Now that we have explored why respiration is important and what we need to do it let’s finally look at how we breathe.


Inspiration occurs when air enters our lungs due to the contraction of our diaphragm and intercostal muscles. During contraction the diaphragm pulls downwards and when contracting the external intercostals pull our ribs outwards expanding the size of our ribcage. Since our lungs are elastic they fill the space and rapidly expand as well. This expansion increases the volume of our lungs and as a result decreases the pressure inside our lungs.

Image: I imagine something or someone pulling down a trampoline like diaphragm it takes effort to pull it down thus contraction and lung expansion go together. I figure we can build off of the previous image and show how this increases the lung cavity size and in turn the lungs.

Neolithic version of diaphragm contraction

This makes sense given Boyle’s Law…

\[P_2= \frac{P_1V_1}{V_2}
…since gases spontaneously flow from higher pressure areas to lower pressure areas air rushes into our lungs. Inspiration stops when the pressure inside our lungs is equal to atmospheric pressure since gas will no longer have net flow in either direction.


Expiration or the movement of air out of our lungs is by contrast a passive process resulting from the relaxation of our diaphragm and external intercostal muscles. During relaxation the diaphragm moves upwards, the intercostals inwards, and ultimately the ribcage contracts. This contraction decreases the volume of our lungs and as a result increases the pressure inside our lungs forcing air out.

Image: Since the diaphragm is springy it snaps back into place without any extra effort. I imagine the diaphragm being released and forcing the lungs to get smaller. Again I think this is just an extrapolation from the other pictures and in this case the lungs are getting “smacked” by the diaphragm.

Again this makes sense given the relationships between the different variables in Boyle’s Law…

\[P_2 \propto \frac{1}{V_2}

…since gases spontaneously flow from higher pressure areas to lower pressure areas air rushes out of our lungs. Expiration stops when the pressure inside our lungs is equal to atmospheric pressure since gas will no longer have net flow in either direction.

The whole process is a bit like using a bicycle pump. When we pull up on the bicycle pump we expand the air chamber and drop the internal pressure causing air to rush into the pump. Which mirrors the contraction of our diaphragm and external intercostals during inspiration. Then when we pump the air into our bike tire we push down on the handle decreasing the volume, increasing the pressure, and forcing air into our flat bike tire. This is analogous to our diaphragm and external intercostals relaxing during exhalation.

Measuring Respirations

It is useful for researchers and doctors alike to quantify the different aspects of respiration. There are a lot of terms here and the MCAT tests them somewhat infrequently. That doesn’t mean this information isn’t worth learning but it is important to note that we don’t need to have detailed knowledge of the different measurements associated with respiration. With that short prelude let’s jump into the material. If you are able try to follow along with the prompts below and “experience” for yourself the different ways we measure respiration.

Okay, take the biggest breath you can. When you can’t breathe in anymore you have reached your total lung capacity (TLC). That is the most air you can fill your lungs with and for most adults is around 6-7 liters.

Now exhale all of the air and keep exhaling till you can’t any more. In turns out that you can’t get every last bit of air out of your lungs and the bit that remains is called the residual volume (RV).

This time take a normal breath.

Respiratory Control

The muscles of respiration aren’t doing everything by themselves though. They are controlled by the respiratory centers of our brain and innervated by a series of nerves. This allows the respiratory system to adapt to changing conditions and respond accordingly. The single biggest trigger for our respiratory system is carbon dioxide.

The levels are first sensed by the carotid bodies and aortic bodies, chemoreceptors that sense the levels of carbon dioxide in the blood and send signals to the respiratory centers in our pons and medulla. Depending on the type of signal these respiratory centers increase or slow down our breathing rate. This mechanism allows for our respiratory system to dynamically respond to changes in pH and help control them.

Image: I picture this as our brain the master controller receiving signals from detectors that then push buttons that tell the respiratory muscle to change how we breathe.

pH Control: Le Châtelier’s In Action

Our bodies need to maintain a tightly controlled pH (7.35 to 7.45) in order to optimally function. Outside of this range, the proteins in our body begin to unfold due to disruption in their ability to form hydrogen bonds. Enzymes that catalyze important metabolic reactions fall apart, the ATPase pumps responsible for maintaining cells resting potential grinds to a halt, and badness ensues.

Image: I am imagining something like the picture below here. I picture the denaturation of enzymes as an enzyme transforming into spaghetti since it is going to lose all structure except for primary structure which is just the linear chain of the amino acids, This explains the importance of maintaining pH so tightly within our bodies.

Thankfully the bicarbonate buffering system helps maintain this tightly controlled range, but if enough acid or base is produced it becomes overwhelmed. Respiration thankfully gives us a fast and effective way of balancing this out by harnessing Le Châtelier’s Principle.

H_2O + CO_2 \leftrightarrow H_2CO_3 \leftrightarrow HCO_3^-+H^+

In the bicarbonate buffering system our lungs can only meaningfully impact the level of CO2 present, which is why our system is so sensitive to it. When the CO2 levels are too high our carotid and aortic bodies send signals to our respiratory centers telling them to increase the rate and depth of breathing. This removes CO2 from our bodies and shifts the whole reaction to the left away from H+ ions.

Image: Although not 100% accurate I was thinking of breathing out acid while Le-Chat is ultimately responsible I think it helps the idea stick more clearly at least in my head.

This is one of the reasons why sick acidotic patients breathe so quickly. They are trying to eliminate CO2 and in the process increase their pH by shifting the bicarbonate buffering system to the left.

The exact opposite occurs when our CO2 levels are too low or our pH is too high. In this scenario, our respiratory centers decrease our respiratory rate and cause CO2 retention which shifts the bicarbonate buffering system to the right towards H+ ions decreasing blood pH.