Breathing physiology plays an important role in the olfactory sensory system. This is because most times, smells are inhaled while breathing. In this page, the breathing physiology and how the olfactory system picks up chemical signals will be reviewed.
Mechanism
Breathing physiology for mammals is simply the act of oxygen and carbon dioxide transport within the animal's body (Dominelli and Sheel 2012). With inhalation comes O2 from the atmosphere and exhalation, CO2 is released. This process is however, highly regulated by the lungs, brain, and the heart to maintain a healthy level of both gases within the body. The act of inspiration allows for a "negative pressure gradient from the mouth to the alveoli" which allows for air to enter the lungs. After contraction, the diaphragm moves "outwards against the ribcage". Expiration occurs as a "passive process" and the cycle repeats itself.
When looking at the canines, during this inhalation process, "12-13%" of air flow travels to the olfactory portion of nose and the rest of airflow is “directed toward the nasopharynx where it exits the nasal cavity” (Jenkins et al. 2018). Due to the structure of their nose where "each nostril samples air separately" the canines are able to obtain "bilateral odor samples". This mechanism of inhalation allows for part of the airflow to reach the olfactory cortex where the smell is processed. In the case of the dog, as they have "macrosmia" or the "enhanced ability to smell" they are able to source the location of the smell. Similar yet different from inhalation, "sniffing is advantageous compared to normal inhalation because it produces unidirectional laminar flow to the dorsal meatus and sensory epithelium" this allows for "increases [in] the sensitivity to odors" it also "drives activity in the olfactory cortex, and affects odorant intensity and identification” which all ultimately allow for dogs to have tremendous senses of smell.
The mechanism of odor detection is similar to the general process of sensory mechanisms where the "odorants bind to receptors" (Rinaldi 2007). This triggers the activation of the olfactory receptor cells, which send an electric signal through the nasal epithelium and glomeruli and are transferred to the olfactory bulb. From the bulb, the signal is passed to the olfactory cortex. Since the olfactory receptors "project directly to the olfactory bulb" and these cells have cilia that significantly increase the surface area of the odor receptors, which allow for higher sensitivity to smells if an animal has a high number of cilia (Jenkins et al. 2018).
Airborne odors also have the capability to stimulate the nasal mucosa nerve which also allows for the animal to sense the temperature (Jenkins et al. 2018. Olfaction is also very closely linked with memories (Nielsen et al. 2015).