term 1 lecture 5 lungs moodle.pdf

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B2050/2051: Human anatomy and Embryology

Lecture 5: Lungs and lung

development



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)



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



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



Mediastinum

Lung

Pleura



Mediastinum

Lung

Visceral pleura

Parietal pleura

Pleural space



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.



Apex

Base

Mediastinal surface

Costal surface

Sharp edges

slide in and out

of costo-diaphragmatic

recess

Cross-section

Costal surface Mediastinal

surface

Lung structure



The right lung

Three lobes

• Upper

• Middle

• Lower

Two fissures

Horizontal

Oblique



The left lung

Two lobes

• Upper

• Lower

One fissure

• Oblique

+

Cardiac notch

+ Lingula (=Middle lobe)



Surface marking of the lungs and their fissures



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



The hilum of the lung

• Pulmonary artery

• 2 pulmonary veins

• Main bronchus• Autonomic nerves

• Lymphatics

• Bronchial vessels



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



Trachea

Main bronchus

Lobar bronchus

Segmental bronchus

.. Terminal bronchusRespiratory bronchus

ALVEOLI

Bronchial tree

Rings of cartilage gradually

break up in more terminal

branches



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



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



Development



Early embryo showing the

developing gut (yellow) still

open to the yolk sac.

Gut

Heart



Step 1 – outgrowth from gut (endoderm)

Lung development

Lung bud

(a median lung bud grows out from the ventral side of the gut tube)



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.



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



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)



At birth (full term infant) – there are around 20-70 million alveoli

In the adult – there are about 300-400 million alveoli



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



Breathing movements



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



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.



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



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



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.



Structure and development of the diaphragm



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



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



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



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



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



Heart

The septum transversum is a sheet of fibrous tissue between the



Heart

Liver bud

Septum transversum

heart and the liver. It gives rise to the central tendon of thediaphragm.



The strange path of the phrenic nerves is explained by embryology



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



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.



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

term 1 lecture 5 lungs moodle.pdf

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