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Chapter 23

The Respiratory

System

Upper respiratory tract

Lower respiratory tract

Respiratory System Anatomy Structurally, the respiratory system is divided into upper

and lower divisions or tracts.

The upper respiratory tract

consists of the nose, pharynx

and associated structures.

The lower respiratory tract

consists of the larynx,

trachea, bronchi and

lungs.

Respiratory System Anatomy Functionally, the respiratory system is divided into the

conducting zone and the respiratory zone.

The conducting zone is involved with bringing air to

the site of external respiration and consists of the

nose, pharynx, larynx, trachea, bronchi, bronchioles

and terminal bronchioles.

The respiratory zone is the main site of gas

exchange and consists of the respiratory bronchioles,

alveolar ducts, alveolar sacs, and alveoli.

Air passing through the respiratory

tract traverses the:

Nasal cavity

Pharynx

Larynx

Trachea

Primary (1o) bronchi

Secondary (2o) bronchi

Tertiary (3o) bronchi

Bronchioles

Alveoli (150 million/lung)

Respiratory System Anatomy

The external nose is visible on the face.

The internal nose is a large cavity beyond the nasal

vestibule.

The internal nasal

cavity is divided by a

nasal septum into

right and left nares.

Respiratory System Anatomy Three nasal conchae (or turbinates)

protrude from each lateral wall into the

breathing passages.

Tucked under each nasal concha is an

opening, or meatus, for a duct that drains

secretions of the sinuses and tears into the

nose.

Receptors in the

olfactory epithelium

pierce the bone

of the cribriform plate.

Respiratory System Anatomy

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Respiratory System Anatomy The pharynx is a hollow tube that starts

posterior to the internal nares and

descends to the opening of the larynx in

the neck.

It is formed by a complex arrangement of

skeletal muscles that assist in deglutition.

It functions as:

a passageway for air and food

a resonating chamber

a housing for the tonsils

Respiratory System Anatomy The pharynx has 3 anatomical regions:

The nasopharynx; oropharynx; and

laryngopharynx

In this graphic, slitting the muscles of the posterior

pharynx shows the

back of the tongue

in the laryngopharynx.

The nasopharynx is separated

from the oropharynx by the

hard and soft palate.

The nasopharynx lies behind the internal nares.

It contains the pharyngeal tonsils (adenoids)

and the

openings of the

Eustachian tubes

(auditory tubes)

which come off

of it and travels

to the middle

ear cavity.

Respiratory System Anatomy Respiratory System Anatomy

The oropharynx lies behind the mouth and

participates in both respiratory and digestive

functions.

The main palatine tonsils (those usually taken

in a tonsillectomy) and small lingual tonsil are

housed here.

The laryngopharynx lies inferiorly and opens

into the larynx (voice box) and the esophagus.

It participates in both respiratory and digestive

functions.

Respiratory System Anatomy Respiratory System Anatomy The larynx, composed of 9 pieces of cartilage,

forms a short passageway connecting the

laryngopharynx with the trachea (the “windpipe”).

The thyroid cartilage (the large

“Adam’s apple”) and the one

below it (the cricoid cartilage)

are landmarks for making an

emergency airway (called a

cricothyrotomy).

Anterior view of the larynx

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The epiglottis is a flap of elastic cartilage covered with a

mucus membrane, attached to the root of the tongue.

The epiglottis guards the entrance of the glottis, the

opening between the vocal folds.

For breathing, it is held

anteriorly, then pulled back-

ward to close off the glottic

opening during

swallowing.

Respiratory System Anatomy Respiratory System Anatomy The rima glottidis (glottic opening) is formed by a

pair of mucous membrane vocal folds (the true

vocal cords).

The vocal folds are situated high in the larynx

just below where the larynx and the esophagus

split off from the pharynx.

Cilia in the upper respiratory tract move mucous

and trapped particles down toward the pharynx.

Cilia in the lower respiratory tract move them up

toward the larynx.

Respiratory System Anatomy

Upper respiratory tract

Lower respiratory tract

Respiratory System Anatomy As air passes from the laryngopharynx into the

larynx, it leaves the upper respiratory tract and

enters the lower respiratory tract.

Air passing through the respiratory

tract

Nasal cavity

Pharynx

Larynx

Trachea

Primary bronchi

Secondary bronchi

Tertiary bronchi

Bronchioles

Alveoli (150 million/lung)

Respiratory System Anatomy The trachea is a semi-rigid pipe made of semi-

circular cartilaginous rings, and located anterior to

the esophagus.

It is about 12 cm long and extends from the

inferior portion of the larynx into the mediastinum

where it divides into right and left primary (1o,

“mainstem”) bronchi.

It is composed of 4 layers: a mucous secreting

epithelium called the mucosa, and three layers of

CT (submucosa, hyaline cartilage, and

adventitia).

The tracheal cartilage rings are incomplete

posteriorly, facing the esophagus.

Esophageal masses can press into this soft

part of the trachea and make it difficult

to breath, or even

totally obstruct

the airway.

Respiratory System Anatomy

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Respiratory System Anatomy The right and left primary (1o or “mainstem”) bronchi

emerge from the inferior trachea to go to the lungs,

situated in the right and left pleural cavities.

The carina is an internal

ridge located at the junction

of the two main stem

bronchi – a very sensitive

area for triggering the

cough reflex.

Respiratory System Anatomy The 1o bronchi divide to form 2o and 3o bronchi which

respectively supply the lobes and segments of each lung.

3o bronchi divide into

bronchioles which in

turn branch through

about 22 more divisions

(generations).

The smallest are the

terminal bronchioles.

Respiratory System Anatomy The bronchi and bronchioles go through structural

changes as they branch and become smaller.

The mucous membrane changes and then

disappears.

The cartilaginous rings become more sparse,

and eventually disappear altogether.

As cartilage decreases, smooth muscle (under

the control of the Autonomic Nervous System)

increases. Sympathetic stimulation causes airway dilation, while

parasympathetic stimulation causes airway constriction.

Respiratory System Anatomy All the branches from the trachea to the

terminal bronchioles are conducting

airways – they do not

participate in gas

exchange.

Respiratory System Anatomy The cup-shaped outpouchings which participate in gas

exchange are called alveoli.

The first alveoli don’t appear until

the respiratory

bronchioles

where they are

rudimentary and

mostly

nonfunctioning.

Respiratory System Anatomy

Respiratory bronchioles give way to alveolar

ducts, and the epithelium (simple cuboidal)

changes to simple squamous, which comprises

the alveolar ducts, alveolar sacs, and alveoli.

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Taken together, these structures form the

functional unit of the lung, which is the

pulmonary lobule.

Wrapped in elastic

C.T., each pulmonary

lobule contains a

lymphatic vessel, an

arteriole, a venule

and a terminal

bronchiole. The pulmonary lobule

Respiratory System Anatomy Respiratory System Anatomy As part of the pulmonary lobule, alveoli are delicate

structures composed chiefly of type I alveolar cells,

which allow for exchange of gases with the pulmonary

capillaries.

Alveoli make up a large

surface area (750 ft2).

Type II cells secrete a

substance called surfactant

that prevents collapse of the

alveoli during exhalation.

Respiratory System Anatomy Alveoli macrophages (also called “dust cells”) scavenge

the alveolar surface to engulf and remove microscopic

debris that has made it past the “mucociliary blanket” that

traps most foreign particles higher in

the respiratory tract.

The alveoli (in close proximity

to the capillaries) form the

alveolar-capillary membrane

(“AC membrane”).

Blood Supply to the Lungs

The lungs receive blood via two sets of

arteries

Pulmonary arteries carry deoxygenated blood

from the right heart to the lungs for oxygenation

Bronchial arteries branch from the aorta and

deliver oxygenated blood to the lungs primarily

perfusing the muscular walls of the bronchi and

bronchioles

Ventilation-Perfusion Coupling

Ventilation-perfusion coupling is the coupling of

perfusion (blood flow) to each area of he lungs to

match the extent of ventilation (airflow) to alveoli in

that area

In the lungs, vasoconstriction in response to

hypoxia diverts pulmonary blood from poorly

ventilated areas of the lungs to well-ventilated

regions

In all other body tissues, hypoxia causes dilation

of blood vessels to increase blood flow

As organs, the lungs are divided into lobes by fissures.

The right lung is divided by the oblique fissure and the

horizontal fissure into 3 lobes .

The left lung is divided into

2 lobes by the oblique fissure.

Each lobe receives it own 2o

bronchus that branches into

3o segmental bronchi (which

continue to further divide).

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The apex of the lung is superior, and extends slightly

above the clavicles. The base of the

lungs rests on the diaphragm.

The cardiac notch –

in the left lung (the

indentation for the

heart) makes the left

lung 10 % smaller

than the right lung.

Respiratory System Anatomy The lungs are separated from each other

by the heart and other structures in the

mediastinum.

Each lung is enclosed by a double-layered

pleural membrane.

The parietal pleura line the

walls of the thoracic cavity.

The visceral pleura adhere

tightly to the surface of

the lungs themselves.

Respiratory System Anatomy

Respiratory System Anatomy On each side of the thorax, a pleural cavity is formed.

The integrity of this space (really potential space)

between the parietal and visceral pleural layers is

crucial to the mechanism of breathing.

Pleural fluid reduces friction and produces a surface

tension so the layers can slide across one another.

The pleura, adherent to the chest wall and to the lung,

produces a mechanical coupling for the two layers to

move together.

Understanding Gases

To understand how this mechanical

coupling between the lungs, the pleural

cavities and the chest wall results in

breathing, we first need to discuss some

physics of gases called the

gas laws.

Understanding Gases The respiratory system depends on the

medium of the earth’s atmosphere to

extract the oxygen necessary for life.

The atmosphere is composed of these

gases:

Nitrogen (N2) 78%

Oxygen (O2) 21%

Carbon Dioxide (CO2) 0.04%

Water Vapor variable, but on average

around 1%

Understanding Gases The gases of the atmosphere have a

mass and a weight (5 x 1018 kg, most

within 11 km of the surface).

Consequently, the atmosphere exerts a

significant force on every object on the

planet (recall that pressure is measured as

force applied per unit area, P = F/A.)

We are “accustomed” to the tremendous

force pressing down on every square inch of

our body.

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Understanding Gases A barometer is an

instrument that

measures atmospheric

pressure.

Baro = pressure or

weight

Meter = measure

Air pressure varies

greatly depending on the

altitude and the

temperature.

Understanding Gases There are many different units used to

measure atmospheric pressure. At sea

level, the air pressure is:

14.7 lb/in2 = 1 atmosphere

760 mmHg = 1 atmosphere

76 cmHg = 1 atmosphere

29.9 inHg = 1 atmosphere

At high altitudes, the atmospheric pressure

is less; descending to sea level,

atmospheric pressure is greater.

Understanding Gases Gases obey laws of physics called the

gas laws.

These laws apply equally to the gases of the

atmosphere, the gases in our lungs, the

gases dissolved in the blood, and the gases

diffusing into and out of the cells of our body.

To understand the mechanics of ventilation

and respiration, we need to have a basic

understanding of 3 of the 5 common gas

laws.

Understanding Gases Boyle’s law applies to containers with

flexible walls – like our thoracic cage.

It says that volume and pressure are

inversely related.

If there is a decrease

in volume – there will

be an increase in

pressure: V ∝ 1/P

Understanding Gases Dalton’s law applies to a mixture of

gases.

It says that the pressure of each gas is

directly proportional to the percentage of that

gas in the total mixture: PTotal = P1 + P2 + P3

…

Since O2 = 21% of atmosphere, the partial

pressure exerted by the contribution of just

O2 (written pO2 or PAO2) = 0.21 x 760 mmHg

= 159.6 mmHg at sea level.

Understanding Gases Henry’s law deals with gases and solutions.

It says that increasing the partial pressure of a

gas “over” (in contact with) a solution will

result in more of the gas dissolving into the

solution.

The patient in this picture is getting

more O2 in contact with

his blood - consequently,

more oxygen goes

into his blood.

Medicimage/Phototake

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Understanding Gases

Gas will always move from a region of

high pressure to a region of low pressure.

Applying Boyle's law: If the volume inside

the thoracic cavity , the pressure .

Ventilation and Respiration Pulmonary ventilation is the movement of air

between the atmosphere and the alveoli, and

consists of inhalation and exhalation.

Ventilation, or

breathing, is made

possible by

changes in the

intrathoracic

volume.

Ventilation and Respiration In contrast to ventilation,

respiration is the exchange of

gases.

External respiration

(pulmonary)

is gas exchange between the

alveoli and the blood.

Internal respiration (tissue)

is gas exchange between

the systemic capillaries and

the tissues of the body.

Ventilation and Respiration External respiration in the lungs is possible

because of the implications of Boyle’s law:

The volume of the thoracic cavity can be

increased or decreased by the action of the

diaphragm, and other muscles of the chest

wall.

By changing the volume of the thoracic cavity

(and the lungs – remember the mechanical

coupling of the chest wall, pleura, and lungs),

the pressure in the lungs will also change.

Ventilation and Respiration

Changes in air pressure result in

movement of the air.

Contraction of the diaphragm and external

intercostal (rib) muscles increases the size of

the thorax. This decreases the intrapleural

pressure so air can flow in from the

atmosphere (inspiration).

Relaxation of the diaphragm, with/without

contraction of the internal intercostals,

decreases the size of the thorax, increases the

air pressure, and results in exhalation.

Ventilation and Respiration Certain thoracic

muscles participate

in inhalation;

others aid

exhalation.

The diaphragm

is the primary

muscle of

respiration – all

the others are

accessory.

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Ventilation and Respiration The recruitment of accessory muscles

greatly depends on whether the respiratory

movements are quiet (normal), or forced

(labored).

Airflow and Work of Breathing Differences in air pressure drive airflow, but 3

other factors also affect the ease with which we

ventilate:

1. The surface tension of alveolar fluid causes

the alveoli to assume the smallest possible

diameter and accounts for 2/3 of lung elastic

recoil. Normally the alveoli would close with

each expiration and make our “Work of

Breathing” insupportable.

Surfactant prevents the complete collapse

of alveoli at exhalation, facilitating

reasonable levels of work.

Airflow and Work of Breathing

2. High lung compliance means the lungs and

chest wall expand easily.

Compliance is decreased by a

broken rib, or by diseases such

as pneumonia or emphysema.

Airflow and Work of Breathing

Measuring Ventilation Ventilation can be measured using spirometry.

Tidal Volume (VT) is the volume of air inspired (or

expired) during normal quiet breathing (500 ml).

Inspiratory Reserve Volume (IRV) is the volume

inspired during a very deep inhalation (3100 ml –

height and gender dependent).

Expiratory Reserve Volume (ERV) is the volume

expired during a forced exhalation (1200 ml).

Measuring Ventilation

Spirometry continued

Vital Capacity (VC) is all the air that can be

exhaled after maximum inspiration.

It is the sum of the inspiratory reserve + tidal volume

+ expiratory reserve (4800 ml).

Residual Volume (RV) is the air still present in

the lungs after a force exhalation (1200 ml).

The RV is a reserve for mixing of gases but is not

available to move in or out of the lungs.

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Measuring Ventilation

Old and new spirometers used to measure ventilation.

Measuring Ventilation

A graph of spirometer volumes and capacities

Measuring Ventilation Only about 70% of the tidal volume reaches the

respiratory zone – the other 30% remains in the

conducting zone (called the anatomic dead space).

If a single VT breath = 500 ml, only 350 ml will

exchange gases at the alveoli.

In this example, with a respiratory rate of 12, the

minute ventilation = 12 x 500 = 6000 ml.

The alveolar ventilation (volume of air/min that

actually reaches the alveoli) = 12 x 350 = 4200ml.

Exchange of O2 and CO2 Using the gas laws and understanding the

principals of ventilation and respiration,

we can calculate the

amount of oxygen and

carbon dioxide

exchanged between

the lungs and

the blood.

Exchange of O2 and CO2

Dalton’s Law states that each gas in a mixture of gases

exerts its own pressure as if no other gases were present.

The pressure of a specific gas is the partial pressure

Pp.

Total pressure is the sum of all the partial pressures.

Atmospheric pressure (760 mmHg) = PN2 + PO2 + PH2O

+ PCO2 + Pother gases

Since O2 is 21% of the atmosphere, the PO2 is

760 x 0.21 = 159.6 mmHg.

Exchange of O2 and CO2 Each gas diffuses across a permeable membrane (like

the AC membrane) from the side where its partial

pressure is greater to the side where its partial pressure is

less.

The greater the difference, the faster the rate of

diffusion.

Since there is a higher PO2 on the lung side of the AC

membrane, O2 moves from the alveoli into the

blood.

Since there is a higher PCO2 on the blood side of the

AC membrane, CO2 moves into the lungs.

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Exchange of O2 and CO2

PN2 = 0.786 x 760 mmHg = 597.4 mmHg

PO2 = 0.209 x 760 mmHg = 158.8 mmHg

PH2O = 0.004 x 760 mmHg = 3.0 mmHg

PCO2 = 0.0004 x 760 mmHg = 0.3 mmHg

Pother gases = 0.0006 x 760 mmHg = 0.5 mmHg

Total = 760.0 mmHg

Partial pressures of gases in inhaled air for sea level

Exchange of O2 and CO2 Henry’s law states that the quantity of a gas

that will dissolve in a liquid is proportional to the

partial pressures of the gas and its solubility.

A higher partial pressure of a

gas (like O2) over a liquid (like

blood) means more of the gas

will stay in solution.

Because CO2 is 24 times more

soluble in blood (and soda pop!) than in O2,

it more readily dissolves.

Exchange of O2 and CO2 Even though the air we breathe is mostly

N2, very little dissolves in blood due to its

low solubility.

Decompression sickness (“the bends”) is a

result of the comparatively insoluble N2 being

forced to dissolve into the blood and tissues

because of the very high pressures associated

with diving.

By ascending too rapidly, the N2 rushes out of the

tissues and the blood so forcefully as to cause

vessels to “pop” and cells to die.

Transport of O2 and CO2 In the blood, some O2 is dissolved in the

plasma as a gas (about 1.5%, not enough

to stay alive – not by a long shot!). Most

O2 (about 98.5%) is carried attached to

Hb.

Oxygenated Hb is called oxyhemoglobin.

Transport of O2 and CO2

CO2 is transported in the blood in three different

forms:

1. 7% is dissolved in the plasma, as a gas.

2. 70% is converted into carbonic acid through the

action of an enzyme called carbonic anhydrase.

CO2 + H2O H2CO3 H+ + HCO3

-

3. 23% is attached to Hb (but not at the same binding

sites as oxygen).

Transport of O2 and CO2 The O2 transported in the blood (PO2 = 100 mmHg) is

needed in the tissues to continually make ATP (PO2 = 40

mmHg at the capillaries).

CO2 constantly diffuses

from the tissues

(PCO2 = 45 mmHg) to

be transported in

the blood

(PCO2 = 40 mmHg) Internal Respiration occurs at

systemic capillaries

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Transport of O2 and CO2 The amount of Hb saturated with O2 is called

the SaO2.

Each Hb molecule can carry 1, 2, 3, or 4

molecules of O2. Blood leaving the lungs

has Hb that is fully saturated (carrying 4

molecules of

O2 – oxyhemoglobin).

The SaO2 is close to 95-98% .

When it returns, it still has 3 of

the 4 O2 binding sites occupied.

SaO2 = 75%

Transport of O2 and CO2 The relationship between the amount of O2

dissolved in the plasma and the saturation

of Hb is called the oxygen-hemoglobin

saturation curve.

The higher the PO2

dissolved in the plasma,

the higher the SaO2.

Transport of O2 and CO2

Measuring SaO2 has

become as

commonplace in

clinical practice as

taking a blood

pressure.

Pulse oximeters

which used to cost

$5,000 can now be

purchased at your local

pharmacy.

3660 Group,

Inc/NewsCom

Transport of O2 and CO2

Although PO2 is the most important

determinant of SaO2, several other factors

influence the affinity with which Hb binds O2

Acidity (pH), PCO2 and blood temperature shift

the entire O2 –Hb saturation

curve either to the left

(higher affinity for O2), or

to the right (lower affinity

for O2).

Transport of O2 and CO2

Transport of O2 and CO2 As blood flows from the lungs toward the tissues,

the increasing acidity (pH decreases) shifts the

O2–Hb saturation curve “to the right”, enhancing

unloading of O2 (which is just what we want to

happen).

This is called the Bohr effect.

At the lungs, oxygenated blood has a reduced

capacity to carry CO2 ,and it is unloaded as we

exhale (also just what we want to happen).

This is called the Haldane effect.

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Fetal and Maternal Hemoglobin Fetal hemoglobin (Hb-F) has a higher

affinity for oxygen (it is shifted to the left)

than adult hemoglobin A, so it binds O2

more strongly. The fetus is thus able

to attract oxygen

across the placenta

and support life,

without lungs.

The medulla rhythmicity area, located in the brainstem, has centers

that control basic respiratory patterns for both inspiration and

expiration.

The inspiratory center

stimulates the diaphragm

via the phrenic nerve, and

the external intercostal

muscle via intercostal nerves.

Inspiration normally lasts about 2 sec.

Control of Respiration

Control of Respiration Exhalation is mostly a passive process,

caused by the elastic recoil of the lungs.

Usually, the expiratory center is inactive

during quiet breathing (nerve impulses

cease for about 3 sec).

During forced exhalation,

however, impulses from this

center stimulate the internal

intercostal and abdominal

muscles to contract.

Control of Respiration Other sites in the pons help the medullary centers manage

the transition between inhalation and exhalation.

The pneumotaxic center limits inspiration to prevent

hyperexpansion.

The apneustic

center coordinates

the transition between

inhalation and exhalation.

Control of Respiration Other brain areas also play a role in respiration:

Our cortex has voluntary control of breathing.

Stretch receptors sensing over-inflation arrests

breathing temporarily (Herring Breuer reflex).

Emotions (limbic system) affect respiration.

The hypothalamus, sensing a fever, increases

breathing, as does moderate pain (severe pain

causes apnea.)

Initial Response

Mucous layer thickens.

Goblet cells over-secrete

mucous.

Basal cells proliferate.

Advanced Response to Irritation

Mucous layer and goblet cells disappear.

Basal cells become malignant & invade deeper tissue.

Normal columnar epithelium

in the respiratory tract

Response to Pollutants

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Diseases and Disorders Asthma is a disease of hyper-reactive airways

(the major abnormality is constriction of smooth

muscle in the bronchioles, and inflammation.) It

presents as attacks of wheezing, coughing, and

excess mucus production.

It typically occurs in response to allergens;

less often to emotion.

Bronchodilators and anti-

inflammatory corticosteroids

are mainstays of treatment.

Pulse Picture Library/CMP mages /Phototake

Diseases and Disorders Chronic bronchitis and emphysema are

caused by chronic irritation and

inflammation leading to lung destruction.

Patients may cough up

green-yellow sputum due to

infection and increased mucous

secretion (productive cough).

They are almost exclusively

diseases of cigarette smoking.

Diseases and Disorders

Pneumonia is an acute infection of the

lowest parts of the respiratory tract.

The small bronchioles and alveoli become filled

with an inflammatory fluid exudate.

It is typically caused by infectious agents such as

bacteria, viruses, or fungi.

Diseases and Disorders Normal Lungs Pneumonia

Patient

Du Cane Medical Imaging, Ltd./Photo Researchers, Inc