Breathing Mechanics


Have you ever taken manual control of your breathing and had the hardest time trying to get your body to breathe on it’s own again? Did you do it right now? If you did, you’re in luck because we’re going to be going over breathing mechanics and manually controlling your breathing is going to be super helpful in setting up the foundation for understanding this phenomenon.

First, we need to remember that the main function of respiration (breathing) is to take in \(O_2\) and get rid of \(CO_2\). Our bodies do this on two scales:

1. External respiration: Inspiration and expiration of air in the lungs and the transport of \(O_2\) and \(CO_2\) through the blood to the organs.

a. This is the scale of respiration that we will be starting to discuss in this article!

2. Internal respiration: a.k.a. Cellular Respiration

Next, let’s review of bit of anatomy:

I. Lungs

Once we pass the respiratory airways (mouth, pharynx, and larynx), we reach our lungs through a series of structures; in order of air passage these structures are: the trachea, the larger bronchi, the bronchioles, and the alveoli. What is most important to grasp here is that the respiratory airways bring air to the alveoli, which we will discuss next.


A. Alveoli

Inflatable sacs within the lungs; the site of gas exchange (exchange of \(O_2\) and \(CO_2\))

Note here that the alveoli are INFLATABLE, meaning they stretch and deflate with the inhale and exhale of air from the lungs. Also note in the picture that the alveoli are surrounded very closely by capillaries! This proximity is super important because it allows for oxygen-poor blood coming back from the body to be re-oxygenated by the \(O_2\) entering the alveoli from the atmosphere.

II. Thoracic structures supporting the lungs

A. Diaphragm

The diaphragm is skeletal muscle that sits at the bottom of our lungs. Before inspiration the diaphragm sits in an arched position and during inspiration the diaphragm flattens (moves downward), giving the lungs more room to expand vertically.

B. Intercostal Muscles

The intercostal muscles are muscles that exist between the ribs to create the chest wall. Before inspiration these muscles are relaxed and during inspiration these muscles contract causing the ribs to move up and out, giving the lungs more room to expand horizontally.

Finally, let’s get into the thick of breathing mechanics:

A. Pulmonary Ventilation

If you’re like me and you keep hearing the term “pulmonary ventilation” in class or reading it in the text and the two giant words take a second to process, just remember that your professor means breathing! Pulmonary ventilation = Breathing or the movement of gas (\(O_2\) and \(CO_2\)) from the outside air into the alveoli in your lungs.

B. How does breathing work?

This might seem like a silly question at first glance since we all know what it feels like to breathe and, if we take a moment to think about it, we can feel our lungs expanding and contracting inside our body. But what’s the point of that expansion/contraction? How does that physical movement allow our body to get the oxygen it needs? To understand this have to talk about Boyle’s Law.

1. Boyle’s Law

Boyle’s Law states that the pressure created by a gas in a contained space is inversely related to the volume of the gas (if the temperature is constant). In other words, if you have a small container, you have a smaller volume for a gas to exist in and therefore the gas exerts a greater pressure because it is packed more tightly. Oppositely, if you have a large container with the same amount of gas as our small container, then you have a larger volume and more space for your gas to spread out, creating less pressure. Our lungs follow this concept and use it for their function. Take a second to take a deep breath in, do you feel your lungs expanding? Or do you at least see your chest rising? Inhaling (or inspiration) occurs when we take a deep breath in, expanding the volume of our lungs which, according to Boyle’s Law, decreases the air pressure in our lungs.

2. Air moves from high pressure to low pressure

Let’s take a second to refresh our minds on the concept of gradients. When I think of gradients, I tend to first think of osmosis and diffusion - if you remember, these 2 phenomena comply with the concept that water and other substances move through membranes from areas of high concentration to areas of low concentration in order to try and reach equilibrium with each other. Our lungs do the same with the air entering from the atmosphere.

If you look at the picture above you will see three “lungs” representing the 3 parts of the breathing cycle. Looking at the first one, we see a lung before inspiration (before inhaling). If you look at the three numbers, you’ll see that the top and middle numbers are both 760 mmHg. The top number is representative of atmospheric pressure, a.k.a the pressure of the air outside our bodies. Note that this number is a constant and will always equal 760. The second number is the air pressure inside our alveoli, it is a value that changes as we move through the breathing cycle.

Okay, so going back to our gradient and Boyle’s Law, look at the 2nd lung - see how it is larger than the other two? This is representative of the expansion of our lungs we feel when we breathe in. (Remember intercostal muscles move up and out and the diaphragm moves downward to create a larger space for the lungs to expand in) As we said before, this expansion decreases the pressure of air in our alveoli (see how the middle number in the pink is now 759 instead of 760?). So now we have an atmospheric pressure (\(P_{atm}\)) of 760 and a \(P_{alveoli}\) of 759 - enough to establish an air gradient! Air will move from the higher pressure atmosphere into the lower pressure alveoli, a.k.a. Inspiration! Now look at the 3rd lung, this lung is representative of expiration (breathing out). Note the two numbers again, \(P_{atm}\) is till 760 and the \(P_{alveoli}\) is now 761. Since 761 > 760 the air gradient is now in the opposite direction and air moves outward.

And that’s the basic gist of breathing! Though there are more complexities and further processes that follow these concepts, hopefully this will give you a solid foundation as you continue to learn about the respiratory system. I’ve attached some practice questions from previous quarters below for you to try and work out! Try to think through each answer choice and use your manual breathing if you get stuck! Try not to look at the answers at the bottom until you settle on a choice! Happy studying :)

Question 1

Inspiratory events occur because

a. the air pressure in the alveoli is greater than that of the atmosphere

b. the volume of the thoracic cavity increases

c. the air pressure in the alveoli is less than that of the atmosphere

d. of conditions a and b

e. of conditions b and c

Question 2

The pressure within the alveoli is greater than atmospheric pressure when

a. The thoracic cavity expands

b. The thoracic cavity contracts

c. The external intercostal muscles contract and the diaphragm contracts

d. The abdominal muscles relax and the diaphragm contracts

Question 3

In response to an increased demand for ventilation such as during intense exercise, which of the following would you predict based on our lectures?

a. Intrapleural pressure should be higher than intra-alveolar pressure to help move air in and out of the lungs more efficiently.

b. There should be strong contraction of the internal intercostal muscles and the abdomen to help to expand the thorax to permit more air into the lungs.

c. Atmospheric and intra-alveolar air pressure would never equal at any point during the respiratory cycle in this situation.

d. Shallow, rapid breathing would be advantageous to overcome the effect of airway dead space.

e. There should be more frequent and stronger contractions of the diaphragm and external intercostal muscles to optimize alveolar ventilation rate to match metabolic demands.


1. E, 2. B, 3. E