Transcript Chapter 20

How does air get
into our lungs ?
Why must we
use our nose to
breathe, instead of
our mouth ?
Chapter 20
Gaseous Exchange
 20.1 Respiratory Surfaces
 All aerobic organisms must obtain regular
supplies of oxygen from their environment
and return to it the waste gas carbon dioxide.
 The movement of these gases between the
organism and its environment is called
gaseous exchange.
 Gaseous exchange always occurs by
diffusion over part or all of the body surface
- the respiratory surface.
Large organisms: use
special respiratory
structures, eg.
Gills in fish
Skin in frog
Lungs in mammals
In order to maintain the maximum possible rate of
diffusion respiratory surfaces have a number of
characteristics:
 1. Large surface area to volume ratio
This may be the body surface in small organisms or
infoldings of the surface such as lungs, gills
 2. Permeable
 3. Thin - Diffusion is only efficient over very short
distances, e.g. 1 mm
 Rate of diffusion is inversely proportional to the
square of the distance between the concentrations on
the two sides of the respiratory surface.
 4. Moist - since oxygen and carbon dioxide
diffuse in solution form
 5.Efficient transport system - This is
necessary to maintain a diffusion gradient
and may involves a vascular system.
 Diffusion is proportional to:
 surface area x difference in concentration
thickness of membrane
 Organisms can obtain their gases from the
air or from water.
TABLE 20.1: Water and Air as Respiratory Media
Property
Oxygen content
O2 Diffusion Rate
Density
Viscosity
Water
Air
TABLE 20.1: Water and Air as Respiratory Media
Property
Water
Oxygen content
Less than 1%
O2 Diffusion Rate
Density
Viscosity
Air
TABLE 20.1: Water and Air as Respiratory Media
Property
Water
Air
Oxygen content
Less than 1%
21%
O2 Diffusion Rate
Density
Viscosity
TABLE 20.1: Water and Air as Respiratory Media
Property
Water
Air
Oxygen content
Less than 1%
21%
O2 Diffusion Rate Low
Density
Viscosity
TABLE 20.1: Water and Air as Respiratory Media
Property
Water
Air
Oxygen content
Less than 1%
21%
O2 Diffusion Rate Low
Density
Viscosity
High
TABLE 20.1: Water and Air as Respiratory Media
Property
Water
Air
Oxygen content
Less than 1%
21%
O2 Diffusion Rate Low
High
Density
>1000 than that
of air
Viscosity
Density of water
TABLE 20.1: Water and Air as Respiratory Media
Property
Water
Air
Oxygen content
Less than 1%
21%
O2 Diffusion Rate Low
High
Density
Density of water
>1000 than that
of air
Viscosity
Water much greater, about 1000 times
than that of air
20.2 Mechanisms of Gaseous Exchange
 As animals increase in size most of their
cells are some distance from the surface and
cannot receive adequate oxygen.
 Many larger animals also have an increased
metabolic rate which increases their oxygen
demand.
 These organisms need to develop specialized
respiratory surfaces such as gills & lungs.
 These surfaces allow gases to enter and leave
the body more rapidly.
20.2.1 Small organisms
 Small organisms have a large surface area
to volume ratio and do not require
specialized structures for gaseous exchange.
 In amoeba, gases diffuse over their whole
surface. Obelia have all their cells in contact
with water.
 Platylelminthes rely on diffusion over the
whole body surface.
 All these organisms must live in water from
which they obtain dissolved oxygen; they
would rapidly desiccate in a terrestrial
environment.
20.2.2 Flowering plants
20.2.2 Flowering plants
 Plants have a low metabolic rate, requiring
less energy per unit volume than animals.
 Unicellular algae employ the whole body
surface for gaseous exchange.
In flowering plants:
 Gases pass through stomata in leaves and
green stems
 Woody stems have lenticels
 Lenticels on woody tree trunk
 Within the plant oxygen diffuses through the
intercellular air spaces & moist cell walls into
the respiring cells; with
carbon dioxide moves in the opposite direction
 Rate of photosynthesis (producing oxygen;
absorbing carbon dioxide) is affected by light
intensity,
 thus varying the amount of these gases during
the day
20.2.3 Insects
 Gases enter and leave through pores called
spiracles.
 Each spiracle is surrounded by hairs which
help to retain water vapour and may be
closed by muscular valves.
 Respiring cells give out carbon dioxide
which accumulates to stimulate the
chemoreceptors to open the spiracles.
 Spiracles open into tubes called tracheae
which are supported by rings of chitin to
prevent collapse.
 Tracheae divide to form smaller tracheoles
extending right into the tissues.
 The tracheal system carries oxygen rapidly to
the cells and allows the insects to develop
high metabolic rates.
 The ends of the fine tracheoles are fluid-filled.
 As activities increase, fluid will be drawn into
the muscle cells to draw air further into the
tracheoles in order to increase oxygen supply.
 The system is ventilated by contractions of
the abdominal muscles of the insect
flattening the body,
 thus reduces the volume of the tracheal
system.
 The volume increases again by elasticity of
the body and system to return to original
shapes.
 Larger insects, e.g. locusts, have some of
the tracheae expanded to form air-sacs for
blowing air in and out.
Limitations of the tracheal system:
1 Insects cannot attain a large size because it
relies entirely on diffusion for the gases to
move from the environment to the respiring
cells.
2 The chitinous linings of the tracheae must be
moulted before the rest of the exoskeleton.
20.2.4 Bony fish - not required in syllabus
 20.2.5 Mammals
Structures of the respiratory
nasal cavity
system:
nose
nostril
pharynx
epiglottis
trachea
bronchiole
Intercostal muscle
rib
alveolus
Inner pleural membrane
outer pleural membrane
pleural fluid
vocal cords
larynx
cartilage rings
left lung
bronchus
heart
diaphragm
Structures of the respiratory
nasal cavity
system:
nose
nostril
pharynx
epiglottis
trachea
bronchiole
Intercostal muscle
rib
alveolus
Inner pleural membrane
outer pleural membrane
pleural fluid
vocal cords
larynx
cartilage rings
left lung
bronchus
heart
diaphragm
20.2.5 Mammals
 Lungs are the site of gaseous exchange in
mammals.
 Rib cage encloses and protects the lungs. There
are 12 pairs of ribs.
 The ribs are moved by a series of intercostal
muscles.
 Diaphragm separates the thorax and the
abdomen.
Regions of the respiratory system
 Air passes into the lungs through a series of tubes
in the following order:
 Nose nostril, hairs (filter dust):
 nasal cavity  pharynx
 larynx
 trachea
 bronchi (bronchus)
 bronchioles
 air sacs
 alveoli (alveolus)
Move up the throat
Unwanted particles
Mucus-secreting cell
Mucus
Cilia
Ciliated epithelium
Cells
inside the
nasal
cavity
 Nasal Cavity
 Wall is lined with a ciliated epithelium and
mucus-secreting cells.
 Bacteria & dust, trapped by mucus, are sent
towards the throat by the beating cilia.
 The mucus is then swallowed or coughed up.
 Numerous blood vessels warm and moisten
the incoming air.
 Olfactory cells give the sense of smell of
the incoming air.
Pharynx
 belongs to both the respiratory & digestive systems
 epiglottis (a cartilage) covers the glottis (opening to
larynx) to prevent food from entering the trachea
Larynx (voice box)
 produces voice when air is forced through its vocal
cords
Larynx
(voice box)
- with
cartilage &
vocal cords
Larynx
Vocal
cords
 Trachea and Bronchi
 further divide into bronchioles which finally
end into alveoli
 dirt particles & germs are trapped by mucus
and sent upwards by its cilia
 wall of trachea is strengthened by C-shaped
cartilages which keep it open
How large are the respiratory
surfaces provided by the lungs ?
About half the size
of a tennis court.
Alveoli - a respiratory surface with a total
area of about 100 m2
Features for efficient gas exchange
1. Very thin so that gases can diffuse through
very quickly
2. A large surface area to diffuse more gases
per unit time
3. Moist so that gases can pass through in
solution forms
4. An excellent transport system of blood
capillaries to transport gases
Lung
protected by the thoracic basket which
consists of the vertebrae, ribs, and sternum
Vertebral column
Sternum
Intercostal
muscles
Rib
 Lungs covered by two pleural membranes
which secrete pleural fluid to reduce
friction during breathing movements
Outer & inner
pleural
membranes
with pleural
fluid
reduce
friction during
breathing.
GASEOUS EXCHANGE IN THE ALVEOLI
 Pulmonary artery delivers deoxygenated blood
to the lungs.
 Oxygen from the incoming air diffuses across
the walls of the alveoli and the capillaries and
passes into the blood because of a higher
concentration:
 O2 + haemoglobin  oxyhaemoglobin
 Oxygenated blood then goes to the heart
through the pulmonary vein
Gaseous Exchange in the alveoli
Red blood cell
Owing to concentration
differences:
Capillary from
pulmonary artery
Oxygen diffuses
into RBCs
Epithelium of
alveolus (1-cell thick)
Film of
moisture
Carbon dioxide
diffuses into
alveolus
Capillary to
pulmonary vein
 Carbon dioxide in the form of hydrogen
carbonate ions in plasma diffuses to the
alveoli because of its higher concentration
in blood
MECHANISM OF BREATHING
(a) Inspiration
MECHANISM OF BREATHING
(a) Inspiration
 1) Thoracic basket is raised
 2) Diaphragm flattens
 3) Volume of thoracic cavity increases
 4) Air is drawn into the lungs
(b) Expiration
 5) Thoracic basket drops down
 6) Diaphragm moves up
 7) Volume of thoracic cavity decreases
 8) Air is forced out of the lungs
1 Movement of the ribs –
external & internal intercostal
Ribs raised upwards &
muscles
Ribs fall
outwards
Volume of thoracic
cavity & lungs
increases
sternum
Rubber
band
shortened
(intercostal
muscles
contract)
downwards &
inwards
V
Lung air pressure
lower than
atmosphere
vertebral
column
Inspiration
Air flows
from
atmosphere
to the lungs
Rubber
band
lengthened
(intercostal
muscles
relax)
P
Air flows out of
the lungs into
the atmosphere
Expiration
2 Movement of the
diaphragm Air drawn in
Pleural membranes
Pleural fluid
rub
Lungs
expanded
Diaphragm
muscles
contract
Diaphragm
lowered
Inspiration
Air pressure
becomes
lower than
that of the
atmosphere
2 Movement of the
diaphragm Air expelled
Vertebral column
Lung
returns to
original
volume
Diaphragm returns
to dome shape
Expiration
Air pressure
becomes higher
than that of the
atmosphere
Diaphragm
muscles relax
20.3 Control of Ventilation in Man
20.3 Control of Ventilation in Man
- Rate and depth of breathing is controlled by the
respiratory centre in the medulla oblongata of
the hind-brain by changes in blood CO2
concentration:
Blood CO2 in blood
 detected by chemoreceptors
 nerve impulses
 respiratory centre in medulla
20.3 Control of Ventilation in Man
20.3 Control of Ventilation in Man
- Rate and depth of breathing is controlled by the
respiratory centre in the medulla oblongata of
the hind-brain by changes in blood CO2
concentration:
Blood CO2 in blood
 detected by chemoreceptors
 nerve impulses
 respiratory centre in medulla
 phrenic & thoracic nerves
 diaphragm & intercostal muscle contractions
 Inspiration
20.3 Control of Ventilation in Man
 stretch receptors in lungs stimulated
 vagus
 expiratory centre in medulla to switch off
the inspiratory centre
 expiration takes place
20.3 Control of Ventilation in Man
 stretch receptors in lungs stimulated
 vagus
 expiratory centre in medulla to switch off
the inspiratory centre
 expiration takes place
 stretch receptors not stimulated
 expiratory centre switched off
 inspiratory centre switched on
 inspiration again
20.3 Control of Ventilation in Man
 The ventral portion of the breathing centre
is the inspiratory centre;
 the remainder is the expiratory centre
 Chemoreceptors in the carotid and aortic
bodies of the blood system
 The breathing centre may also be stimulated
by impulses from the forebrain resulting in
a conscious increase or decrease in
breathing rate.
 The main stimulus for ventilation is carbon
dioxide;
 Changes in oxygen concentration have
relatively little effect.
 At high altitudes the reduced atmospheric
pressure makes it more difficult to load the
haemoglobin with oxygen.
 In an attempt to obtain sufficient oxygen a
mountaineer takes very deep breaths.
 This forces more carbon dioxide out of the body
and the level of carbon dioxide in the blood
therefore falls.
 The inspiratory centre is no longer stimulated
and breathing becomes increasingly laboured,
causing great fatigue.
 Given time, man can adapt to these
conditions by excreting more alkaline urine.
 This causes the pH of the blood to fall,
 i.e. more acidic
 the chemoreceptors are stimulated and so is
the inspiratory centre.
Lung volumes
Vital capacity
Volume of air
in lung
Tidal volume
Residue
volume
Time
20.4 Measurement of Lung Capacity
20.4 Measurement of Lung Capacity
 Tidal volume is the volume of air breathed
in or out during each respiratory cycle
 Vital capacity is the total amount of air that
can be forcibly inspired or expired
 Residue volume is the amount of air that
remains in the lungs even after maximum
expiration
 Ventilation rate is the process of exchanging
gases in the lungs/gills with gases from the
environment per unit time.