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GASEOUS EXCHANGE
17 JULY 2013
Lesson Description
In this lesson we:
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Discuss what is gaseous exchange?
Consider requirements of an efficient gaseous exchange surface.
Look at diversity in gas exchange systems.
Discuss structure and adaptations of the human gaseous exchange system.
Look at mechanism of breathing.
Discuss gaseous exchange at lung surface
Look at gaseous exchange at tissue level
Look at transport of gases in the blood
Take a look at Homeostasis.
Consider relevant terminology
Key Concepts
What is Gaseous Exchange?
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The main function of the respiratory system is to take in oxygen from the atmosphere and
make it available to the cells for cellular respiration.
Carbon dioxide is produced during respiration and must be removed.
Terms that are important that you can differentiate between:
o Breathing – the mechanical process whereby air moves into and out of the lungs.
(inhalation and exhalation)
o Gaseous exchange – the exchange of oxygen and carbon dioxide across a gaseous
exchange surface.
o Cellular respiration – the release of energy from glucose in the presence of oxygen
Requirements of an Effective Gaseous Exchange Surface
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To enable oxygen and carbon dioxide to diffuse through it easily, it must be:
o Large – to ensure the maximum exchange of gases can take place.
o Thin and permeable – so that diffusion can take place easily and rapidly.
o Moist – so that gases can dissolve to form a solution.
o Well ventilated – good oxygen supply and carbon dioxide removal. (maintain a
diffusion gradient)
o Efficient transport system – for effective transport of gases.
o Well protected - to prevent desiccation (drying out) and mechanical injury.
Diversity in Gas Exchange Systems
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The way in which gaseous exchange is brought about depends on the body form of the
organisms and on the environment in which it lives
Organism
Dicot leaf
Earthworm
Terrestrial
Terrestrial
Gaseous Exchange Structure
 Gaseous exchange takes
place in the leaves.
 The main gaseous
exchange surface is the
spongy mesophyll.
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These have a large outer
surface and moist cell walls.
 Gases enter and leave
through the stomata – their
opening and closing
regulates gaseous
exchange
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Insect e.g. Locust
Terrestrial
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Gaseous exchange takes
place though the skin from
the air.
The skin is kept mist by
mucous secretions from the
skin
Oxygen dissolves in the
moisture of the skin to the
surface blood vessels to the
rest of the body
Carbon dioxide is
transported to the skin and
then diffuses out of the body
The locust has a tracheole
system.
Air enters the body through
small openings known as
spiracles.
They open into tracheas
that form extensive network
of tubes that become even
smaller to form tracheoles
Oxygen diffuses out of the
tracheoles, which have very
thin walls into the cells. air
sacs are also present as a
ventilation system
Air sacs are neat to muscles
and ensure air moves
quickly to and from the
tissues.
Exoskeleton prevents water
loss
Fish
Aquatic
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Mammals
Terrestrial
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Fish have gills for gaseous
exchange
Gills consist of a large
number of filaments and are
richly supplied with blood
capillaries
Water flows into the mouth
and over the gills
Dissolved oxygen in the
water diffuses into the blood
of the capillary vessels
Carbon dioxide diffuses into
the water.
Mammals have lungs and a
blood supply
Structure and Adaptation of the Human Gaseous Exchange Surface
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The ventilation is made up of :
o Air passages through which the air enters and leaves the lungs
o Two lungs that provide the gaseous exchange surface area
o Breathing muscles that move the chest up and down, causing air to enter and leave
the lungs
o Breathing centre in the brain which controls the rate and depth of breathing
Trachea
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Lined with ciliated epithelium
C-shaped cartilaginous rings
hold the trachea open
Dust particles trapped by
mucous and transported to the
exterior by mucous
Bronchi and Bronchiole
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The trachea divides into the right and
left bronchus which enter the lung.
Inside the lung they divide into smaller
branches called bronchiole
Lined with mucous membrane and kept
open by O-shaped cartilaginous rings
Smaller the bronchiole the cartilage is
no longer present
External Structure of the Lungs
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Two lungs. The right lung has three lobes and the
left two
Protected by the intercostal muscles between the
ribs.
Spongy and elastic
Surrounded by a double pleural membrane with
pleural fluid in between to stop friction
The lungs rest on a dome shaped muscular plate,
the diaphragm
Internal Structure of the Lungs
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Bronchiole end in small air sacs called alveoli
Gaseous exchange takes place in the alveoli
The wall of the alveoli is thin and consists of a
single layer of squamous epithelium.
The alveoli are surrounded by a system of
capillary blood vessels
Mechanism of Breathing
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INHALATION
Active process
Diaphragms contracts and flattens
Thoracic cavity enlarges
External intercostals muscles contract
The ribs move up and outward and
enlarges the thoracic cavity
Abdominal muscle relax to
accommodate the intestines that are
pushed down by the diaphragm
Total volume of the thoracic cavity
increases
Decrease in air pressure
The elastic lungs expand and air flows
into the lungs
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EXHALATION
Passive process
Diaphragm relaxes and returns to dome
shape
External intercostals relax
The ribs move down and inwards
Decrease in the volume of the thoracic
cavity
Increase in air pressure
Air flows out of the lungs
Composition of Inhaled Air vs. Exhaled Air
Lung Capacity
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Lung capacity refers to the amount of air that enters or leaves the lungs. And is measured by
a spirometer
o Total volume of a humans lungs is about 5l
o The amount of air that is normally inhaled and exhaled is about 0,5l – this is known as
the tidal volume or tidal air
o When air is expelled with force after a deep inhalation is about 3,5l of air is exhaled –
this known as the vital capacity
Gaseous Exchange at Lung Surface
Gaseous Exchange at Tissue Level
Transport of Gases in the Blood
Oxygen
 Small portion dissolves in the blood
plasma
 Most oxygen combines with haemoglobin
to form oxyhaemoglobin
Carbon dioxide
 Small portion dissolves in the blood
plasma
 Most CO2 combines with haemoglobin to
form carbaminohaemoglobin
 Most CO2 is transported as bicarbonate
ions
Homeostasis of Breathing
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The control of respiration is tied to the principle of homeostasis.
Recall that the body maintains homeostasis through homeostatic control mechanisms, which
have three basic components:
1. receptors
2. control centers
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3. effectors
The principal factors which control respiration are chemical factors in the blood.
Changes in arterial PCO2, PO2 and pH are monitored by sensory receptors called
chemoreceptors.
The chemoreceptors send sensory input to respiratory centers in the
brainstem, which determine the appropriate response to the changing variables.
These centers then send nerve impulses to the effectors, the respiratory muscles, to control
the force and frequency of contraction.
This changes the ventilation, the rate and depth of breathing.
Ventilation changes restore the arterial blood gases and pH to their normal range.
Important Terms
Breathing
Gaseous exchange
Cellular respiration
Ventilation
Diffusion
Diffusion gradient
Trachea
Bronchi
Bronchiole
c-shaped cartilaginous rings
Ciliated epithelium
Inhalation
Exhalation
Oxyhaemoglobin
Carbominohaemoglobin
Bicarbonate ions
Homeostasis
Chemoreceptor
Tidal volume
Vital capacity
Total volume
Questions
Question 1
The diagram alongside shows a section through an alveolus (air sac) of a human lung and its
surrounding capillary.
a.)
b.)
c.)
d.)
Name the tissue forming the wall of the alveolus (labeled 1)
Name the gases shown at A and B in the diagram.
For which process do the cells need a constant supply of gas A?
State and explain three ways in which the alveolus is structurally suited to the
process of gaseous exchange.
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Question 2
A spirometer can be connected to an instrument that draws a graph. In an experiment, someone was
asked to breathe normally for 1 minute, then to breathe very slowly for 30 seconds, and again to
breathe normally. The graph below represents the results obtained
a.)
b.)
c.)
d.)
e.)
f.)
What volume of air was breathed in during normal breathing?
How many times did the person inhale during the first period of normal breathing?
What was the total amount of air taken into the lungs during 1 min of normal breathing?
How much more air was inhaled during vital capacity than in normal breathing?
Name the muscles that contract during inhalation.
After the subject breathes out fully there is still air left in the lungs:
i.
How much air is still left in the lungs?
ii.
Why is there still air left in the lungs?
iii.
What term is used to name this air?
Links
http://www.biologycorner.com/worksheets/lungcapacity.html
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