Gas Exchange
Exchange surfaces
All organisms require nutrients and the ability to excrete
waste. Many simple organisms, such as bacteria and sea
anemones, can exchange substances directly across their
external surfaces.
Larger organisms require specialized
gas exchange and transport systems
to transport substances such as oxygen
and nutrients to their cells efficiently.
Fish exchange these substances across
gills, while insects have openings called
spiracles on their surfaces.
In mammals, gas exchange occurs in
the lungs, and in particular the alveoli.
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Crop
photo
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Adaptations for gas exchange

Issue: Requirements are proportional to
volume of organism however diffusion is
proportional to surface area
v.
Diffusion:
◦ In large organisms the surface area to volume
ratio is much less than in very small organisms
Larger organisms
Have a small surface area to volume ratio, so need
adaptations to increase gas exchange:
 Ex:

◦ gills for aquatic environments
◦ lungs for terrestrial environments

Have:
◦
◦
◦
◦
◦
large surface area
thin (short diffusion distance)
permeable surface
moist
good blood supply (carries gas away quickly, maintaining
the concentration difference)
◦ good ventilation (pumping mechanism-like lungs)
Small organisms

the surface area to volume ratio is so large that
diffusion through the body surface is sufficient to
supply their needs. (as is the distance it needs to
travel in the body)
Example

Amoeba:
◦ lives in fresh water allowing for easy diffusion of
nutrients
◦ Small body (single celled) so diffusion can supply
requirements
Water Loss

large moist area for gaseous exchange is a
region of potential water loss for land
animals
Strategies for gas exchange

Earthworms (annelids)
earthworms
have an increased efficiency of gaseous
exchange (sufficient for a slow moving
animal)
 are multicellular, terrestrial animals
restricted to damp areas (for gas exchange)
 long tube shape for high surface area
 moist body surface (mucus) for diffusion of
gases

Earthworms
use their outer surfaces as gas exchange
surfaces
 have a series of thin-walled blood vessels
known as capillaries

have a closed circulatory system (blood
vessels- which is more efficient) and blood
pigments (haemoglobin) to bind oxygen
 gas exchange occurs at capillaries located
throughout the body tissues as well as those
in the respiratory surface

Insects
Have a hard exoskeleton which is not
suitable for gas exchange
 have evolved a different system of gaseous
exchange to other land animals (need lots of
energy for rapid flight)

Insects
Do not use a transport system to carry
oxygen
 Use a branched, chitin-lined system of
tracheae with openings called spiracles (that
can open and close)

insects

Main features:
◦ Large surface area using a network of tubes
◦ Small bodies so that diffusion from tubes to
tissues is sufficient
◦ Thin, fluid filled tracheoles to allow gases to
dissolve and diffuse to/from tissues efficiently
◦ Some species have rhythmical muscle contraction
to assist the diffusion of gases (ventilation)
insects

Can control the rate of gas exchange using lactic
acid



High respiration rate means more lactic acid made and
stored in tissues
This causes fluid to move in by osmosis from the fluid
filled tracheoles
Gases can now diffuse faster from tracheoles with less
fluid in them.
Bony fish
larger and active animals so high demand for
oxygen
 use gills with a large surface extended by gill
filaments with lamellae (ie highly folded)

gills
Each gill is composed of many filaments
that are each covered in many lamellae to
increase surface area.
 The lamellae contain blood capillaries, which
have blood flowing in the opposite direction
to the water.


The lamellae are thin, ensure that the
diffusion distance between the blood, in the
lamellae, and the water is short
mouth cavity (buccal cavity) and the chamber
at the side (operculum cavity) help to
increase ventilation over gills (like a pump)
 Steps:

1.
2.
3.
4.
Mouth opens
Operculum (gill cover) closes
Floor of buccal cavity lowers
These all increases the volume inside, thus
lowering pressure
5. Water is drawn in
Gill ventilation
http://www.youtube.com/watch?v=kf7vBjhjwec
http://www.youtube.com/watch?v=YLsmEhnYdM0&feature=related
Counter current flow

The lamellae contain blood capillaries, which
have blood flowing in the opposite direction
to the water.
Figure 4
Countercurrent system
The blood flows through the lamellae in the opposite
direction to the water. This is a countercurrent system.
It ensures the maximum exchange possible occurs.
Counter current vs. Parallel Flow

Counter flow allows continuous diffusion of oxygen
into the blood as there is always a concentration
gradient across the gill lamella (plate) even when
the blood is very saturated with oxygen
Reptiles and birds



have more efficient lungs than amphibians
ribs assist ventilation
Birds have air sacs to keep lungs always inflated
(like a bellows) that takes the dead air from the
lungs during the next breathe to ensure fresh
air goes into the lungs each time
Gills can also be external

External gills generally have a higher surface
area but are less protected
Amphibians
larval form (tadpole) develops in water (uses
gills) and undergoes metamorphosis into
the adult form
 inactive frog uses its moist skin as a
respiratory surface but when active uses
lungs

Terrestrial vertebrates
have adapted for exchange with air, a less
dense medium (air) so have internal lungs
(gills don’t work in air)
 internal lungs minimise loss of water and
heat

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The human respiratory system:
ventilation and exchange of gases
 ventilation involves creating volume and
pressure changes that allow a continuous
exchange of gases inside the body, so
maintaining concentration gradients

Human
Structure of the lungs
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Gas exchange in the alveoli
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Maintaining the structure of the alveoli
During inhalation, the chest cavity increases in volume,
lowering the pressure in the lungs to draw in fresh air.
This decrease in pressure leads to a tendency for the lungs
to collapse. Cartilage keeps the trachea and bronchi open,
but the alveoli lack this structural support.
Lung surfactant is a
phospholipid that coats the
surfaces of the lungs.
Without it, the watery lining
of the alveoli would create a
surface tension, which would
cause them to collapse.
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alveoli
surfactant
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Keeping the airways clear
The walls of the trachea
and bronchus contain
goblet cells, which
secrete mucus made of
mucin. This traps microorganisms and debris,
helping to keep the
airways clear.
The walls also contain ciliated epithelial cells, which are
covered on one surface with cilia. These beat regularly to
move micro-organisms and dust particles along with the
mucus. They contain many mitochondria to provide
energy for the beating cilia.
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Structures of the human lung
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Why do we breathe?
Animals need to maintain a concentration gradient across
their exchange surfaces so that oxygen will diffuse into the
blood and carbon dioxide will diffuse out.
Fish manage this by keeping a
continuous stream of oxygenated
water moving over their gills.
In animals such as mammals and
birds, a concentration gradient is
maintained in the alveoli by the
mechanism of ventilation.
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The mechanism of ventilation
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The pleural cavity
Each of the lungs is enclosed in a double membrane known
as the pleural membrane. The space between the two
membranes is called the pleural cavity, and is filled with a
small amount of pleural fluid.
lung
This fluid lubricates the
lungs. It also adheres to
the outer walls of the
lungs to the thoracic
(chest) cavity by water
cohesion, so that the
lungs expand with the
chest while breathing.
pleural membranes
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Composition of inhaled/exhaled air
composition (%)
In one breathing cycle, the air in the lungs loses only some
of its oxygen content. This is why mouth-to-mouth
resuscitation can be effective.
90 78% 78%
80
70
60
50
40
30
20
10
0
N2
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inhaled air
exhaled air
21%
15%
0.04% 4%
O2
CO2
<1% 3%
<1% <1%
H 2O
other
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What’s the keyword?
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Structures involved in gas exchange
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Multiple-choice quiz
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Comparing gas exchange surfaces
Plants



rely entirely on diffusion for the exchange of gases.
Leaves are thin to shorten distances for diffusion and have a
large surface area and are permeated by air spaces
Leaves have a cuticle to prevent water loss which also
reduces gaseous exchange.
The air spaces between mesophyll
cells allow carbon dioxide and oxygen
to diffuse to and from all the cells.
 The cells are moist so gases can
dissolve.
 The presence of pores, stomata,
allow water vapour and gases to pass
through
 Irregular arrangement of spongy
mesophyll cells creates a large surface
area for gaseous exchange
 Cell wall is thin – short diffusion path

Guard cells change shape because of changes
in turgor; in the light, water flows in by
osmosis so the cells expand.
 The inner wall is inelastic and thicker so the
pairs of cells curve away from each other as
water enters and the pore opens
 Pores close due to the reverse process.

malate theory
potassium ions move from the epidermal
cells into the guard cells by active transport
 This causes starch to change to malate
(water soluble)
 This creates a negative water potential in
the guard cells.
 Water moves in by osmosis.

Xerophytes

may open stomata at night instead of during
the day in order to conserve water
Heat/water loss
Loss in large organisms than in small
 This is because the organism has :

◦
◦
◦
◦
a low surface area : volume
longer diffusion pathways
longer distances in general
probably more insulation so it is harder for the
heat to escape.
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