The than the right one. On the other hand

The lungs
are part of the lower respiratory system starting from trachea which is an air
passageway and is found at the front of the esophagus extending from the larynx
to the border of the 5th thoracic vertebra. The trachea is divided
into the right and left primary bronchi. The trachea has various different
layers (which can be seen in the picture below). Firstly the mucosa which is
made out of ciliated pseudostratified columnar epithelium and has various
mucus-secreting goblet cells, with a thin layer of lamina propria (mostly
collagenous) underneath, having elastic and reticular fibers. The submucosa has
areolar connective tissue has seromucous glands while the adventitia contains
cartilaginous rings that are interconnected by connective tissue which joins
the trachea to surrounding tissue.   

figure 1 – Microscopic picture
of trachea showing different layers: mucosa, submucosa, cartilage. (Lungs, 1. 2017).

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By the fifth
thoracic vertebra, the trachea divides into left and right primary bronchus with
one going into the right and the other to the left. The bronchi will then enter
the lungs and divide into smaller bronchi creating the secondary lobar bronchi,
one for each lobe of the lung. Each lung has different lobes with the right
having three and left having two. The secondary bronchi will carry on branching
out until it creates smaller bronchi, called tertiary bronchi. This will then
divide into bronchioles which lead to terminal bronchioles. The left lung will
be slightly smaller in size due to having a cardiac notch where the heart lies,
making the left lung about 10% smaller than the right one. On the other hand
the right lung is shorter than the left lung but that is due to the diaphragm
being higher so we have enough space for the liver below it. The lung itself
can be split into different lobes seen in image below (Lungs, 2017)

Figure 2 –
images lung and cross sectional lobes. (Lungs, 1. 2017)

Each lobe
has its own secondary bronchus, with the right primary bronchus gives rise to
three secondary bronchi in the lung and the secondary bronchi gives rise to the
tertiary bronchi with each lung having 10 tertiary bronchi. Each lobule is
wrapped in elastic connective tissue and contains a lymphatic vessel,
arteriole, venule and a branch of terminal bronchiole. Terminal bronchioles
will be divided into microscopic branches known as respiratory bronchioles. As
they go deeper in the lung the epithelial lining changes from cuboidal to
simple squamous. The lungs are coned shaped and get separated by the
heart/other structures. They spread from the diaphragm to the calvicles, lying
against the ribs. The lung is concaved by the base which allows it to be placed
in the concaved area of the diaphragm. The medial surface of both lungs is made
of the hilus which is classed as the root of the lung. Nerves, bronchi,
pulmonary blood vessels, lymphatic vessels and nerves enter and exit. All these
structures are held together by pleura/ connective tissue. The lungs get
protected by a serous membrane which covers the lungs and is called the pleural
membrane. In the superficial layer it lines with the thoracic cavity and is
called the parietal pleura with the deep layer being known as the pleura
visceral which is attached to the lungs. Between these two spaces there is a
space known as the pleural cavity which is made of lubricating fluid and is
secreted by the layers of the lung. This fluid allows the membranes to slide
against each other and reduces friction. If more fluid than needed is
accumulated in the pleural space it will cause a pleural effusion. Inside the
lungs they are specialised cells known as the alveoli which are the main site
of gas exchange. The alveolus is a cup shaped lined by simple squamous
epithelium. It is supported with a thin elastic basement membrane consisting of
2 types of alveolar. Type 1 alveolar cells being the main site of gas exchange
have predominant cells and form constant lining of the wall and are made out of
simple squamous epithelial cells. Type 2 alveolar cells are septal cells which
are found in between type 1 cells. They are rounded and cuboidal epithelial
cells. On the surface they contain microvilli and secrete alveolar fluid which
keeps the surface between cells and air moist (This can be seen in figure 2).
The alveolar fluid contains phospholipids and lipoproteins. It also has
surfactant which lowers surface tension and helps to reduce the alveolar not
collapsing. (Tortora and Derrickson, n.d.)

Figure 3 (Philschatz.com, 2017)

The alveolar
wall contains macrophages which engulf phagocytes and removes any debris and
dust particles that may be in the walls. The pulmonary arterioles and the
venules separate into capillaries in the outer surface of the alveoli. They
have a single layer of endothelial cells and a basement membrane. (Nhlbi.nih.gov, 2017)

During inhalation
the air pressure inside the lungs gets equal to the atmospheric pressure. When
air will be flowing into the lungs it would cause the pressure inside the
alveolus to come lower than the atmospheric pressure, which is done by the
increase in the volume of lungs. So inhalation can take place the lungs must
expand. This is done by the alveoli expanding with the participation of the
diaphragm /external intercostal.
The diaphragm is dome shaped having skeletal muscles that contract, allowing it
to flatten which will increase the diameter of the thoracic cavity and is accountable
for the movement of about 75% of the air that enter the lungs. As the diaphragm
increase its dimeter the external intercostal contract causing the ribs to be
pulled up and the sternum to be pushed down. As this happens the walls of the
lungs are pulled outward due to low atmospheric pressure the parietal and
visceral pleurae will start to keep close to each other due to the surface tension
that gets created by the touching surfaces. As the thoracic cavity gets more
expanded so does the parietal and pleura lining (getting pulled in all
direction) which in return increases lung volume. But during exhalation the
pressure in the lungs is greater than the atmosphere. Exhalation has two main
factors which is the recoil of the elastic fibers and the inward pull of the
surface tension from the alveolar fluid. Exhalation is the process of
inhalation but reversed. The external intercostal, ribs, diaphragm and thoracic
cavity go back to their original space and also the surface tension will exert
and inward pull between the parietal and visceral pleurae and the elastic membrane
which would recoil. This will make the lung volume decrease and the alveolar
pressure to increase, causing the air from the alveoli to flow into the low pressure
air in the atmosphere. (Nhlbi.nih.gov, 2017)

During
breathing there’s a process called gas exchange that takes place. This is
Oxygen and Carbon Dioxide moving between the lungs and blood. Diffusion
occurs during gas exchange as the blood in the capillaries
surrounding the alveoli has a lower oxygen concentration compared to the air in
the alveoli which has just been inhaled. The walls of the alveoli and
capillaries are only one cell thick, this allows gasses to diffuse across them.
The blood in capillaries will have a higher concentration of CO2 than the
inspired air due to CO2 being a waste product. CO2 will then diffuse the other
way and into the alveoli where it can be exhaled.