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Functional anatomy of the respiratory system

Respiratory System Divisions

? Upper Airway

? Nose,

pharynx,

larynx and

associated

structures

? Lower Airway

? trachea,

bronchi,

lungs
Midsagittal section through the upper airway

3

Functions of the upper airway

? Passageway for gas flow

? Filter

? Heater

? Humidification

? Sense of smell and taste

? Phonation

? Protection of lower airways

4
Size of Particles Entrapped in the Respiratory Passages.

No particles larger than 6 micrometers in diameter enter the lungs through the nose.

Of the remaining particles, many that are between 1 and 5 micrometers settle in the smal er

bronchioles as a result of gravitational precipitation. For instance, terminal bronchiolar

disease is common in coal miners because of settled dust particles.

Some of the stil smal er particles (smal er than 1 micrometer in diameter) diffuse against

the wal s of the alveoli and adhere to the alveolar fluid.

But many particles smal er than 0.5 micrometer in diameter remain suspended in the

alveolar air and are expel ed by expiration. For instance, the particles of cigarette smoke are

about 0.3 micrometer.

Almost none of these particles are precipitated in in the respiratory passageways before

they reach the alveoli. Unfortunately, up to one third of them do precipitate in the alveoli by

the diffusion process, with the balance remaining suspended and expel ed in the expired air.

Many of the particles that become entrapped in the alveoli are removed by alveolar

macrophages

5

Nasal breathing v/s oral breathing

Breathing is normally possible through either the nose or the mouth, the two

alternative air passages converging in the oropharynx.
Deflection of gas into either the nasal or the oral route is under voluntary control and
accomplished with the soft palate, tongue and lips.

Nasal breathing is the norm and has two major advantages over mouth breathing:
1.

filtration of particulate matter by the vibrissae hairs and

2.

Better humidification of inspired gas.

Humidification by the nose is highly efficient because the nasal septum and

turbinates greatly increase the surface area of mucosa available for evaporation.
Nasal resistance to airflow when obstructed by polyps, adenoids or congestion of the

nasal mucosa.
With increasing levels of exercise in normal adults, the respiratory minute volume

increases, and at a level of ~35 L/min-1 the oral airway comes into play.

6
Lower respiratory tree

1. The airways of the tracheobronchial tree extend from the larynx down to the

airways participating in gas exchange.

2. Each branching of an airway produces subsequent generations of smaller airways.

3. The first 15 generations are known as conducting airways because they convey gas

from the upper airway to the structures that participate in gas exchange with

blood.

4. The microscopic airways beyond the conducting airways that carry out gas

exchange with blood are classified as the respiratory airways.

7

Tracheobronchial Tree

Conducting zone: 1st 16 generations (Trachea to

terminal bronchioles)

? Passageway for air movement; No gas exchange occurs in the

conducting zone.

? The smallest airways in the conducting zone are the terminal

bronchioles.

? Trachea and main bronchi: Cartilage consists of U-shaped rings.

The lobar and segmental bronchi: small plates of cartilage.

? Bronchioles: Cartilage disappears. Bronchioles are suspended by

elastic tissue in the lung parenchyma, and the elasticity of the lung

tissue helps keep these airways open.

? The first four generations of the conducting zone are subjected to

changes in negative and positive pressures and contain cartilage to

prevent airway collapse.

? The conducting zone has its own separate circulation, the

bronchial circulation, which originates from the descending aorta

and drains into the pulmonary veins.
9

Tracheal: generation 0

The normal trachea as viewed during a

rigid bronchoscopy. The ridges of the

cartilage rings are seen anteriorly and the

longitudinal fibres of the trachealis muscle

are seen posteriorly, dividing at the carina

and continuing down both right and left

main bronchi.
Major airways of the tracheobronchial tree

The trachea bifurcates asymmetrically, and the right bronchus is wider and makes a

smal er angle with the long axis of the trachea.

The trachea is positioned midline in the upper mediastinum and branches into right and left

main stem bronchi.

At the base of the trachea, the last cartilaginous ring that forms the bifurcation for the two

bronchi is cal ed the carina.

The carina is an important landmark used to identify the level where the two main stem bronchi

branch off from the trachea; this is normal y at the base of the aortic arch.

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Trachea

? extends from its connection to the cricoid cartilage down through the neck

and into the thorax to the articulation point between the manubrium and

body of the sternum (angle of Louis).

? The adult trachea is approximately 12 cm long and has an inner diameter of

about 2 cm.

? The cartilaginous rings support the trachea so it does not collapse during

exhalation. Some compression occurs when the pressure around the trachea

becomes positive. During a strong cough, the trachea is capable of some

compression and even collapse when the decreased diameter increases the

linear velocity of gas flow and therefore the efficiency of removal of

secretions.

? The negative pressure generated around the trachea during inhalation causes

it to expand and lengthen slightly.

? The part of the trachea in the neck is not subjected to intrathoracic pressure

changes, but it is very vulnerable to pressures arising in the neck . An external

pressure of the order of 4 kPa (40 cmH2O) is sufficient to occlude the

trachea.
Cross-sectional view through the trachea and esophagus

? The outermost layer is a thin connective tissue sheath.

? Below the sheath are 16 to 20 C-shaped cartilaginous rings that provide support

and maintain the trachea as an open tube.

? The inner surface of the trachea is covered with a mucous membrane.

? In the posterior wall of the trachea is a thin band of tissue, called the trachealis

muscle that supports the open ends of the tracheal rings.

? The esophagus lies just behind the trachea.

13

Course of trachea and right and left main stem bronchi,

superimposed on a standard chest radiograph

? The right bronchus branches off from the trachea at an angle of approximately 20 to 30

degrees, and the left bronchus branches with an angle of about 45 to 55 degrees.

? The lower angle branching (closer to mid-line) of the right bronchus results in a greater

frequency of foreign body passage into the right lung because of the more direct pathway

14
The placement of an endotracheal tube through the upper airway and

into the trachea is a common airway management technique to

facilitate artificial airway placement.

? PROBLEM: After placement of an endotracheal tube in a patient with a 70-kg

predicted body weight (PBW), it is noted that breath sounds are heard in the

right chest only and that the patient 's oxygenation is deteriorating. Is the

airway placement the cause of the problem? How can this problem be

avoided?

? ANSWER: An endotracheal tube (ET) of proper diameter should be placed in

the trachea so the tip is 3 to 5 cm above the carina. If the ET is advanced too

far, it often enters the right main stem bronchus because of the straighter path

this bronchus offers. A right main stem intubation results in right lung

ventilation only. The left lung continues to receive pulmonary blood flow but

does not oxygenate adequately. To avoid this problem the ET generally should

not be advanced more than 24 cm past the lips in a 70-kg PBW patient. At this

point, auscultation is done with a stethoscope to confirm breath sounds in

both lungs. A chest radiograph can be taken to confirm the ET position.

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RULE OF THUMB : The 60-to-40 Rule

? The right lung is

slightly larger than the left lung

because of the location of the heart.

? The right lung has a sizable middle lobe and the left lung has a

smaller lingular segment in the left upper lobe.

? For purposes of estimating the contribution of the right

and left

lungs to ventilation and gas exchange, the 60-to-40 rule is used.

? The right lung is assumed to provide 60% of the ventilation/gas?

exchange capacity, and the left lung is assumed to provide the

remaining

40%.

?

If a patient requires removal of the entire left lung

(pneumonectomy), a 40%

decrease in lung volume

would

be expected.

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Lobar and Segmental Pulmonary Anatomy

? The lungs: an apex and a base and are subdivided by fissures into lobes. The

lobes are subdivided further into bronchopulmonary segments (functional

anatomic unit of the lung

? Each segment is supplied with gas from a single segmental bronchus.
? the right lung has 10 and the left lung has 8 segments.
? The airways continue to divide as they penetrate deeper into the lungs. The

segmental bronchi bifurcate into approximately 40 subsegmental bronchi, and

these divide into hundreds of smaller bronchi. Thousands of bronchioles branch

from the smaller bronchi. Tens of thousands of terminal bronchioles arise from

the bronchioles.

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Bronchopulmonary

segmental

divisions

of

the

lungs

18
Bronchi down to generation 4 are sufficiently regular to

be individual y named.

Total cross-sectional area of the respiratory tract

is minimal at the third generation.

The smal bronchi (5-11) extend through about seven

generations with their diameter fal ing from 3.5 to 1

mm.

At the level of the smal est true bronchi, air passages lie

close to branches of the pulmonary artery in a sheath

containing pulmonary lymphatics, which can be

distended with oedema fluid giving rise to the

characteristic `cuffing' responsible for the earliest

radiographic changes in pulmonary oedema.

These smal er bronchi rely for their patency on cartilage

within their wal s and on the transmural pressure

gradient, which is normal y positive from lumen to

intrathoracic space.

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Respiratory epithelium: Cell types

Ciliated Epithelial Cel s:

? The most abundant cell type in the respiratory epithelium.

? In the nose, pharynx and larger airways: are pseudostratified, gradually changing bronchi: single layer

of columnar cells; bronchioles : cuboidal cells and finally type I alveolar epithelial cells.

? are characterized by the presence of around 300 cilia per cell.

? The ratio of secretory to ciliated cells in the airway decreases in more distal airways from (equal in the

trachea and three quarters ciliated in the bronchioles).

Goblet Cel s : A density of approximately 6000 per mm2 (in the trachea) and produce the thick layer of

mucus that lines all but the smallest conducting airways.

Submucosal Secretory Cel s

? Submucosal glands occur in the larger bronchi and in the trachea (10 submucosal openings per mm2 ).

? Comprise both serous cells (gland acinus) and mucous cells (closer to the collecting duct).

? The serous cells have the highest levels of membrane-bound cystic fibrosis transmembrane

conductance regulator in the lung, Antiprotease enzymes and a variety of other proteins of uncertain

function.

Neuroepithelial Cel s

? found throughout the bronchial tree, (larger numbers in the terminal bronchioles).

? found individually or in clusters.

? Present in foetal lung tissue in a greater number (might control lung development).

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Cross-sectional view through a

Microscopic view of

bronchiole

mucous

membrane

21

Tracheal Anatomy

Under the electron microscope, the surface of the mucus membrane looks like a "shag carpet"

of cilia with approximately 1 to 2 bil ion cilia per square centimeter.
? The cross-sectional area of the conducting system increases exponential y.

At the level of the terminal bronchioles, the cross-sectional area is

approximately 20 times greater than that at the trachea.

? Increased cross-sectional area reduces the velocity of gas flow during

inspiration.

? At the level of the terminal bronchiole, its average velocity falls to speed of

diffusing gas molecules which is physiologically important for two reasons.

1. Laminar flow develops minimizing resistance in the smal airways and

decreases the work associated with inspiration.

2. Low gas velocity facilitates rapid mixing of alveolar gases. This mixing

provides a stable partial pressure of O2 and CO2 in the alveolar

environment that supports stable diffusion and gas exchange.

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Mucus secretion...

? The various secretory cells (primarily goblet cells) of the mucosa and bronchial

glands of the submucosa contribute to the production of mucus.

? Approximately 100 ml of mucus per day is produced. Most of the mucus

formed in the larger airways is produced by the bronchial glands.

? The amount and composition of mucus produced can increase and change with

airway irritation and diseases such as chronic bronchitis and asthma.

? Mucus is spread over the surface of the mucus membrane to a depth of

approximately 7 ?m and is propelled by the ciliated epithelia toward the

pharynx.

? The outer layer of mucus is more gelatinous and is called the gel layer. The inner

layer is much more fluid-like and is referred to as the sol layer.

? The mucus normally produced is a nearly clear fluid with greater viscosity than

water. It is a mixture of 97% water and 3% solute.

? The solute portion is produced primarily by goblet cells and bronchial glands; it

is called mucin and is composed of protein and minerals. The glycoprotein, lipid,

and water content of mucus provide its viscoelastic gel properties.

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Mucus secretion...

? Mucus functions to protect the underlying tissue.
? It helps prevent excessive amounts of water moving into and out of the

epithelia.

? It shields the epithelia from direct contact with potentially toxic materials and

microorganisms.

? It acts like sticky flypaper to trap particles that make contact with it. This

makes mucus an important part of the pulmonary defenses.

? The production of mucus is stimulated by local mechanical and chemical

irritation, release of proinflammatory mediators (e.g., cytokines), and

parasympathetic (vagal) stimulation.

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Ciliary epithelium

Ciliated cel s are found in the nasal cavity and al the airways from the larynx to the terminal

bronchioles.

Each of the pseudostratified cel s possesses approximately 200 cilia on its luminal surface.

Each cilium is an extension of the cel with an average length of about 6 ?m and diameter of

about 0.2 ?m.

The cilia "stroke" at a rate of approximately 15 times per second, producing a sequential motion

of the cilia cal ed a metachronal wave whose "wavelength" is approximately 20 ?m and

propels surface material in a specific direction.

Ciliary beating can be effectively slowed or stopped if the viscosity of the sol layer is increased

by exposure to dry gas.

Ciliary motion is also slowed or stopped after exposure to smoke, high concentrations of

inhaled O2, and drugs such as atropine.

26
Bronchioles (Generations 12 to 14)

? The internal diameter is ~1 mm.

? Patency : Cartilage disappears from the airway wall below this level . The air

passages are directly embedded in the lung parenchyma, the elastic recoil of

which holds the air passages open .

? The total cross-sectional area increases which makes the flow resistance of

these smaller air passages (less than 2 mm diameter) negligible under normal

conditions.

? The resistance of the bronchioles can increase to very high values when their

strong helical muscular bands are contracted.

? The conducting airways derive their nutrition from the bronchial circulation

and are influenced by systemic arterial blood gas levels.

? The acinar airways and rely upon the pulmonary circulation for their nutrition.

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Transition to Respiratory Bronchioles (Generations 15 to 18)

? The functions of the air passages are solely conduction and humidification. Beyond

this point there is a gradual transition from conduction to gas exchange.

? In the four generations of respiratory bronchioles there is a gradual increase in the

number of alveoli in their walls.

? Like the earlier bronchioles, the respiratory bronchioles are embedded in lung

parenchyma.

? There is no significant change in calibre of advancing generations of respiratory

bronchioles (~0.4 mm diameter).

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Alveolar Ducts (Generations 19 to 22) and Alveolar Sacs

(Generation 23)

? Arise from the terminal respiratory bronchiole, from which they differ by having

no walls other than the mouths of mural alveoli (approximately 20 in number).

? The alveolar septa comprise a series of rings forming the walls of the alveolar

ducts and containing smooth muscle.

? Approximately 35% of the alveolar gas resides in the alveolar ducts and the

alveoli that arise directly from them.

? The last generation of the air passages differs from alveolar ducts solely because

they are blind.

? It is estimated that about 17 alveoli arise from each alveolar sac and account for

about half of the total number of alveoli which occurs in larger airways. Millions

of alveolar ducts branch off the respiratory bronchioles.

? Alveolar ducts are tiny airways only 0.3 mm in diameter, and their walls are

composed entirely of alveoli.

? Each alveolar duct ends in a cluster of alveoli, which is frequently referred to as an

alveolar sac.

? Each alveolar sac opens into about 16 or 17 alveoli, and about one-half the total

number of alveoli are found in this region.

29

Cel ular transition from conducting airway to the alveolus. The epithelial layer transitions from

pseudostratified layer with submucosal glands to a cuboidal and the to a squamous epithelium.

The underlying mesynchyme tissue and capil ary structure also changes with the airway

transition. The cel s of the respiratory mucosa change as they progress into the smal er airways.

As the thickness of the airway wal s decreases, bronchial glands become fewer in number. At the

bronchiolar level, the number of ciliated cel s decreases.

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Transition from conducting airway to the alveolus

? As the conducting airway transitions to terminal and transitional bronchioles, the

histological appearance of the conducting tubes change.

? Secretory glands are absent from the epithelium of the bronchioles and terminal

bronchioles, smooth muscle plays a more prominent role and cartilage is largely

absent from the underlying tissue.

? Clara cells, nonciliated cuboidal epithelial cells that secrete important defense

markers and serve as progenitor cells after injury, make up a large portion of the

epithelial lining in the latter portions of the conducting airway

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Epithelial cel s in the conducting airway secrete a variety of molecules that aid in lung

defense: Secretory immunoglobulins (IgA), collectins (including surfactant protein (SP) ?A

and SP-D), defensins and other peptides and proteases, reactive oxygen species, and

reactive nitrogen species.

These secretions can act directly as antimicrobials to help keep the airway free of infection.

Airway epithelial cel s secrete a variety of chemokines and cytokines that recruit traditional

immune cel s and other immune effector cel s to site of infections.

The smal er particles that make it through the upper airway, 2?5 m in diameter, generaly

fal on the wal s of the bronchi as the airflow slows in the smal er passages.

There they can initiate reflex bronchial constriction and coughing.

32
Circulation in lung

? The conducting zone has its own separate circulation, the

bronchial circulation, which originates from the descending aorta

and drains into the pulmonary veins.

? The respiratory zone has its own separate and distinct circulation,

the pulmonary circulation.

? The lungs have the most extensive capillary network of any organ

in the body. Pulmonary capillaries occupy 70% to 80% of the

alveolar surface area.

? The pulmonary circulation receives all of the cardiac output and,

therefore, blood flow is high.

? One pulmonary arterial branch accompanies each airway and

branches with it.

? Red blood cells can pass through the pulmonary capillaries in less

than 1 second.

33

Smooth muscle of the airways

? The smooth muscle of the airways varies in location and structure. In the

large airways (e.g., the trachea), smooth muscle is bundled in sheets. In

smaller airways, smooth muscle forms a helical pattern that wraps the airway

in bundles in decreasing quantities as the airways branch and become

smaller.

? Muscle fibers crisscross and spiral around the airway walls. This placement

reduces the diameter of the airway and shortens it when the muscle

contracts.

? This pattern of smooth muscle continues but thins out on reaching the

smallest bronchioles.

? The tone of the smooth muscle is increased and results in bronchospasm by

the activity of the parasympathetic nervous system (release of acetylcholine)

and proinflammatory mediator release from mast cells, inflammatory cells,

and neuroendocrine cells.

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Nervous and Local Control of the Bronchiolar Musculature

The wal s of the bronchi and bronchioles are innervated by the autonomic nervous system.

Nerve cel s in the airways sense mechanical stimuli or the presence of unwanted substances

in the airways such as inhaled dusts, cold air, noxious gases and cigarette smoke.

These neurons can signal the respiratory centers to contract the respiratory muscles and

initiate sneeze or cough reflexes.

The receptors show rapid adaptation when they are continuously stimulated to limit sneeze

and cough under normal conditions.

-- "Sympathetic" Dilation of the Bronchioles. Direct control of the

bronchioles by sympathetic nerve fibers is relatively weak because few of these fibers

penetrate to the central portions of the lung. However, the bronchial tree is very much

exposed to norepinephrine and epinephrine released into the blood by sympathetic

stimulation of the adrenal gland medul ae. Both these hormones, especial y epinephrine

because of its greater stimulation of beta-adrenergic receptors, cause dilation of the

bronchial tree.

Parasympathetic Constriction of the Bronchioles. A few parasympathetic

nerve fibers derived from the vagus nerves penetrate the lung parenchyma. These nerves

35

secrete acetylcholine and, when activated, cause mild to moderate constriction of the

Local Control of the Bronchiolar Musculature

? Local Secretory Factors Often Cause Bronchiolar Constriction.

? Several substances formed in the lungs are often quite active in causing

bronchiolar constriction.

? Two of the most important of these are histamine and slow reactive substance of

anaphylaxis. Both of these are released in the lung tissues by mast cells during

allergic reactions, especially those caused by pollen in the air.

? Therefore, they play key roles in causing the airway obstruction that occurs in

allergic asthma; this is especially true of the slow reactive substance of

anaphylaxis.

? The same irritants that cause parasympathetic constrictor reflexes of the airways--

smoke, dust, sulfur dioxide, and some of the acidic elements in smog--often act

directly on the lung tissues to initiate local, non-nervous reactions that cause

obstructive constriction of the airways.

36
The Alveoli

? The mean total number of alveoli has been estimated as 400 mil ion, but ranges from about

270 to 790 mil ion, correlating with the height of the subject and total lung volume.

? The size is dependent on lung volume but due to gravity they are normal y larger in the

upper part of the lung, except at maximal inflation when the vertical gradient in size

disappears.

? At functional residual capacity the mean diameter of a single alveolus is 0.2 mm and the

total surface area of the alveoli is ~130 m2.

? Intrinsic shape of alveoli (Al) is maintained from FRC to TLC.

? Alveolar wal s are flat with sharp corners where the adjacent walls meet.

37

A 30 cm H2O [TLC]).

B 8 cm H2O 50% TLC).

C 4 cm H2O [FRC]).

D 0 cm H2O (minimum volume).

Scanning electron photomicrographs at the same magnification of perfusion-fixed normal

38

rat lung at different degrees of inflation pressure.
The Alveolar Septa

They are general y flat, making the alveoli polyhedral rather than spherical.

The septa are perforated by small fenestrations known as the pores of Kohn,

which provide collateral ventilation between alveoli.

Collateral ventilation also occurs between small bronchioles and neighbouring

alveoli, adjacent pulmonary acini and occasionally intersegmental

communications, (Canals of lambert) and is more pronounced in patients with

emphysema.

The `active' side: The alveolar wall, the capillary endothelium and the alveolar

epithelium are closely apposed, with almost no interstitial space, such that the

total thickness from gas to blood is ~0.3 m.

The `service' side: is more than 1- to 2-m thick and contains a recognisable

interstitial space containing elastin and collagen fibres, nerve endings and

occasional migrant polymorphs and macrophages.

The distinction between the two sides of the capillary has considerable

pathophysiological significance as the active side tends to be spared in the 39

accumulation of both oedema fluid and fibrous tissue.

Highly magnified cross-sectional sketch

of the cel s

and organization

of the

alveolar septa

40

This post was last modified on 08 April 2022