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Anatomy and physiology of the respiratory system

Anatomy and physiology of the respiratory system


The main job of the lungs is gas exchange,
pulling oxygen into the body and getting rid of carbon dioxide. Normally, during an inhale – the diaphragm
contracts to pull downward and chest muscles contract to pull open the chest, which helps
suck in air like a vacuum , and then during an exhale – the muscles relax, allowing the
lungs to spring back to their normal size pushing that air out. When you breathe in, air flows through the
nostrils and enters the nasal cavity which is lined by cells that release mucus. That mucus is salty, sticky, and contains
lysozymes, which are enzymes that help kill bacteria. Nose hairs at the entrance of the nasal cavity
get coated with that mucus and are able to trap large particles of dust and pollen as
well as bacteria, forming tiny clumps of boogers. The nasal cavity is connected to four sinuses
which are air-filled spaces inside the bones that surround the nose, there’s the frontal,
ethmoid, sphenoid, and maxillary sinus. The paranasal sinuses help the inspired air
to circulate for a bit so it has time to get warm and moist. The paranasal sinuses also act like tiny echo-chambers
that help amplify the sound of your voice, which is why you sound so different when they’re
clogged with mucus during a cold! So the relatively clean, warm, and moist air
goes from the nasal cavity into the pharynx or throat, the region connecting the two is
called the nasopharynx, and the part connecting the pharynx to the oral cavity is called – you
guessed it – the oropharynx. The soft palate, the softer portion of the
roof of your mouth behind the hard part that you can feel with your tongue, and the pendulum-like
uvula hanging at its end move together to form a flap or valve that closes the nasopharynx
off when you eat to prevent food from going up into the nasopharynx. Finally, there’s the laryngopharynx, the
part of the pharynx that’s continuous with the larynx or the voice box. Up to this point, food and air share a common
path. But at the top of the larynx sits a spoon-shaped
flap of cartilage called the epiglottis which acts like a lid that seals the airway off
when you’re eating, so that the food can only go one way – down the esophagus and towards
the stomach. If anything other than air enters the larynx,
then there’s a cough reflex to kick it right out. Now, once air makes it’s way into the larynx,
it then continues down as the trachea or the windpipe, which splits into the two mainstem
bronchi. The point at which they split is called the
carina. They then enter the lungs, and the right lung
has three lobes – upper lobe, middle lobe, and lower lobe, and the left lung has just
an upper lobe and lower lobe. The right mainstem bronchus is wider and more
vertical than the left which is why if you accidently inhale something big that can’t
get coughed out like a peanut, then it’s more likely to go into the right lung than
the left. The mainstem bronchi then divide into smaller
and smaller bronchi. The trachea and the first three generations
of bronchi, are all pretty wide and use cartilage rings for support. Taking a look at a cross section chunk, there’s
also a layer of smooth muscle which has nerves of the autonomic nervous system within it. The autonomic niervous system is made up of
two basic types of nerves – sympathetic nerves which are involved in ‘fight or flight’
mode like running from a turkey and parasympathetic nerves which are involved in the ‘rest and
digest’ mode – like eating ice cream on the beach. Smooth muscle along the trachea and the first
few branches of bronchi have beta 2 adrenergic receptors. Going back to that turkey, when you’re running,
the sympathetic nerves stimulate those beta 2 adrenergic receptors and increase the diameter
of the airways. But – those same airways also have muscarinic
receptors which can get stimulated by parasympathetic nerves, causing a decrease in the diameter
of airways. The large airways are lined mostly by ciliated
columnar cells and a handful of goblet cells which get their name from looking like a wine
goblet or glass, and secrete mucus. That mucus helps trap particles, and then
the ciliated columnar cells beat rhythmically together to move the mucus and any trapped
particles from the air towards the pharynx where they can either be spit out or swallowed-
this mechanism is known as the mucociliary escalator. After the first three generations of bronchi,
however, the airways become more narrow, called bronchioles – ‘little bronchi’, and these
can stay open without the need for cartilage. Air is conducted through smaller and smaller
bronchioles for about 15-20 generations, and collectively they’re known as conducting
bronchioles.. Now the walls of the conducting bronchioles
are similarly lined by ciliated columnar cells and mucus secreting goblet cells, as well
as a new cell type called club cells because they look like tiny clubs. These club cells secrete glycosaminoglycans
which is a material that protects the bronchiolar epithelium. These guys can transform into ciliated columnar
cells, so they help regenerate and replace damaged ciliated columnar epithelial cells
if needed. These conducting bronchioles receive oxygenated
blood from the bronchial arteries The
last part of the conducting bronchioles are the terminal bronchioles, and then after that
air gets to the respiratory bronchioles, which are unique because they have tiny outpouchings
that bud off of their walls. These outpouchings are called alveoli, and
there are about 500 million of them within the lungs. Eventually the respiratory bronchioles ends
when there are nothing but alveoli, and at that point the airway is called an alveolar
duct rather than a respiratory bronchiole. This is the final destination of the inhaled
air. The alveolar wall has a completely different
structure from the bronchioles – there are no cilia or smooth muscle, and instead the
wall is lined by thin epithelial cells called pneumocytes. Most are regular pneumocytes called type I
pneumocytes, but some, called type II pneumocytes, have the ability to secrete a substance called
surfactant, which helps decrease the surface tension within the alveoli and keeps them
open. Like the club cells, the type II pneumocytes
are capable of transforming into type I pneumocyte, so they can also help regenerate and replace
damaged cells. Finally, if a tiny particle ever makes it
deep into the lungs, there are alveolar macrophages that can gobble it up and then physically
move up to the conducting bronchioles where they can ride the mucociliary escalator all
they way up to the pharynx to be either coughed up or swallowed down. Free from particles, the inhaled air is now
in the alveolus surrounded by mostly type I pneumocytes. On the other side of the pneumocytes are endothelial
cells that line the capillary walls – which is where that sweet sweet blood is. This time, though, that blood comes from the
pulmonary arteries, carrying deoxygenated blood. The pneumocytes and the capillaries are glued
together with a protein layer called basement membrane. So the alveolar wall, the basement membrane,
and the capillary wall is really all that separates the air from the blood, and this
is called the blood-gas barrier. At this point, carbon dioxide diffuses out
from the deoxygenated blood and into the air of the alveoli, which then gets breathed out. And with each breath in, oxygen enters the
alveoli and freely diffuses into the blood. That freshly oxygenated blood then heads off
to the pulmonary veins, the heart, and then to the body’s tissues! All right, as a quick recap, the respiratory
system facilitates gas-exchange. Oxygen in the air is inhales and makes it’s
way through the pharynx, larynx, trachea, large upper airways, conducting bronchioles,
respiratory bronchioles, the alveoli, and finally the capillary to be sent to the body’s
tissue. Then Carbon dioxide makes the reverse journey
to eventually be exhaled into the world. Thanks
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