term 1 lecture 5 lungs moodle.pdf
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B2050/2051: Human anatomy and Embryology
Lecture 5: Lungs and lung
development
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Most of the human respiratory system is
involved in carrying air to or from
the respiratory surface
Respiratory surfaces – for gas exchange
– must be:
• large (large surface to volume ratio)
• wet• well supplied with blood
• thin
• aerated (open to a source of oxygen)
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The thorax is divided into two pleural
cavities by a central wall of tissue
(mainly consisting of the heart + fat
+ great vessels + remnants of thymus) – called the MEDIASTINUM
This can also be seen in
transverse section
Mediastinum
Pleural cavities
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Each of the lungs lies within a separate pleural cavity. This reaches abovethe clavicle in the neck and well down to the lower margin of the rib cage at
the back. At the lower margin of the thorax, there is a gap between the
base of the lung and the base of the pleural cavity. This is the
costodiaphragmatic recess – into which the lungs can slide when fully
inflated.
Costodiaphragmaticrecess
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Mediastinum
Lung
Pleura
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Mediastinum
Lung
Visceral pleura
Parietal pleura
Pleural space
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Normally the visceral and parietal pleura are in contact so that the
pleural cavity is only a potential space, containing a small amount of
lubricating fluid – but perforating wounds of the thorax can allow
air or blood to enter the cavity. This in turn causes the lung to collapse.
The parietal pleura is
well supplied bysensory nerves. The
visceral pleura is not.
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Apex
Base
Mediastinal surface
Costal surface
Sharp edges
slide in and out
of costo-diaphragmatic
recess
Cross-section
Costal surface Mediastinal
surface
Lung structure
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The right lung
Three lobes
• Upper
• Middle
• Lower
Two fissures
Horizontal
Oblique
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The left lung
Two lobes
• Upper
• Lower
One fissure
• Oblique
+
Cardiac notch
+ Lingula (=Middle lobe)
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Surface marking of the lungs and their fissures
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Each lobe of the lung is further divided into
sub-lobes called bronchopulmonary segments
Each segment is supplied by a
A segmental artery
A segmental vein
and a
Segmental bronchus
( You don’t need to learn the names of the segments)
Bronchopulmonary segments
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The hilum of the lung
• Pulmonary artery
• 2 pulmonary veins
• Main bronchus• Autonomic nerves
• Lymphatics
• Bronchial vessels
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Trachea
Dissection showing the structures that enter and leave the lung at the hilum
Lobar bronchiPrimary
bronchus
Trachea
Left bronchus is
long and
horizontal
Right bronchus is
shorter and vertical.
‘Foreign objects’ usually end up
here
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Trachea
Main bronchus
Lobar bronchus
Segmental bronchus
.. Terminal bronchusRespiratory bronchus
ALVEOLI
Bronchial tree
Rings of cartilage gradually
break up in more terminal
branches
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Resin filled cast of human bronchial tree
Bronchopulmonary
segments:
Segmental artery
Segmental vein
Segmental bronchus
You will see this in the Dissecting Room – each coloured area is a
different bronchopulmonary segment
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Fine structure of the bronchioles and alveoli
Notice that the cartilage disappears in the fine bronchioles but there is still
muscle In their walls until you get to the alveoli. This is smooth muscle –
with autonomic control
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Development
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Early embryo showing the
developing gut (yellow) still
open to the yolk sac.
Gut
Heart
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Step 1 – outgrowth from gut (endoderm)
Lung development
Lung bud
(a median lung bud grows out from the ventral side of the gut tube)
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Step 2: Proliferation and budding.The endodermal bud divides. Endoderm forms the lining and glands of the
system, but it is surrounded by splanchnic LP mesoderm that will form the
smooth muscle, cartilage, blood vessels etc in the lung and the walls of
the bronchial tree.
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Time
Fine branching of the bronchial tree and the terminal parts and alveoli. The branches
become finer and finer through development as the lungs mature.
De elopment of the terminal bronchioles and al eoli
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Development of the terminal bronchioles and alveoli
A) 5-17 weeks
Terminal bronchiolesbegin to form about
week 16.
No alveoli –
Cannot survive
B) 16-26 weeks
Respiratory
bronchiolesdevelop with
increased
Vasculature
A little surfactant
wks 20-22, a few
alveoli ~wk 24
Poor prognosis
C) 24 weeks-birth
Increase in alveoli
and vasculature.Increased surfactant.
Can survive with
help
D) 29 wks-8 years
Alveoli mature
and increase
(Full term = 38 weeks, breathing movements from about 32 weeks)
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At birth (full term infant) – there are around 20-70 million alveoli
In the adult – there are about 300-400 million alveoli
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b) Fluid in the lungs: tissue fluid in the lungs is reabsorbed in the last
2 months and then defence mechanisms against inhaled pathogens
begin to develop. Infants born before 26-28 weeks can therefore have a
problem because of the fluid and the danger of infection
c) Lack of surfactant: Surfactant is a lipoprotein substance secreted by
alveolar cells that coats the alveolar surfaces and stops them sticking
together during expiration. Without it, airways will collapse in air.
It begins to form around week 20 but there is not enough to
prevent airway collapse until much later. In a premature baby
there therefore a danger of Respiratory distress syndrome where the
airways collapse and become inflamed.
Premature babies often need help with breathing for three main reasons:
a) Alveoli are few and immature, lung capacity is low. Even at 34 weeks
lung capacity can be half that at birth
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Breathing movements
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Breathing movements - inspiration:
The goal of inspiratory movements is to increase the volume of the
thorax so that internal pressure falls and air is sucked in.
This can be done in one of three ways:
b) Increase bilateral diameter
a) Increase superior-inferior height
c) Increase anteroposterior diameter
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1. Anteroposterior increase – pump handle. Mainly upper ribs
Requires flexion at
manubrio-sternal joint
Because the upper ribs slope downwards at rest, raising them (ext. intercostals)
increases the anteroposterior diameter of the chest and pushes the sternum
forwards.
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Increase vertical diameter by flattening the diaphragm
Also aids in raisingintra-abdominal
pressure (requires
air held against
closed glottis)
Domes of the diaphragm
contract and flatten out
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The external intercostal muscles can be assisted by pectoralis (ribs to
arm), scalenes, sternocleidomastoid (skull to clavicle and sternum).
These muscles contract faster than external intercostals.
The scalenes (neck vertebrae to first rib) can fix the first rib so that the
other ribs are pulled towards it when the intercostals contract
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Expiration:
Normal – recoil, aided by elasticity of lung tissue + some contraction of
abdominal wall
Active expiration involves contraction of internal intercostals pulling ribs down
towards fixed lower ribs (fixed by back and abdominal muscles), aided by
increased contraction of abdominal wall muscles.
Contraction of abdominal muscles also helps push diaphragm up.
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Structure and development of the diaphragm
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The motor supply of the
diaphragm (and much of its
sensory supply) comes from
a pair of phrenic nerves –
one to each dome. These nerves
are derived from spinal nerves
from the cervical part of the spinal
cord – C3,4&5. The diaphragm is
skeletal muscle so these are
somatic not autonomic nerves
Phrenic nerves: run from the neck into the thorax in front of the root
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Phrenic nerves: run from the neck into the thorax, in front of the root
of the lung, and then over the pericardium of the heart which they supply
with sensory fibres. The nerves are then motor to all the diaphragm and
sensory to the central part of the diaphragm and associated pleura
(and peritoneum on the undersurface of the diaphragm)
The periphery of the diaphragm
and overlying parietal pleura
has a sensory nerve supply
from the intercostal nerves
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Caval opening (T8): IVC, right phrenic nerve (left phrenic has its own opening)
In central tendon
Oesophageal opening (T10): oesophagus, right & left vagus, oesophagealbranches from left gastric artery).
Guarded by crura
Aortic opening (T12): aorta, thoracic duct, azygous vein
Behind diaphragm
Caval opening
Oesophageal
opening
Aortic opening
Muscle slips converge on the central tendon from many
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Central tendon
Costal slips
Right and left crura
(from lumbar vertebrae)
p g y
different origins: ribs (costal slips), lumbar vertebrae
(crura), sternum (sternal slips) and from thick fascia
over the muscles in the back
Diaphragm from below
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Development of the diaphragm
The diaphragm is derived from four main embryonic components:a) The septum transversumb) The pleuroperitoneal folds (from the membranes lining the body wall)c) The mesentery (suspensory peritoneum) of the oesophagusd) Muscle cells from the somites (paraxial mesoderm) of the cervical (neck) region
(c) Mesentery of
oesophagus
(d) Muscle fibres from
neck mesoderm
(cervical somites)
(b) Folds from lining
of body wall
(pleuroperitoneal folds)
(a) Septum transversum
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Heart
The septum transversum is a sheet of fibrous tissue between the
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Heart
Liver bud
Septum transversum
heart and the liver. It gives rise to the central tendon of thediaphragm.
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The strange path of the phrenic nerves is explained by embryology
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The strange path of the phrenic nerves is explained by embryology.
The skeletal muscle that makes up the diaphragm comes from muscle blocks
(myotomes of somites) in the neck region.
The myotomes drag their nerve supply with them into the thorax.
Cervical muscle blocks
(myotomes)
Cervical spinal nerves.
C3, C4, C5 make up the phrenic
nerve
Muscle from neck migrates to diaphragm – nerve supply migrates with it
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Remember: First dissection practical on Thursday
Surnames L-Z only
Be on time 2-4 (+). You will not be admitted after the class has started.
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Objectives
By the end of this lecture you should be able to: Answer MCQ questions or write short notes on
• The adult diaphragm
• Diaphragm development
• Lung structure
• Lung development
• The phrenic nerve• The bronchial tree
• Breathing movements