7 Gas exchange in
humans
7.1 Human breathing system
1 Gas exchange (氣體交換) between an organism and the external environment ensures a
continuous supply of oxygen for respiration in cells and the removal of carbon dioxide
produced in the process.
2 Gas exchange always takes place by diffusion across a moist respiratory surface (呼吸表面).
3 Unicellular organisms (e.g. Amoeba) have a large surface area to volume ratio. Their body
surface acts as the respiratory surface.
4 Large multicellular organisms (e.g. humans) have a smaller surface area to volume ratio.
For efficient gas exchange, they have:
- a breathing system (呼吸系統) which provides a larger respiratory surface and allows gases
to move into and out of the body.
- a transport system (運送系統) which carries gases between the respiratory surface and body
cells.
5 Structure of the human breathing system:
6 The path of air flowing along the respiratory tract:
7 Adaptive features of the respiratory tract for keeping the inhaled air clean, moist and warm:
Adaptive feature Function
In the nostrils Hairs - Filter large dust particles
In the linings of Mucus-secreting cells - Secrete mucus (黏液) to trap
nasal cavity, dust particles and
trachea, bronchi microorganisms
and larger - The mucus also moistens the
bronchioles inhaled air
Ciliated epithelial cells - Their cilia (纖毛) beat to sweep
(纖毛上皮細胞) the mucus towards the
pharynx
Capillaries (微血管) - The blood in the capillaries
warms the inhaled air
7.2 Gas exchange in the air sacs
1 Gas exchange between the air in an air sac and the blood in a capillary:
2 Adaptive features of air sacs for gas exchange:
Adaptive feature Explanation
Large in number Provides a very large surface area for diffusion
Very thin walls (one-celled thick) Reduce the diffusion distance
Moist inner surfaces Allow oxygen in inhaled air to dissolve in the water film
before diffusion
Rich supply of capillaries Allows rapid transport of gases to and away from the
air sacs to maintain a steep concentration gradient of
gases for diffusion
In close contact with capillaries Reduces the diffusion distance
3 Differences between inhaled air and exhaled air:
Inhaled air Exhaled air Explanation
Oxygen content 21% 16% - Oxygen in inhaled air diffuses from the air
sacs into the blood during gas exchange.
- It is used by body cells for respiration.
Carbon dioxide 0.04% 4% - Carbon dioxide produced by body cells
content during respiration diffuses from the blood
into the air sacs during gas exchange.
Water vapour Variable Saturated - Exhaled air is moistened by the water
content film lining the air sacs and the mucus
lining the respiratory tract.
Temperature Lower Higher - Exhaled air is warmed by blood in the
capillaries near the inner surface of the
respiratory tract.
4 The content of nitrogen (78%) and other gases (1%) remains the same in inhaled air and
exhaled air. These gases are not used nor produced by body cells.
7.3 Transport of respiratory gases
1 Adaptive features of red blood cells for carrying oxygen:
Adaptive feature Explanation
Packed with haemoglobin (血紅蛋白) Greatly increases the oxygen-carrying
capacity of blood
No nucleus when mature Provides space for more haemoglobin
Biconcave disc shape (雙凹圓盤狀) Provides a large surface area to volume
ratio and a short distance for the diffusion of
oxygen
2 The uptake of oxygen from the air sacs and the release of oxygen to the body cells:
3 The uptake of carbon dioxide from the body cells and the release of carbon dioxide to the
air sacs:
7.4 Ventilation
1 Ventilation (換氣) is the flow of air into and out of the lungs.
2 Ventilation is brought about by breathing actions, which include inhalation (吸氣) and
exhalation (呼氣).
3 Mechanism of inhalation:
4 Mechanism of exhalation:
5 Differences between inhalation and exhalation:
Inhalation Exhalation
Intercostal muscles Contract Relax
Movement of rib cage Upwards and outwards Downwards and inwards
Diaphragm muscles Contract Relax
Shape of diaphragm Flattened Dome shape
Volume of thoracic cavity Increases Decreases
Volume of the lungs Increases Decreases
Air pressure in the lungs Lower than atmospheric Higher than atmospheric
pressure pressure
Direction of airflow Into the lungs Out of the lungs
6 The change in air pressure in the lungs during inhalation and exhalation:
7 The change in volume of the lungs during inhalation and exhalation:
