Author's Note
While I did a bit of research for this, I'm not an expert on the respiratory system. If anything described in this story is wrong, please assume things are just different in the MLP universe. ;)
Breathing Is Fundamental
Twilight Sparkle stood in the library of her castle, looking at her bookshelves in quiet contemplation, trying to decide which book to read next. She wasn’t even thinking about breathing, and yet it was still happening.
As Twilight stood, her brain was unconsciously considering the levels of oxygen and carbon dioxide in her blood. In the human world, the oxygen level in the air was approximately 21% and the normal arterial pressure of oxygen for humans ranged from 75 to 100 millimeters of mercury at sea level. This had been the case for Twilight as well when she visited the human world and changed into human form.
In Equestria, however, things were just a bit different. The oxygen level was slightly higher, at 26%, and the normal oxygen levels of ponies could vary depending on whether they were unicorns, pegasi, earth ponies, or alicorns. As Twilight was an alicorn, her own normal arterial pressure of oxygen at sea level ranged from 85 to 110 millimeters of mercury. This could go as high as 150 millimeters of mercury if she was in the middle of casting an intense spell.
Of course, Twilight’s brain didn’t actually measure oxygen levels in such literal terms as partial pressures or millimeters of mercury. But it measured it nonetheless, and right now it was telling Twilight's body that her levels were just a bit low. It was time to take a nice, relaxing breath, time to inhale more of the oxygen that her body needed. Despite the minor differences in air composition between worlds, the minor differences between the oxygen requirements for humans and ponies, and even the obvious anatomical differences, breathing itself worked in essentially the same way.
To begin, Twilight’s medulla oblongata - a part of her brain stem - sent signals down her phrenic nerves to her diaphragm and to her intercostal muscles, telling them to contract.
Upon receiving its signal, Twilight’s diaphragm contracted, pulling down and straightening out so as to allow more room for her lungs to take in air. Her intercostal muscles contracted as well upon receiving their own signal, thus pulling on her ribs and helping to create still more room. If Twilight was in the middle of exercising or in the middle of an intense spell session, she might have needed even more help in the form of her accessory muscles, but as she was in a comparatively relaxed state and in good shape, that wasn’t necessary at this time.
And thus Twilight inhaled some of the surrounding air, with it first entering her nose and then traveling through her trachea to her lungs. Equestria’s air was mostly pollution-free, and while there was just the tiniest bit of dust in the library’s air from old books, all that she inhaled was trapped in the mucus of her nose. (If it had reached her lungs, then her body would have undergone additional processes involving macrophages and cilia in order to remove it.)
Once the air entered her lungs, it traveled through her bronchi, then her bronchioles, then finally into the microscopic air sacs known as her alveoli. Within the alveoli, the inhaled air and Twilight’s blood exchanged gases, with oxygen from the air diffusing into her bloodstream and carbon dioxide from her bloodstream diffusing out. The hemoglobin in Twilight’s blood would help in the transport of oxygen that was about to occur.
Now oxygen-rich, the blood left the capillaries of Twilight’s alveoli and traveled through her pulmonary veins to the left atrium of her heart. From there, it was pumped through her mitral valve into her left ventricle, and then pumped into her aorta. The pumping of Twilight’s heart was regulated by her cardiac conduction system, with some help from other sources such as her sympathetic and parasympathetic nervous systems. (Just as Twilight’s medulla oblongata helped to regulate her breathing, it also helped to regulate her heart rate.) The cardiac conduction system was a special system involving an electrical signal traveling from the pacemaker cells of the sinoatrial node in her right atrium, through her atria, to other pacemaker cells at the atrioventricular node between her atria and ventricles, and finally across her ventricles before starting again anew.
From the aorta, the blood then traveled all over Twilight's body through her arteries, which soon narrowed into tiny capillaries to allow oxygen to diffuse into her cells. The cells would use the oxygen for a complicated process known as cellular respiration, in which chemical reactions would lead to the creation of adenosine triphosphate (or ATP), which would itself later be used to provide energy for other metabolic processes. As the oxygen diffused into her cells, carbon dioxide in the form of bicarbonate ions diffused back out into her bloodstream.
The now-oxygen-depleted blood traveled from Twilight’s capillaries into her veins, which eventually joined together into her vena cava. The vena cava took the blood back to her heart, where this time it entered her right atrium, was pumped through her tricuspid valve into her right ventricle, and was then pumped into her pulmonary artery. From there, it traveled back to the alveoli of her lungs, where it again exchanged oxygen and carbon dioxide with the air that Twilight had inhaled.
Finally, Twilight’s diaphragm and intercostal muscles relaxed and thus the now-oxygen-depleted air was exhaled back out through her nose. (Though presented here as not happening until the blood followed by this story traveled through her body and back, this is for the sake of narrative simplicity. The events described were in reality a more continuous process.)
“Hmm…” Twilight said to herself. “Maybe this one?”
She took a book down from the shelf with her magic and looked at the cover. It was titled Respiration, Volume 1: Inhalation and featured a unicorn pony looking to the left with her mouth open and a diagram of her lungs, diaphragm, and other features important to the respiratory system labeled on the picture. Twilight skimmed the book and decided that it was the one.
“I’ve always wondered how exactly breathing works,” Twilight thought. Though she was very knowledgeable about magic, her knowledge of anatomy and physiology was rather limited, and it was time to change that. Closing the book, she walked over to a sofa located within the library and got into a comfortable position.
“'Chapter 1,'” Twilight read, having opened the book again. “'The Medulla Oblongata.' Interesting.” She took a deep breath, this time on purpose, inhaling and exhaling and trying to imagine what might be happening inside her body as she did so. Then she began to read.
The End
