Professor Dave here, let’s check out some lungs.
We’ve gone through many of the important systems of the human body, but the one that
we are possibly most acutely aware of at all times is the respiratory system.
Every couple of seconds, we breathe in air from the atmosphere, and then breathe out.
This is to supply every cell in our bodies with one particular component of the atmosphere, oxygen.
Our cells need oxygen to perform cellular respiration, which generates the energy we
use to think and move around.
Without oxygen, we can’t produce energy, and we don’t last very long.
So how exactly does the respiratory system work?
Let’s take a closer look now.
As we said, the major function of the respiratory system is to supply every nook and cranny
of the body with oxygen, and it also serves to collect carbon dioxide, a byproduct of
cellular respiration, and expunge it from the body when we exhale.
We already learned about part of this process when we talked about the circulatory system.
There we learned about how oxygen can bind to hemoglobin and travel through the bloodstream,
diffusing into tissue at capillary beds.
But we also have to learn about how oxygen gets into the bloodstream in the first place,
and for that we need to understand pulmonary ventilation, also called breathing, whereby
oxygen enters the lungs from our surroundings, as well as external respiration, which is
how oxygen gets from the lungs to the blood, and how carbon dioxide gets from the blood
to the lungs.
To understand this, let’s take a look at this system as a whole.
The respiratory system includes the nose, nasal cavity, paranasal sinuses, larynx, trachea,
bronchi, and lungs, which contain little sacs called alveoli.
All of these components belong to one of two zones.
There is the respiratory zone, which is where gas is exchanged within the lungs, and the
conducting zone, which is all the rest, where air comes in and out of the body.
Starting with the nose, we probably know that air enters through the nostrils, and since
we’ve already touched on other structural aspects of the nose earlier in the series,
we will jump straight to the nasal cavity.
This is separated from the oral cavity by the palate, which has two sections, the bony
hard palate and the muscular soft palate.
We can also see the nasal vestibule, with hairs for filtering, as well as three nasal
conchae, which are mucosa-covered projections, separated by nasal meatuses.
These serve to filter, heat, and moisten the air.
The nasal cavity is surrounded by the paranasal sinuses, which produce mucus that flows into
the nasal cavity.
Next we see the pharynx, or throat.
This connects the nasal cavity with the mouth.
This has three regions, the nasopharynx, the oropharynx, and the laryngopharynx.
This passageway continues down into the esophagus when eating food, but as far as air goes,
the next stop is the larynx, or voice box.
This provides an airway, and also serves as the site of voice production.
The larynx is comprised of a variety of cartilages, including the epiglottis, which stays open
for air flow, but when swallowing, it will cover up the laryngeal inlet so that we don’t
end up breathing our food.
Then we see the vocal folds and vestibular folds, which are the true vocal cords and
false vocal cords respectively.
More on these structures another time.
Next we find the trachea, or windpipe, which descends and divides into two main, or primary
bronchi, one to the right, and one to the left.
Each bronchus will quickly subdivide into lobar, or secondary bronchi, which in turn
branch into segmental, or tertiary bronchi, and this continues even further, until we
get to passageways less than a millimeter in diameter, which are called bronchioles.
As these tubes get smaller, the epithelium gets thinner, the amount of cartilage decreases,
and the amount of smooth muscle increases.
This ends at the terminal bronchioles, which feed into the respiratory bronchioles, and
this marks the boundary between the conducting zone and the respiratory zone.
The respiratory bronchioles have structures that protrude called alveoli, which are collected
in alveolar sacs, like grapes in a bunch.
Because of the extensive branching in the bronchi, this results in hundreds of millions
of alveoli, and this is where gas exchange takes place.
These structures have very thin walls made of a single layer of squamous epithelial cells,
which is what allows for the diffusion of gases, and they are covered by a cobweb of
pulmonary capillaries, so blood vessels are right up against the alveoli, ready to make a trade.
The air in the sacs has plenty of oxygen and the blood doesn’t, so diffusion occurs spontaneously,
and the blood has a good amount of carbon dioxide, and that will diffuse in the opposite
direction, so that it can be exhaled.
Now let’s discuss the lungs, which house all of this internal structure.
These cone-shaped organs occupy almost all of the thoracic cavity, and they are surrounded
by the visceral pleura and parietal pleura, while lying in close contact with the ribs,
from the apex down to the base.
The hilum is where blood vessels and nerves enter and leave the lungs, and the cardiac
notch is a small cavity in the left lung that accommodates the heart.
We can see different fissures in each lung that divide them into different lobes, with
two fissures and three lobes in the right lung, and one fissure and two lobes for the left.
Lung tissue itself is called stroma, which is made of elastic connective tissue.
When we looked at the circulatory system, we mentioned the pulmonary arteries and veins
that bring blood to and from the lungs, arriving oxygen-poor, and leaving oxygen-rich, so we
can imagine those connecting to the pulmonary capillaries to complete the circuit.
Within the lungs, there is the intrapulmonary pressure, or the pressure in the alveoli,
and the intrapleural pressure, the pressure in the pleural cavity that sits between the
two pleurae.
The intrapleural pressure is negative, which is what keeps the lungs open.
Ventilation, or breathing, is the result of the activity of inspiratory muscles, namely
the diaphragm and intercostal muscles, which produces a change in volume, and thereby a
change in pressure, allowing air to enter along the pressure gradient.
As they relax, expiration occurs.
In between, oxygen diffuses and binds to hemoglobin present in the blood, so that it can then
be transported to tissues all around the body.
And with that, we have a basic understanding of the respiratory system, and combining this
with previous knowledge of the circulatory system, we have a clear picture of how oxygen
enters the body and gets to where it needs to go.
Let’s continue with a few more systems now.