breath to keep us alive! (2) Just how big is that number? The oxygen molecules you need in every breath is 100 times greater than the number of grains of sand in all the beaches and deserts of the world! (3) Four seconds later we’ll need that many again, and that never changes, even while we’re sleeping. Keep in mind that this is the amount of oxygen needed when we’re at rest (sitting down). If we’re doing something really active like playing soccer or working out our demand for oxygen can increase by as much as eight times!
Let’s imagine the oxygen molecules which enter our bloodstream from one breath are lined up in a single file line. For every inch of the line there would be over 100 million oxygen molecules. How long would our line be? Three feet? Three football fields? Three miles? Our imaginary line of oxygen molecules, even as small as they are, would stretch over 113 million miles! That is farther than the distance to the sun (93 million miles) and would wrap around the Earth over 4,500 times! (4) So, the question we want to answer is: “How can our lungs possibly handle that many molecules every 4 seconds?”
Imagine you work at Disneyland and are responsible to load 732,000,000,000,000,000,000 passengers onto your ride every four seconds. It would really be more like every 2 seconds, because you will need 2 seconds to first unload the passengers who are getting off. Your plan is to have each passenger placed directly in front of their car when the train pulls up. How long would that train of cars have to be? Assuming each car on the ride can hold 4 passengers, you would need a train with 1.83 x 10^20 cars. If each car is 6 ft long the entire train of cars would stretch 2.1 x 10^17 miles! That is equal to over 22 million trips to Pluto and back. Hmmm. How would that train fit on Earth? Can you imagine the chaos and impossibility involved with having that many passengers get off the train and then reloading the same amount every four seconds?
Just like all our passengers loading onto a car at the same time, each one of our 7.32 x 10^20 molecules of oxygen must have their own space or surface area to pass through the tissue of the lungs into the bloodstream at the same moment. The oxygen molecules, however, are by no means alone within the lungs. For each oxygen molecule there are 4 nitrogen molecules milling around, plus an assortment of CO2, argon, and water vapor molecules all mixed up with each other. Imagine trying to load your 7.32 x 10^ 20 passengers in four seconds when they are fighting their way through a crowd four times their size. To make matters worse, there are roughly the same number of CO2 molecules which have to exit from the blood into the lungs in each breath. We’ll talk more about that when we get to “Solving the Trash Problem”.
Most people are familiar with the approximate size of their lungs, each one being about the size of a small football, but they think of them as simple bags which can inflate and deflate. If that were true, the total surface area of our lungs would be around 340 sq. inches or 2. 4 sq. feet. (think of cutting open 2 paper lunch bags and spreading them flat open and taping them together to see how much area they cover). Would there be enough surface area for the oxygen molecules from each breath to find a place to enter through the lung tissue and into our bloodstream? The answer is, “NOT EVEN CLOSE!” Less than 1% of our needed oxygen could find its way into our bloodstream if our lungs were like empty sacs. We would quickly die. How much surface area is needed? If the oxygen molecules from one breath were to suddenly fall to the ground and huddle next to each other they would take up around 500 sq. feet, which is roughly the size of a two-car garage and rougly 200 times too small an area for our oxygen molecules to find space to enter our bloodstream (5). God’s ingenious solution to providing enough surface area for the necessary oxygen to enter the bloodstream comes in the form of tiny grape-like sacs called alveoli.
Illustration of alveoli in lungs
When the air we breathe in enters our right and left lungs it is not an empty sack it finds, but a network of smaller and smaller bronchiole tubes which all end in a cluster of microscopic spheres which resemble tiny grapes. These tiny air sacs are each roughly the size of one grain of salt, but there are an average of 480 million alveoli in adult lungs. (6)
If you were to take each one of them and open them up to spread them out like our paper sacks their combined surface area would be not 2.4 sq. ft, but somewhere between 700 – 1,000 sq. ft! That is almost the size of 1/2 of a tennis court! The use of alveoli in our lungs does not just double the surface area capability of our lungs, or make it 10 times greater, or even 100 times greater, but around 300 times greater! (7) Can you imagine this perfect design solution coming about through the blind chance mechanisms that evolution relies on? See End Notes: Blind Chance
Capillaries cover the alveoli waiting to transport their oxygen passengers to our cells.
A bronchiole ending in alveoli taken with electron scanning microscope. Around 1.5 trillion oxygen molecules will find their way through each of our 480 million alveoli in each breath!
OK, but can the oxygen molecules pass through these alveoli into our blood? Yes! The expression “paper-thin” is not adequate to describe alveoli! Scientists estimate that the alveoli walls are super-thin, around 25 nanometers (nm). making them around 4,000 times thinner than a piece of paper. (8) Gas molecules like oxygen are able to diffuse through this without any problem. The outside of the alveolar walls are covered with capillaries, very small blood vessels carrying their cargo of carbon dioxide which when unloaded will make room for the precious oxygen molecules.
Whew! That was a lot of big numbers! What does it all mean? If our lungs were just like empty bags and we tried dumping all the oxygen molecules in each breath into them, less than 1% would find their way into our bloodstream. We’d never make it. Thank God, that’s not the case. Instead our lungs come equipped with a surface area of nearly 1/2 a tennis court through which each of the 7.32 x 10^20 oxygen molecules you breathe in during every breath can easily pass through to enter your blood.
Take another deep breath - breathe in and as you feel your chest expand, think about the nearly 1/2 billion tiny air sacs that are filling with their precious cargo of oxygen. Each of these salt grain-sized air sacs provides entry for an average of 1.5 trillion oxygen molecules every 4 seconds of your life. Rest in peace. God holds your life-breath in His hands.
Questions to consider: What chance of survival would a species have if it had to wait for a random genetic change (mutation) which would alter proteins enough to create alveoli at the end of the bronchi? Do the alveoli appear designed for the purpose they fulfill? If a blind process created this system, how do you explain the many capillaries surrounding each alveolus?
Endnotes:
(1)Living In Space: https://www.nasa.gov/pdf/146558main_RecyclingEDA(final)%204_10_06.pdf
(2) One way to calculate is to use NASA’s estimate: daily needs of O2 is 0.84 kg. At 15 breaths per minute that equals 0.0389 g/breath. 840 g/ 21,600 = 0.0389 g/breath
(3) 0.0389 g divided by 32.0 grams per mole = 0.00122 moles of oxygen x 6.022 x 10 ^23= 7.32 x 10 ^20 0xygen molecules which need to enter bloodstream every breath.
(4) Dr. Keith Murphy; Are Nitrogen Molecules Really Larger Than Oxygen Molecules? The correct answer, with respect to “permeation”, is yes; retrieved at: https://www.getnitrogen.org/pdf/graham.pdf
Diameter of oxygen molecule - estimates range from 2.3 angstroms to 2.9 angstroms so we’ll use 2.5 angstroms = 2.5 x 10^-10 m
(2.5x10^-10m x 7.32x10^20 x 1 mile/1609.34 m) = 113,711,211 miles
Divided by distance to moon and back (480,000 miles) = 237round trips to moon or over 4848 trips around Earth.
(5) Surface area calculations: Imagine a skinny rectangle that is 2.5 x 10-10 m x 7.32 x 10 ^ 20 molecules = 1.83 x 1011 m long by 2.5 x 10-10 m wide = 45.75 m2 x 10.7639 ft2 /1 m = 492.5 ft sq. ft.
(6) Ochs, M., Nyengaard, J. R., Jung, A., Knudsen, L., Voigt, M., Wahlers, T., ... & Gundersen, H. J. G. (2004). The number of alveoli in the human lung. American journal of respiratory and critical care medicine, 169(1), 120-124.
(7) Fröhlich, E., Mercuri, A., Wu, S., & Salar-Behzadi, S. (2016). Measurements of Deposition, Lung Surface Area and Lung Fluid for Simulation of Inhaled Compounds. Frontiers in pharmacology, 7, 181. doi:10.3389/fphar.2016.00181
According to this study the estimated surface area of lungs is 700 – 1000 ft2
This is around the size of one half a tennis court (78 ft x 27 ft = 2106 ft2 / 2 = 1,053 ft2)
Our lungs have a surface area which is approx. 300 x greater than if they were nothing more than elastic bags (average size is 850 ft2 / 2.78 ft2 = 305 times larger
(8) Eduardo A Celis, MD; Lung Anatomy 2017; retrieved at: https://emedicine.medscape.com/article/1884995-overview
Blind Chance: Many people believe natural selection can design new structures in organisms. This is not true as nature only can to a small degree influence change in organisms by “selecting” for survival those organisms which already have the appropriate design. Changes to the structure/design of an organism must come about through changes to the DNA instructions (genes). These carry information to make the proteins and plans responsible for the design of new structures. These changes to the genes are usually the result of random mutations. Mutations are 99.995% harmful to organisms because they mess with existing genes. The creation of elastic tiny sacs in the numbers needed and in the exact right location by blind chance is not something I would put any hope in, nor do I believe it is plausible scientific explanation for the design of lungs along with the automatic systems which run and maintain them.