Gas Exchange in Mammals
Aim – to understand the structure and function of the lungs.
Objectives- by the end of this lesson you should be able to
• Name and identify parts of the gas exchange system.
• Explain how the alveoli are adapted for efficient gas exchange.
• Describe the structure and function of ciliated epithelium, goblet cells, cartilage, smooth muscle and elastic fibres
• Explain how the lungs are ventilated.
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 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
This photomicrograph of a transverse section through the trachea showsthe C-shaped ring of cartilage
Magnify
C-shapedcartilage ring
This magnified view of the wall of the trachea shows the cartilage cells together with the cells that line the lumen of the trachea – ciliated epithelium
Cartilagecells
Ciliatedepithelium
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
Move the cursor over the area of lung (yellow circle) to show the alveoli...
The Gas Exchange SurfaceThe Gas Exchange Surface
Section of lung A Single alveolusThorax
The Gas Exchange SurfaceThe Gas Exchange Surface
Each alveolus is a hollow, thin-walled sac that is surrounded by a
dense network of capillaries and is the site of gas exchange
in the lungs
The bronchioles divide many times forming
respiratory bronchioles, which in turn divide to
form alveolar ducts that terminate in groupsof sacs – the alveoli
Alveolarduct
Respiratorybronchiole
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
Alveolar TissueAlveolar TissueA photomicrograph of a section of alveolar tissueshowing the delicate nature of the lungs and the
'one cell thick' walls of the alveoli which facilitate diffusion of respiratory gases.
A photomicrograph of a section of alveolar tissueshowing the delicate nature of the lungs and the
'one cell thick' walls of the alveoli which facilitate diffusion of respiratory gases.
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 contraction
Diaphragm
The Mechanics of BreathingThe Mechanics of Breathing
During inspiration, the external intercostal muscles
contract and raise the rib cage upwards and
outwards; the diaphragm also contracts and flattens
The volume of the thorax increases, lowering the air pressure in the chest cavity
to less than that of the atmosphere outside
A pressure gradient is created between the atmosphere and the lungs, and air rushes in via the trachea to equalise the pressure
difference
Air moves from a higher to a lower pressure region andinflates the lungs as inspiration takes place
Inspiration - Breathing InInspiration - Breathing In
During an expiration, the external intercostal muscles relax and lower the rib cage; the diaphragm relaxes and becomes dome-shaped
The volume of the thorax decreases, raising the air pressure in the chest cavity to above that of the atmosphere outside
A pressure gradient is created between the lungs and the atmosphere, and air rushes out via the trachea to equalise the pressure difference
Air moves from a higher to a lower pressure region and deflates the lungs as expiration takes place
The mechanism described is that for breathing at rest
At rest, inspiration is an active process involving contraction of the muscles of breathing
Expiration is a purely passive process involving relaxation of the muscles of breathing together with elastic recoil of the lungs
During forced breathing, as in exercise, expiration becomes an active process
Expiration - Breathing OutExpiration - Breathing Out
Expiration is assisted by the elastic recoil of the lungs following the stretching of elastic fibres during the process of inspiration
ExpirationInspiration
External intercostal muscles contract andraise the ribs upwards and outwards
External intercostal muscles relax andthe ribs move downwards and inwards
The diaphragmmuscle contracts
and flattens
The diaphragm muscle relaxes and
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