You can be surrounded by a planet full of oxygenated air, but it does no good at all until that air enters your lungs. Our next job is to tackle the question, “How does the oxygen in our atmosphere get inside of our lungs so it can perform its life-saving work?”  “That’s easy ”, you say.   “We breath it in!”  You are right, but it’s not as simple as that. 

         Take a deep breath and let it out.  There’s nothing as natural as breathing, right?  Tell that to someone who has just been hit right below their ribs and is on the ground gasping for breath.  What happened? Air stopped entering their lungs. Getting the wind knocked out of you reveals in a very dramatic way the amazing process of breathing which we take for granted. 

           What do you think: Can an uninflated balloon sitting on your kitchen counter inflate itself? Of course not!  Nor do lungs become inflated with air all by themselves.  There is plenty of air around the balloon and your lungs, but something more than air is needed. To blow up the balloon you have to create high pressure air in your mouth which you blow into the balloon.   It is the same with our lungs.   Air will move inside our lungs only when the air pressure outside our lungs is higher than the air pressure inside our lungs.  Air always moves from high pressure to low pressure, like when you open a soda can you hear a fizzing sound – high pressure air from inside the can rushing out to the lower pressure air outside.  What we need is a way to change the air pressure inside our lungs, since the air pressure outside does not change very much.

Activity – Blow Up A Balloon - see end notes

          God’s ingenious solution to the problem of how to get air inside our lungs is the diaphragm muscle.   This dome-shaped muscle sits right below our lungs, separating the thoracic (chest) cavity from the abdomen (stomach).  It attaches at the back to the spinal column, and in the front to the lower end of the sternum.  When relaxed this muscle looks like a dome or umbrella.  When contracted it flattens out.  This flattening pulls down the whole chest cavity, drawing the lungs down also.  At the same time small muscles between the ribs known as external intercostal muscles contract, pulling the ribs upwards and outwards causing the chest cavity to get larger.   

          This increased volume results in lowering the air pressure in the lungs, as now you have the same amount of air, but in a larger volume.  Now the higher pressure air from outside the lungs flows naturally towards the low pressure air in the lungs.   When the diaphragm relaxes it again resumes its dome shape, drawing up the floor of the chest cavity with it, decreasing lung volume and creating higher air pressure in lungs.  Air flows out of the lungs. For those of you who want to know more about the amount of air pressure difference created by our lungs expanding see the end notes (1).

          The person on the ground gasping for breath has an air pressure problem.  His diaphragm muscle, upon being hit, became paralyzed and stopped moving.   The chest cavity stopped its expansion and contraction.  Air pressure inside and out of the lungs became equal, bringing to a quick halt the flow of air into and out of the lungs.  What a horrible feeling!  Thankfully, after a few seconds which seem like an eternity, the diaphragm resumes its contractions, allowing the precious air to again enter our lungs.

How well would we do without a diaphragm muscle?  A frog survives well without a diaphragm muscle, but it has been designed with several other processes to bring in oxygen.  First, its moist skin allows oxygen to pass through it into the blood stream.  Second, it is able to bring in oxygen through its nostrils by lowering and raising the bottom of its mouth with a pump-like action.

The diaphragm of polio victims was many times affected by partial paralysis, making it very difficult to breath.  Those suffering had to spend years of their lives in an 800 pound machine known as the “Iron Lungs” which used motors to continually change the air pressure inside the machine.  First high pressure air was generated  so that air would flow into the patient’s lungs.  Then the air pressure had to be reduced mechanically so that air would flow out of the patient’s lungs. 

Again, take a deep  breath.  Can you feel your chest getting larger?  Can you feel air filing your lungs?  Now breathe out. See how your chest shrinks.  This would be impossible without your diaphragm muscle working to increase and decrease the size  of your chest cavity.  Let’s stop and take another deep breath, thanking God for our diaphragm muscle. Watch this video to see how we can use air pressure to suck an egg in a bottle.

How is the Diaphragm Muscle Controlled?

          Most muscles are voluntary, meaning they won’t move unless you choose to make them contract by firing a motor neuron from your brain to the muscle.  That is what I’m doing as I type this, with the muscles in my forearm responding to messages from my motor cortex.  What if your diaphragm muscle was voluntary, meaning it wouldn’t contract unless you directed it to do so? Every four seconds you would have to remember, “Oh, yeah, I need some oxygen!” Then you would choose to send the signal from your motor cortex in your brain to the diaphragm to cause it to contract.  What would your life be like?  One word describes this– interrupted!   Imagine trying to hold a conversation or playing a game of tag while still remembering to signal the diaphragm to contract every few seconds! A good night’s sleep would be impossible. Fortunately for us, the diaphragm muscle has hybrid abilities, meaning it operates automatically most of the time, but can still be controlled through choice.  Muscles that work automatically are called involuntary muscles. Can you think of other involuntary muscles in your body? See end notes (2).

Of the 21,600 breaths you breathe in one day, how many do you actually think about? Not too many, right? So, if you don’t choose to send the signal for the diaphragm to contract, how does it get sent? The good news for us is that just like clockwork, every four seconds or so, a life-giving signal is sent out from the small respiratory control center in the medulla which is located in the brain stem.  This lower part of the brain is not the part we think with; it’s more like a deeply buried super computer with built-in software that automatically controls vital processes like breathing, heart rate, blood pressure and digestion. That frees us to think of other things like what we’re going to eat for breakfast or whether we should make our bed or not. (YES!)

Once the signal is initiated by the respiratory control center the phrenic nerve carries this message to the diaphragm, causing it to contract. The contraction phase may last two seconds, during which air flows into the lungs.  When the contraction ceases the diaphragm relaxes and resumes its dome-like shape, bringing up the floor of the thoracic cavity with it and forcing air out.   Exhaling is also helped by the elasticity of our lungs, which act like inflated and stretched balloons when let go, forcing air out as they returns to their unstretched shape. 

          There are times when the diaphragm muscle gets help form other muscles.  When you are going to try to swim across the pool underwater what do you do first? That’s right, you take a deep breath. This breath is voluntary, meaning you made it happen because you need lots of oxygen in your lungs. These deep breaths require more force than your diaphragm can deliver alone.  That help comes from muscles between your ribs called intercostals.  The outer layer, called external intercostals, when contracted are able to raise each rib, thereby expanding the chest to lower air pressure and bring in a greater volume of air.   The diaphragm needs help at times with expiration (breathing out) as well, such as when you are blowing up a balloon or blowing out candles.  Forceful expiration is aided by stomach muscles like the rectus abdominus (your abs) pushing up on the diaphragm/thoracic cavity to force air out. 

          The diaphragm muscle has the perfect location, attachments, shape, and nerve connections to do its job.  It also comes equipped with the perfect openings or pathways through which pass essential structures such as the aorta which brings oxygenated blood to the lower body, the esophagus, bringing food to the stomach, the inferior vena cava, bringing deoxygenated blood back to the heart and several very important nerves such as the phrenic  and vagus nerves. More importantly, it also comes equipped with its own control center in the brain to make it automatic!

We’ve learned that air does not automatically rush inside our lungs; nor do we have to suck it in, though we can.  It is the imperceptible (unseen) actions of the diaphragm muscle which keeps us alive.

Take a deep breath - inhale and then exhale. Think about your diaphragm muscle working behind the scenes to draw air into your lungs, and then to push it out. It does its job so well we rarely think about it. When was the last time you thanked God for your life-saving diaphragm muscle which is always on duty, even while you sleep?

Things to think about: Could random chance processes such as mutations design thie diaphragm? How did the diaphragm muscle get in the exact right location to do its job? Who set the automatic timing for it to contract every 4 seconds? How did the diaphragm muscle “know” to be an involuntary so it could be controlled automatically? Where did the respiratory control center in the medulla come from? Could blind chance design have designed this perfect system?

Activity - Blowing Up a Balloon

Before you blow up the balloon answer: 1) Where is the air pressure greater - inside or outside the balloon, or are they the same? 2) How is this like your lungs when the diaphragm muscles stops working?

Blow up the balloon and pinch it shut. 1) Where is air pressure higher - inside or outside the balloon? W hat will happen when you let the balloon go? Why?

End Notes

(1) Partial Pressures

Air pressure has to do with how hard air molecules are pushing on a surface. Think of your bike tire when you pump it up. The added air molecules you pumped in increase the pressure pushing on the tire. Inside the tire are a combination of mostly nitrogen and oxygen molecules. They both have energy and push on the side of the tire, but since there are more nitrogen molecules in air these exert a greater force. We call the force each kind of molecule is exerting a partial pressure. Air pressure at sea level is enough to push down on a surface of liquid mercury and support a column of mercury that is 760 mm high.

The partial pressure of oxygen in atmospheric air will support a column of mercury 159 mm high (159 mm Hg).  We call it partial pressure because the other gases in the atmosphere such as nitrogen and carbon dioxide are not included in this measurement.  Total atmospheric pressure is around 760 mm Hg.  As the chest expands due to the diaphragm contracting, the partial pressure of oxygen inside the lungs decreases to 100 mm Hg.  When this happens the higher pressure oxygen outside the body (159 mm) now is drawn into the lower pressure oxygen in the lungs.

(2) Involuntary Muscles - Muscles in your digestive system which move food along - found in esophagus, stomach, intestines. Also in circulatory system in blood vessels and heart to keep blood circulating.

Videos to Watch

Lamb Lung Inflating: https://youtu.be/r8sBhsAeDf8

Animation of Respiration - note the cutaway showing oxygen and carbon dioxide diffusing through alveoli.:https://youtu.be/kacMYexDgHg?t=80