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The human gas exchange systemconsists of the nasal passages, thepharynx or throat, the larynx orvoice box, the trachea, the rightand left bronchus and the lungs

Larynx

Trachea(with rings of cartilage)

Left lung

Ribs

Diaphragm(a powerful sheet of muscle

separating the thorax from the abdomen)

Intercostalmuscles

Section throughribs

Rightbronchus

Bronchioles

The Human Gas Exchange SystemThe Human Gas Exchange System

The lungs are containedwithin the thoracic cavity,

the sides of which arebounded by 12 pairsof ribs that articulate

(join) with the vertebrae towards

the back of the body,and the sternum or

breast bone, towardsthe front

The portions of the ribsthat articulate with the

breastbone are composedof cartilage rather than bone

Cartilage

Cartilage is softerand more pliable thenbone and thus assists

the movements ofthe rib cage during

breathing

Two sets ofantagonistic musclesare located between

the ribs – theseare the intercostal

muscles

The trachea orwindpipe is about10 cm long and is

supported byC-shaped ringsof cartilage to

prevent the tubefrom collapsing

during breathing

Trachea

The tracheasubdivides to giverise to the right

and left bronchus –these tubes are

also strengthenedby cartilage

The two bronchisubdivide to form

an extensivenetwork of

bronchioles thatdeliver air to

the gas exchangesurfaces – the

alveoli

Right and Leftbronchus

Bronchioles

Air enters the bodythrough the nasal

passages and mouth,and passes via the

pharynx and larynxto the trachea

Air is delivered tothe alveoli as the trachea branchesinto bronchi and

bronchioles

The TracheaThe Trachea

Lumen oftrachea

This highly magnified view of the lining of the trachea shows the cilia andmucus-secreting goblet cells that make up the epithelium

Goblet cell that secretesmucus to trap dust andother foreign material

that may enter the respiratory system

The wafting of these ciliaremoves the mucus and

trapped foreign materialfrom the respiratory

system

The bronchioles dividemany times forming

respiratory bronchioles,which in turn divide toto form alveolar ducts

that terminate in groupsof sacs – the alveoli

Respiratorybronchioles

Alveolarduct

Alveoli

A single alveolus

Each alveolus is ahollow, thin-walled

sac that is surroundedby a dense network ofcapillaries and is thesite of gas exchange

in the lungs

The Gas Exchange SurfaceThe Gas Exchange Surface

Gases are exchanged across the alveoli by diffusion

According to Fick’s Law...

Rate of diffusion =surface area x difference in concentration

thickness of exchange surface

Maximum rate of diffusion of respiratory gases is achieved by:

• the large surface area presented by the alveoli (there are about 350 millionalveoli in the two lungs presenting an enormous surface area of

approximately 90 square metres – about the area of a tennis court)

• the large differences in concentration of metabolites between the alveoli and the blood capillaries

• the thinness of the diffusion barrier (alveolar and capillary walls providea total thickness of only 0.005 mm)

As deoxygenated blood from the body tissues flows through the network ofcapillaries surrounding each alveolus, oxygen diffuses into the blood and carbondioxide diffuses from the blood into the alveolus; oxygenated blood travels from

the lungs to the left of the heart for delivery to the body tissues

The Gas Exchange SurfaceThe Gas Exchange Surface

Breathing in (inspiration) and breathing out (expiration) are

mechanical processes involving the ribs, intercostal muscles

and the diaphragm

Two sets of antagonistic muscles are located between

the ribs; these are the externaland internal intercostal muscles

External intercostalmuscles

Internal intercostalmuscles

The intercostal muscles are antagonistic in the

sense that contraction of the external muscles raises

the rib cage, whereas contraction of the internal muscles

lowers the rib cage

The diaphragm is a powerful sheet of

muscle thatseparates the

thorax from theabdomen; it is

dome-shaped when relaxed and flattens

on contractionDiaphragm

The Mechanics of BreathingThe Mechanics of Breathing

ExpirationInspirationExternal intercostal muscles contract and

raise the ribs upwards and outwardsExternal intercostal muscles relax andthe ribs move downwards and inwards

The diaphragmmuscle contracts

and flattens

The diaphragmmuscle relaxesand becomes dome-shaped

The volume of the thorax increasesThe air pressure in the thoracic cavityfalls below that of the atmospheric air

Air rushes into the lungs along a pressure gradient

The volume of the thorax decreasesThe air pressure in the thoracic cavityrises above that of the atmospheric air

Air rushes out of the lungs along a pressure gradient

SummarySummary

The lungs are sealed in anairtight, fluid-filled, double-

membrane sac called the pleura

Two pleural membranessurround the lungs and the cavity between themis filled with pleural fluid to protect the lungs from

the bony ribs

The pressure withinthe pleural cavity is

known as the intrapleural pressure and this pressure is always below that of

the atmosphere(sub-atmospheric)

The pressure within the airways of the

respiratory system is known as the

intrapulmonary pressure

Intrapulmonarypressure within

the airways

The magnitude of these pressures varies during the breathing cycle

Pressure Changes During the Breathing CyclePressure Changes During the Breathing Cycle

0 1 2 3

tim e (s)

+2

+1

inspiration expiration

4

0

-1

-2

-3

-4

-5

-6

0.6

0.4

0.2

0

volum e

(dm )3

pressure(m m Hg)

intrapulmonarypressure

intrapleuralpressure

volum e ofbreath

The pressure and volume changes occurringduring the breathing cycle can be represented as agraph where zero, on the pressure axis, represents

atmospheric pressure

As inspiration begins and the ribs move upwards andoutwards, the pressure within the airways (the intra-pulmonary pressure) falls below that of atmospheric

air (shown as 0 mm Hg)Air rushes into the lungs to equalise the pressures and

intrapulmonary pressure increases to that of the atmosphere

The intrapleural pressure falls even more below that ofthe atmosphere as the pleural cavity expands on inspiration

As expiration begins and the ribs move downwards andinwards, the pressure within the airways (the intra-

pulmonary pressure) rises above that of atmosphericair (shown as 0 mm Hg)

Air rushes out of the lungs to equalise the pressures andintrapulmonary pressure falls to that of

the atmosphere

The intrapleural pressure rises as the pleural cavity decreases in size on expiration

The volume of air inspired and expired during onebreathing cycle is shown in the lower part of the

graph

Atmosphericpressure

The pressure withinthe pleural cavity

(intrapleural pressure) is always below that of

the atmosphere(sub-atmospheric)

If a lung is pierced, thenair rushes into the pleural

cavity along a pressure gradient

Atmospheric air pressure isGREATER than the fluid

pressure within thepleural cavity

As air rushes into the pleural cavity, the pressure difference across the lungwall is eliminated, and the stretched lung collapses

The Collapsed LungThe Collapsed Lung