3.2 Gas exchange in humans
● Human body is far too large for needs of all body cells to be met by simple
diffusion of gases through surface skin
● Therefore humans have a respiratory system for gas exchange
○ Large, much folded gas exchange surface (alveoli in lungs)
○ Mechanism for ventilation (breathing), moving air in and out of lungs,
allowing air to come into contact with gas exchange surface
○ Close association of gas exchange surface with blood circulatory system to
enable distribution of gases around body to and from cells
Human respiratory system (in order of air in) - of thorax (胸部) -
ventilation
● .
● Trachea (氣管) (windpipe) [1]
o Supported by C-shaped hoops/rings of cartilage (軟骨)
o Front of oesophagus
● Bronchus (支氣管) [2]
o Carries air towards and out of lung
● Lungs (肺) [2]
o Lie inside rib cage
o Provides protection, assists breathing
● Bronchioles (細支氣管) [~60k]
o Branches of bronchi
o Further to carry air towards alveoli
● Alveoli (肺泡) [~600m]
o Sites of gas exchange
● Pleural membrane (胸膜) [2 layers]
o Double layer, closely surrounding each lung
o Between the layers is pleural fluid
1
, o Lines inner wall of chest and outer surface of each lung
● Pleural fluid (胸水) [10~20 mL]
o Between pleural membranes
o Allows smooth movement of lungs as they inflate and deflate, so that they
can slide smoothly against chest wall of thoracic cavity (胸腔)
● Diaphragm (橫膈膜) [1]
o Lies below lungs, base of thoracic cavity
o Sheet of muscle separating thoracic cavity from abdominal cavity
(腹腔)
o When relax, dome-shaped (圓頂); when muscles contract (i.e.: inhale), it is
pulled flatter, increase in size of thoracic cavity
● Ribs (肋骨) [24, 12 on each side]
o Provide physical protection to lungs (& heart lying inside thoracic cavity)
● Intercostal muscles (肋間肌) [22, 11 pairs]
o Lie between each of the ribs
o Relax when lungs expand (inhale) {for internal}, allowing lungs to fill up with
air; contract when exhale {for internal}, putting pressure on lungs, helping
driving air out
● Sternum (胸骨) [8]
o (Along with ribs) protect organs in thorax
Summary***:
Nose → trachea → bronchus → bronchiole → alveoli
Alveoli (air sacs)
● Site of gas exchange between air (in lungs) and blood (in capillaries)
● Oxygenated blood flows from alveolar capillaries into pulmonary vein (肺靜脈)
● Total surface area about a tennis court (large surface area)
● Thin alveoli walls - short diffusion distance between alveoli and blood
capillaries
● Moist air inside lungs, alveoli have film of liquid on surface
2
● Human body is far too large for needs of all body cells to be met by simple
diffusion of gases through surface skin
● Therefore humans have a respiratory system for gas exchange
○ Large, much folded gas exchange surface (alveoli in lungs)
○ Mechanism for ventilation (breathing), moving air in and out of lungs,
allowing air to come into contact with gas exchange surface
○ Close association of gas exchange surface with blood circulatory system to
enable distribution of gases around body to and from cells
Human respiratory system (in order of air in) - of thorax (胸部) -
ventilation
● .
● Trachea (氣管) (windpipe) [1]
o Supported by C-shaped hoops/rings of cartilage (軟骨)
o Front of oesophagus
● Bronchus (支氣管) [2]
o Carries air towards and out of lung
● Lungs (肺) [2]
o Lie inside rib cage
o Provides protection, assists breathing
● Bronchioles (細支氣管) [~60k]
o Branches of bronchi
o Further to carry air towards alveoli
● Alveoli (肺泡) [~600m]
o Sites of gas exchange
● Pleural membrane (胸膜) [2 layers]
o Double layer, closely surrounding each lung
o Between the layers is pleural fluid
1
, o Lines inner wall of chest and outer surface of each lung
● Pleural fluid (胸水) [10~20 mL]
o Between pleural membranes
o Allows smooth movement of lungs as they inflate and deflate, so that they
can slide smoothly against chest wall of thoracic cavity (胸腔)
● Diaphragm (橫膈膜) [1]
o Lies below lungs, base of thoracic cavity
o Sheet of muscle separating thoracic cavity from abdominal cavity
(腹腔)
o When relax, dome-shaped (圓頂); when muscles contract (i.e.: inhale), it is
pulled flatter, increase in size of thoracic cavity
● Ribs (肋骨) [24, 12 on each side]
o Provide physical protection to lungs (& heart lying inside thoracic cavity)
● Intercostal muscles (肋間肌) [22, 11 pairs]
o Lie between each of the ribs
o Relax when lungs expand (inhale) {for internal}, allowing lungs to fill up with
air; contract when exhale {for internal}, putting pressure on lungs, helping
driving air out
● Sternum (胸骨) [8]
o (Along with ribs) protect organs in thorax
Summary***:
Nose → trachea → bronchus → bronchiole → alveoli
Alveoli (air sacs)
● Site of gas exchange between air (in lungs) and blood (in capillaries)
● Oxygenated blood flows from alveolar capillaries into pulmonary vein (肺靜脈)
● Total surface area about a tennis court (large surface area)
● Thin alveoli walls - short diffusion distance between alveoli and blood
capillaries
● Moist air inside lungs, alveoli have film of liquid on surface
2
