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Chapter 7
Gas exchange in humans
General plan of the breathing system
Gas exchange in air sacs
Routes of transport of respiratory gases
Mechanism of ventilation
1.
Why do we need a breathing system?
We are multicellular organisms. The body surface area to volume ratio is very small. Diffusion across body surface cannot
supply enough oxygen to all our body cells. We need a larger surface for gas exchange. Therefore, we need a specialized gas
exchange surface. Second, we have a lot of cells far away from the body surface. If we use simple diffusion to get oxygen,
the rate of diffusion is too slow to supply enough oxygen to support high metabolism of our body.
2.
Give an order of our respiratory tract.
Nostril – nasal cavity – pharynx – larynx – trachea – bronchi – bronchioles – air sacs
3.
How do we clean, warm and moisten the incoming air?
In nostrils: hair to filter large dust
In nasal cavity: mucus on surface moisten the incoming air, so that the air is not too dry to evaporate the water film on air sac
In nasal cavity: blood capillaries lie close to surface to warm the air, so the air is not too cold to damage the air sac
In nasal cavity: mucus trap the germs and dust; cilia beat to move the mucus away from the air sac
In trachea, bronchi, cilia and mucus are also present.
4.
Should we breathe by our mouth?
No. We cannot clean the incoming air by our mouth.
5.
What is the use of C-shaped cartilage in trachea? What is the use of ring-shaped cartilage in bronchus?
C-shaped and ring-shaped cartilage can support the tube; prevent them from collapse during breathing. In trachea, it is
C-shape to allow expansion of oesophagus during eating.
6.
Will the trachea and bronchus change shape?
Yes. Although there is cartilage, the tubes can still change shape.
7.
What are present in our lungs?
Bronchioles, air sacs, blood capillaries and elastic tissues
8.
What is the function of air sac?
It is a site for gases exchange. There is a thin water film on the surface to dissolve the gases. It is surrounded by blood
capillaries to carry the oxygen away and bring carbon dioxide in.
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9.
Explain why our lungs are pink in colour and spongy in touch.
There are a lot of capillaries which contains blood. This makes the lungs look pink. There is air in the air sac and bronchioles.
The air makes the lungs spongy and floats in water.
10. What is the use of rib cage?
Protect heart and lungs. It is formed by ribs, sternum and backbones. These bones can form a thoracic cavity. When ribs
move, the pressure in the thoracic cavity changes for breathing in and out. Therefore, it is used for ventilation. Furthermore,
the bones serve many other uses such as making red blood cells, storage of calcium etc.
11. What is the use of intercostal muscle and diaphragm muscle?
Intercostal muscle contracts. Ribs move upwards and forwards. Volume of thoracic cavity increases. Pressure of thoracic
cavity decreases. Air moves into the lungs. Intercostal muscle relaxes. Ribs move downwards and backwards. Volume of
thoracic cavity decreases. Pressure of thoracic cavity increases. Air is forced out.
Diaphragm muscle in diaphragm contracts. It moves the diaphragm down. Volume of thoracic cavity increases. Pressure of
thoracic cavity decreases. Air moves into the lungs. Diaphragm muscle relaxes. Diaphragm returns to dome-shape. Volume of
thoracic cavity decreases. Pressure of thoracic cavity increases. Air is forced out.
12. What is the use of pleural membrane and pleural fluid?
Outer pleural membrane attaches the rib cage. Inner pleural membrane attaches to lung surface. In between is the
incompressible pleural fluid. The movement of rib cage can pull and compress the pleural membranes which in turn pull or
compress the lungs. The pleural fluid transmits the force onto the lung surface and to maintain pressure of the lungs.
13. Explain what happens when left pleural membrane is punctured.
Pleural membrane and pleural fluid maintain the pressure of the lungs. The left lung will collapse. The lung cannot draw in
air during inspiration. The lung cannot force air out during expiration.
14. How does oxygen in incoming air enter our blood?
It is breathed in. It mixes with the air in the air sacs. It diffuses to the air sac down a concentration gradient. It dissolves in the
water film of air sac. Dissolved oxygen diffuses across the wall of air sac and wall of capillary. It enters the blood plasma and
passes into red blood cells. In the red blood cells, oxygen combines with haemoglobin to form oxyhaemoglobin.
15. What are adaptive features of air sacs?
It is ball-shaped and folded to increase surface area. There are a lot of air sacs to increase surface area. The wall is one-cell
thick to shorten the diffusion distance. The water film is used to dissolve oxygen for gas to diffuse into the cell. It is
surrounded by capillaries. The capillaries carry oxygen away quickly to steepen the diffusion gradient.
16. How can red blood cells help in transport of carbon dioxide?
Red blood cells contain an enzyme to speed up the conversion of carbon dioxide and water to hydrogencarbonate ions. The
ions diffuse out to the plasma. It is this form of carbon dioxide transported in plasma.
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17. Explain what property of haemoglobin helps in oxygen transport.
Haemoglobin can combine with oxygen when oxygen concentration is high. This reaction is reversible. When oxygen
concentration is low. The reaction goes back to release oxygen.
18. In a graph showing the change of air pressure in thoracic cavity with time, what period represents inhalation or exhalation?
When the pressure of the lung is higher than atmospheric pressure, air must goes out. The whole period refers to exhalation.
When the pressure of the lung is lower than atmospheric pressure, air must goes into the lungs. The period is inhalation.
19. How does a rib cage model help in representing rib cage movement in ventilation?
The short vertical wood is sternum. The long vertical wood is backbone. The horizontal woods are ribs. The rubber bands are
intercostal muscle.
When the muscle is shortened, the muscle contracts. The horizontal woods are raised. The area formed by the woods is larger,
meaning the lung volume is bigger. This refers to inspiration.
When the muscle is lengthened, the muscle relaxes. The horizontal woods are lowered. The area formed by the woods is
smaller, meaning the lung volume is smaller. This refers to expiration.
20. How does a bell jar model help in representing diaphragm movement in ventilation?
The bell jar is the thoracic wall. The Y tube is the trachea and bronchi. The balloons are lungs. The rubber sheet is the
diaphragm.
When the rubber sheet is pulled down, this refers to diaphragm contraction. The volume of the bell jar increases. The
pressure inside the bell jar is smaller. Air is pulled into the bell jar and inflates the balloons.
When the rubber sheet is pushed up, this refers to the return of position of diaphragm to dome shape. The volume of the bell
jar decreases. The pressure inside the bell jar is larger. Air tends to move out and it compresses the balloons and forces the air
out of the balloons.
This model is a poor representation because it does not show the movement of rib. It does not show the presence of pleural
fluid. When the rubber sheet is pushed upwards, it cannot show the relaxation of muscle. In actual case, it is the pleural
membrane pulling the lung to lower the pressure so that the air inflates the lungs passively. In this model, it is the air moving
into the lungs to inflate it.
21. Which gas is the most abundant in breathed air? Is oxygen more or less than carbon dioxide in breathed air?
Nitrogen is the most abundant in breathed air. Oxygen is more than carbon dioxide in breathed air. This breathed air is good
in artificial breathing to save human life. It contains 16% oxygen, enough for a person. It contains 4% carbon dioxide which
can stimulate human brain to breathe.
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