3.1 Exchange surfaces and breathing
Exchange surfaces
To respire and grow, living things need to take up:
- Oxygen, water, nutrients, and carbon dioxide (plants)
Living things also need to remove by-products and wastes such as:
- Carbon dioxide, urea, and oxygen (plants)
Exchange - process of taking up and removing substances.
Three factors affect need for exchange system:
- Size - In very small organisms, all cytoplasm very close to environment, so diffusion
supplies enough oxygen and nutrients to keep cells alive and active. Multi-cellular
organisms may have several layers of cells, so oxygen and nutrients diffusing in have
longer diffusion pathway. Diffusion too slow to enable sufficient supply to innermost cells.
- Surface area : Volume ratio - Small organisms have small SA and small V, and large
SA:V ratio. SA large enough to supply all cells with sufficient oxygen. Larger organisms
have larger SA and larger V, and small SA:V ratio as V increases more rapidly than SA.
Most large organisms need range of tissues for support and strength, so volume increases
with more layers of cells.
- Level of activity - Some organisms more active than others. Metabolic activity uses
energy from food and requires oxygen to release energy in aerobic respiration. Cells of
active organism need good supplies of oxygen and nutrients to supply energy for
movement. Need for energy increased in animals that keep themselves warm.
Multicellular Organisms
Multicellular organisms need exchange surfaces to diffuse substances needed.
- Diffusion distance too far due to small SA:V ratio.
- Takes too long for substances to diffuse passively to all cells.
- Multicellular organisms usually more active, so higher demand for energy,
so higher respiration rate needing more substances to be delivered.
For an efficient exchange surface:
- Large surface area - to provide more space for molecules to pass through. Often
achieved by folding walls and membranes.
- Short diffusion distance - barrier must be permeable to substances exchanged, and
thin for ease of diffusion.
- Good blood supply - can bring fresh supplies of molecules to supply side, keeping
concentration high, or may remove molecules from demand side to keep concentration low.
Maintains steep diffusion gradient.
- Steep diffusion gradient - allows faster rate of diffusion as molecules move from more
concentrated to less concentrated side.
Gaseous Exchange System in Mammals
Mammalian gaseous exchange system consists of lungs and airways that carry
gases into and out of lungs.
Lungs - a pair of inflatable sacs in chest cavity, protected by ribcage.
Air can pass into lungs through nose and along trachea, bronchi, and
bronchioles. Finally, it reaches tiny air-filled sacs called alveoli. These are
surfaces where the exchange of gases takes place.
The lungs
Ribcage - surrounds and protects lungs and heart. Left lung smaller (2 lobes) to
create space for heart. Right lobe thicker (3 lobes) but shorter, as liver
underneath.
Intercostal muscles - hold together ribs. Contract to increase volume of chest
cavity to draw air into lungs.
Diaphragm - thin sheet of dome-shaped muscle beneath lungs. Contracts and
flattens during inspiration, decreasing pressure. Relaxes during expiration to
increase pressure. Air entering and leaving lungs is ventilation.
Module 3 - Exchange and transport Page 1
, increase pressure. Air entering and leaving lungs is ventilation.
Pleura - sac around each lung made up of two pleural membranes - one around
lungs and one inside of ribcage with pleural fluid in pleural cavity. Allows lungs to
move smoothly as they expand and contract.
Trachea - rings of cartilage to stop it collapsing. C-shaped to allow food bolus to
pass down oesophagus.
Trachea branches into two bronchi then into bronchioles ending in alveoli.
External intercostal muscles contract in inspiration to expand volume of chest
cavity.
Internal intercostal muscles only contract for forced expiration during
coughing/sneezing/singing (not relaxed breathing).
Lung adaptations
In all mammals, gas exchange takes place in lungs. Adapted to have very large
surface area in contact with bloodstream, so O2 can diffuse into blood, and CO2
can diffuse out.
Lungs adapted to be good exchange surface:
• Large surface area - provides more space for molecules to pass through.
Alveoli lined by thin layer of moisture, which evaporates and is lost as we
breathe out. Lungs must produce surfactant that coats internal surface of
alveoli to reduce cohesive forces between water molecules, as can make
alveoli collapse.
• Exchange surfaces permeable to oxygen and carbon dioxide - barrier for
exchange compromised of alveolus wall and blood capillary wall. Cells and
plasma membranes readily allow diffusion of oxygen and CO2, as molecules
small and non-polar.
• Thin barrier to reduce diffusion distance - alveolus wall one cell thick;
capillary wall one cell thick; both walls consist of squamous cells (very thin);
capillaries in close contact with alveolus walls; capillaries so narrow that red
blood cells squeezed against capillary wall, making them closer to air in
alveoli. Total barrier to diffusion only two flattened cells.
• Good blood supply to maintain steep concentration gradient - blood
transports CO2 from tissues to lungs, ensuring concentration of CO2 in blood
higher than in air of alveoli. Blood transports oxygen away from lungs to
ensure concentration of oxygen in blood lower than in alveoli.
Fick’s Law of diffusion:
Rate of diffusion ∝ surface area x concentration difference
thickness of membrane
Module 3 - Exchange and transport Page 2
Exchange surfaces
To respire and grow, living things need to take up:
- Oxygen, water, nutrients, and carbon dioxide (plants)
Living things also need to remove by-products and wastes such as:
- Carbon dioxide, urea, and oxygen (plants)
Exchange - process of taking up and removing substances.
Three factors affect need for exchange system:
- Size - In very small organisms, all cytoplasm very close to environment, so diffusion
supplies enough oxygen and nutrients to keep cells alive and active. Multi-cellular
organisms may have several layers of cells, so oxygen and nutrients diffusing in have
longer diffusion pathway. Diffusion too slow to enable sufficient supply to innermost cells.
- Surface area : Volume ratio - Small organisms have small SA and small V, and large
SA:V ratio. SA large enough to supply all cells with sufficient oxygen. Larger organisms
have larger SA and larger V, and small SA:V ratio as V increases more rapidly than SA.
Most large organisms need range of tissues for support and strength, so volume increases
with more layers of cells.
- Level of activity - Some organisms more active than others. Metabolic activity uses
energy from food and requires oxygen to release energy in aerobic respiration. Cells of
active organism need good supplies of oxygen and nutrients to supply energy for
movement. Need for energy increased in animals that keep themselves warm.
Multicellular Organisms
Multicellular organisms need exchange surfaces to diffuse substances needed.
- Diffusion distance too far due to small SA:V ratio.
- Takes too long for substances to diffuse passively to all cells.
- Multicellular organisms usually more active, so higher demand for energy,
so higher respiration rate needing more substances to be delivered.
For an efficient exchange surface:
- Large surface area - to provide more space for molecules to pass through. Often
achieved by folding walls and membranes.
- Short diffusion distance - barrier must be permeable to substances exchanged, and
thin for ease of diffusion.
- Good blood supply - can bring fresh supplies of molecules to supply side, keeping
concentration high, or may remove molecules from demand side to keep concentration low.
Maintains steep diffusion gradient.
- Steep diffusion gradient - allows faster rate of diffusion as molecules move from more
concentrated to less concentrated side.
Gaseous Exchange System in Mammals
Mammalian gaseous exchange system consists of lungs and airways that carry
gases into and out of lungs.
Lungs - a pair of inflatable sacs in chest cavity, protected by ribcage.
Air can pass into lungs through nose and along trachea, bronchi, and
bronchioles. Finally, it reaches tiny air-filled sacs called alveoli. These are
surfaces where the exchange of gases takes place.
The lungs
Ribcage - surrounds and protects lungs and heart. Left lung smaller (2 lobes) to
create space for heart. Right lobe thicker (3 lobes) but shorter, as liver
underneath.
Intercostal muscles - hold together ribs. Contract to increase volume of chest
cavity to draw air into lungs.
Diaphragm - thin sheet of dome-shaped muscle beneath lungs. Contracts and
flattens during inspiration, decreasing pressure. Relaxes during expiration to
increase pressure. Air entering and leaving lungs is ventilation.
Module 3 - Exchange and transport Page 1
, increase pressure. Air entering and leaving lungs is ventilation.
Pleura - sac around each lung made up of two pleural membranes - one around
lungs and one inside of ribcage with pleural fluid in pleural cavity. Allows lungs to
move smoothly as they expand and contract.
Trachea - rings of cartilage to stop it collapsing. C-shaped to allow food bolus to
pass down oesophagus.
Trachea branches into two bronchi then into bronchioles ending in alveoli.
External intercostal muscles contract in inspiration to expand volume of chest
cavity.
Internal intercostal muscles only contract for forced expiration during
coughing/sneezing/singing (not relaxed breathing).
Lung adaptations
In all mammals, gas exchange takes place in lungs. Adapted to have very large
surface area in contact with bloodstream, so O2 can diffuse into blood, and CO2
can diffuse out.
Lungs adapted to be good exchange surface:
• Large surface area - provides more space for molecules to pass through.
Alveoli lined by thin layer of moisture, which evaporates and is lost as we
breathe out. Lungs must produce surfactant that coats internal surface of
alveoli to reduce cohesive forces between water molecules, as can make
alveoli collapse.
• Exchange surfaces permeable to oxygen and carbon dioxide - barrier for
exchange compromised of alveolus wall and blood capillary wall. Cells and
plasma membranes readily allow diffusion of oxygen and CO2, as molecules
small and non-polar.
• Thin barrier to reduce diffusion distance - alveolus wall one cell thick;
capillary wall one cell thick; both walls consist of squamous cells (very thin);
capillaries in close contact with alveolus walls; capillaries so narrow that red
blood cells squeezed against capillary wall, making them closer to air in
alveoli. Total barrier to diffusion only two flattened cells.
• Good blood supply to maintain steep concentration gradient - blood
transports CO2 from tissues to lungs, ensuring concentration of CO2 in blood
higher than in air of alveoli. Blood transports oxygen away from lungs to
ensure concentration of oxygen in blood lower than in alveoli.
Fick’s Law of diffusion:
Rate of diffusion ∝ surface area x concentration difference
thickness of membrane
Module 3 - Exchange and transport Page 2
