By Greg Kincheloe
Every year around spring, many people can be heard sniffling and sneezing their way through life because of seasonal allergies. It can be frustrating and borderline infuriating to experience, and as a result, often brings a renewed appreciation for things that we normally wouldn’t notice in any other season. There’s truly nothing like sneezing 8-10 times in a row followed by the sniffles for the rest of the day that makes one think “Wow, I really took breathing through my nose for granted.” And let’s be honest, mouth breathing all day just doesn’t feel as natural. However, gratitude for our nose and nasal cavities shouldn’t stop at allowing comfortable breathing. The functionality of the nasal cavity is much more in depth than one may think, and this may help explain exactly why breathing through our noses is so much better than through our mouths.
Before the science behind our noses can be explained, it may help to do a quick observational experiment. First, walk to the closest window or mirror and take a deep breath. With your face as close to the surface as possible, exhale that deep breath from your mouth. The exhale should create a sizable cloud of perspiration on the glass. Next, take an equally deep breath and this time exhale on a different part of the glass with your nose. The cloud of perspiration on the glass should be noticeably less than the first exhale with your mouth. This is because breathing through your nose is much more efficient at water retention than mouth breathing. Why? Because of its unique shape as well as physics.
Nasal Cavity Shape
Rather than a direct path from the nostrils to the lungs, the nasal cavity has a labyrinthian structure with all sorts of nooks and crannies that air has to pass through (Fig. 1). In the center, these nooks and crannies are created by bony features called turbinates, of which there are three on each side. In addition to these turbinates, there are several sinuses that all open into the nasal cavity as well (yes, we have giant air-filled pockets inside our skull). These can range from smaller ones like the ethmoid air cells to the vast sinuses like the maxillary sinuses. In addition to aiding in the efficiency of breathing, the sinuses are so large that our voices reverberate inside them and help form everyone’s own distinctive voice. This is why when we are sick and the sinuses and nasal cavity are inflamed, our voices sound different and nasally.
Nasal Cavity Physics
The second law of thermodynamics states that heat will always travel to a place with less heat. In simpler terms, we see this on a cold Pennsylvania day when our hot coffee quickly becomes lukewarm and no longer palatable. The heat was transferred from the coffee to the surrounding air. In addition, warmer air also holds more water inside it (if you need proof of this, go to Florida in the summer and feel the humidity). Both of these concepts can be applied to the nasal cavity when breathing occurs. Firstly, it is a highly vascularized region of the body with blood vessels carrying warm blood located very close to the surface. These arteries and the surrounding mucus membranes work together to both warm and humidify the air that is breathed in before it enters the lungs. This is extremely advantageous because cold and dry air causes stress to the cells of the lungs. Because the nasal cavity is so highly vascularized, it can warm up the cold outside air significantly, which results in water evaporating from the mucus membranes and humidifying the air as well. This warmer, more humid air continues becoming warmer and humidifying all the way to the lungs, until it is time to exhale. At the point to exhaling, the air that is in your lungs is warmer and more humid than that of the nasal cavity, so the membranes and arteries in the nasal cavity reabsorb moisture and heat from the air being exhaled, resulting in water and heat retention (Fig 2). This is why the glass in the experiment had less condensation when breathing through the nose as opposed to the mouth, as mouth breathing does not retain water to the extent of the nasal cavity.
Tying it Together
As the cold air making contact with the surface of the nasal cavity is heated and humidified, it would make sense that more surface inside the nasal cavity would mean more heating and more humidity. This is a reason why the nasal cavity is so convoluted and has a labyrinthian maze-like structure. With each extra nook and cranny, the surface area of the nasal cavity increases, as does its efficiency to both heat and humidify the air during inspiration (scientific term for inhalation) as well as cool the air and reabsorb water during expiration. An incredible evolutionary example of this is in desert animals, more specifically the kangaroo rat, which is native to the Southwestern region of the United States. Because these rats have evolved to live in extreme deserts, water retention is of the utmost importance, which is why the surface area of their nasal cavities is absolutely bonkers when compared to humans (Fig. 3). While humans can reabsorb about 16% of the water from the air breathed out their nose, the kangaroo rat can reabsorb anywhere between 54-83% of the air’s moisture depending on the outside temperature. They have been described as being able to breath out cold air because of the heat that was reabsorbed as well4,5.
There is a reason that most of us rely on nose-breathing as a default. Our bodies seem to know that it is more efficient and potentially even less damaging to our airways than mouth-breathing because it can effectively warm and humidify the harsh cold air. But what about the cells inside the nasal cavity? They undoubtedly get damaged too, but our bodies go through what is called the nasal cycle throughout the day, where one side of the nasal cavity will become more congested while the other side opens up6. This allows less air to pass through the congested side and as a result may give the cells in the congested side time to recover (or take a breather, if you will). Every few hours, a majority of the air we breathe will travel through a different nostril. Take a moment and plug your left nostril and breathe through your right. Now do the opposite. One nostril will be much easier to breathe through while the other is harder, and this will most likely reverse multiple times throughout the day! Now that is pretty cool.
TL:DR
Nose breathing is very important for thermoregulation as well as water retention and protection of the lungs.
References
1. http://sniflmd.com/nasalanatomy.html
2. https://spatialexperiments.wordpress.com/2016/09/16/countercurrent-heat-exchange-in-desert-animals/
3. https://www.fairbanksplasticsurgery.com/procedures/nasal-airway-surgery
4. Willmer, P., Stone, G., & Johnston, I. A. (2005). Environmental physiology of animals. Malden, Mass: Blackwell Pub.
5. https://www.open.edu/openlearn/mod/oucontent/view.php?printable=1&id=247
6. Kahana-Zweig R, Geva-Sagiv M, Weissbrod A, Secundo L, Soroker N, Sobel N. Measuring and Characterizing the Human Nasal Cycle. PLoS One. 2016;11(10):e0162918
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