, Exchange surfaces
All living cells need to be able to take up & excrete substances to & from
their environment. Size, surface area to volume ratio & level of activity
of an organism affects the needs for an exchange system.
Single celled organisms- do not need a specialised exchange system as the
cell has a large SA:V & all surfaces are in contact with the external
environment. Diffusion is enough to provide the cell with what is needed.
The metabolic activity is usually low so O₂ demands & CO₂ production is low.
Multicellular organisms- need a specialised exchange surface to increase
SA:V. Diffusion across the outer membrane is too slow as some cells are
too far from the outer environment. Multicellular organisms also have a
higher metabolic rate than single celled organism so have higher O₂
demands & CO₂ production.
Features of a good exchange surface:
Large SA- more space for molecules to pass SA:V=
𝒔𝒖𝒓𝒇𝒂𝒄𝒆 𝒂𝒓𝒆𝒂
through. 𝒗𝒐𝒍𝒖𝒎𝒆
Surface area= 4πr²
Thin barrier- reduces diffusion distance
A good blood supply- maintains concentration Volume= πr²
gradient so diffusion can occur rapidly Express ratio as 1:x
Ventilation-maintains diffusion gradient (for
gases)
, Trachea, bronchi & bronchioles
Trachea
• Contains wide tubes supported by incomplete C shaped rings of
strong, flexible cartilage, stopping the trachea collapsing. The C
shape gives flexibility & allows food to pass down the oesophagus.
• Lined with a ciliated epithelium with goblet cells between & below the
epithelial cells. Goblet cells secrete mucus which traps dust, pathogens &
foreign materials. The cilia then beats the mucus upwards away from the lungs
to be swallowed & digested. Smoking stops cilia beating & so causes a build up
of tar.
Bronchus
Similar structure to trachea but smaller
Bronchioles
• The walls of the bronchioles contains smooth muscle. If smooth muscle
contracts the lumen of the airway constricts & narrows. This is important if
there are harmful substances in the air. If the smooth muscle relaxes the
lumen of the airway dilates & widens. This means there's less resistance to
airflow & air can move in & out of the lungs easier. (happens during exercise).
The star shaped lumen allows dilation.
• Pulmonary artery & pulmonary vein are found
at the terminal bronchiole.
• No cartilage
, Alveoli
Adaptations of the alveoli
Both the alveolus & capillary wall are one cell thick & consist of squamous
cells which are flattened. The total barrier is only 2 flattened cells thick
providing a very short diffusion distance,
The capillaries are so narrow that the red blood cells are squeezed
against the capillary wall making them closer to the air in the alveoli &
reducing their flow rate so there is more chance for diffusion to occur.
The internal surface of the alveoli are lined by a thin layer of moisture
containing water, salts & lung surfactant. The surfactant reduces the
cohesive forces between the water molecules preventing the alveoli
collapsing & making it possible for the alveoli to remain inflated. O₂
diffuses into the water in the moisture before diffusing into the blood.
Large surface area
The capillaries are in close contact with the
alveolus wall reducing diffusion distance
Good blood supply- each alveolus is surrounded
by a large capillary network
The alveoli are elastic to enable shape changes
due to the ventilation movements.
All living cells need to be able to take up & excrete substances to & from
their environment. Size, surface area to volume ratio & level of activity
of an organism affects the needs for an exchange system.
Single celled organisms- do not need a specialised exchange system as the
cell has a large SA:V & all surfaces are in contact with the external
environment. Diffusion is enough to provide the cell with what is needed.
The metabolic activity is usually low so O₂ demands & CO₂ production is low.
Multicellular organisms- need a specialised exchange surface to increase
SA:V. Diffusion across the outer membrane is too slow as some cells are
too far from the outer environment. Multicellular organisms also have a
higher metabolic rate than single celled organism so have higher O₂
demands & CO₂ production.
Features of a good exchange surface:
Large SA- more space for molecules to pass SA:V=
𝒔𝒖𝒓𝒇𝒂𝒄𝒆 𝒂𝒓𝒆𝒂
through. 𝒗𝒐𝒍𝒖𝒎𝒆
Surface area= 4πr²
Thin barrier- reduces diffusion distance
A good blood supply- maintains concentration Volume= πr²
gradient so diffusion can occur rapidly Express ratio as 1:x
Ventilation-maintains diffusion gradient (for
gases)
, Trachea, bronchi & bronchioles
Trachea
• Contains wide tubes supported by incomplete C shaped rings of
strong, flexible cartilage, stopping the trachea collapsing. The C
shape gives flexibility & allows food to pass down the oesophagus.
• Lined with a ciliated epithelium with goblet cells between & below the
epithelial cells. Goblet cells secrete mucus which traps dust, pathogens &
foreign materials. The cilia then beats the mucus upwards away from the lungs
to be swallowed & digested. Smoking stops cilia beating & so causes a build up
of tar.
Bronchus
Similar structure to trachea but smaller
Bronchioles
• The walls of the bronchioles contains smooth muscle. If smooth muscle
contracts the lumen of the airway constricts & narrows. This is important if
there are harmful substances in the air. If the smooth muscle relaxes the
lumen of the airway dilates & widens. This means there's less resistance to
airflow & air can move in & out of the lungs easier. (happens during exercise).
The star shaped lumen allows dilation.
• Pulmonary artery & pulmonary vein are found
at the terminal bronchiole.
• No cartilage
, Alveoli
Adaptations of the alveoli
Both the alveolus & capillary wall are one cell thick & consist of squamous
cells which are flattened. The total barrier is only 2 flattened cells thick
providing a very short diffusion distance,
The capillaries are so narrow that the red blood cells are squeezed
against the capillary wall making them closer to the air in the alveoli &
reducing their flow rate so there is more chance for diffusion to occur.
The internal surface of the alveoli are lined by a thin layer of moisture
containing water, salts & lung surfactant. The surfactant reduces the
cohesive forces between the water molecules preventing the alveoli
collapsing & making it possible for the alveoli to remain inflated. O₂
diffuses into the water in the moisture before diffusing into the blood.
Large surface area
The capillaries are in close contact with the
alveolus wall reducing diffusion distance
Good blood supply- each alveolus is surrounded
by a large capillary network
The alveoli are elastic to enable shape changes
due to the ventilation movements.
