3.1.1 Exchange Surfaces

• What is the structure and function of the nasal cavity?

  Has a large surface area with a good blood supply, which warms air entering the lungs. It has a hairy lining, which secretes mucus to trap dust and bacteria protecting delicate lung tissue from irritation and infection. It has moist surfaces, which increase the humidity of the incoming air reducing evaporation from the exchange surfaces.
• What is the structure and function of the trachea?

  The trachea is a wide tube leading to the bronchi. It is help open by strong, flexible incomplete rings of cartilage. They are incomplete, so that food can move easily down the oesophagus behind the trachea. It also prevents the trachea from collapsing during inspiration.
• What is the structure and function of the bronchi?

  The 2 bronchi connect the trachea to the bronchioles in the lungs. It is similar in structure to the trachea with the same rings of cartilage holding them open. The cartilage plates in the bronchus becoming increasingly irregular as the diameter decreases.
• What is the structure and function of the bronchioles?

  Bronchioles are formed from the division of the bronchi. The smaller ones have no cartilage rings, whereas the large bronchioles will still have some cartilage. The walls contain smooth muscle allowing the bronchioles to constrict (close up) or dilate (open up) when the muscle contracts or relaxes. The smaller bronchioles are lined with a thin layer of squamous epithelium, making some gas exchange possible.
• What is the structure and function of the alveoli?

  The alveoli are tiny air sacs found at the end of the bronchioles, and unlike the airways that conduct air, the alveoli are where gas exchange occurs. They are made up of a thin layer of squamous (flattened) epithelial cells along with some collagen and elastic fibres.
• What is the function of cartilage and Which parts of the gas exchange system contains it?

  Cartilage holds the airways open to allow easy flow of air. It provides support and flexibility to allow easy flow of air.

  Distribution: trachea, bronchi, large bronchioles
• What is the function of smooth muscle and Which parts of the gas exchange system contain in?

  Smooth muscle contracts to narrow the airways.

  Distribution: trachea, bronchi, bronchioles
• What is the function of elastic fibres and Which parts of the gas exchange system contains it?

  Elastic fibres allow tissues to expand when breathing in; recoil when breathing out, helping to force air out of the lungs.

  Distribution: trachea, bronchi, bronchioles and alveoli
• What is the function of goblet cells and Which parts of the gas exchange system contain it?

  Goblet cells secrete mucus, which traps dirt and bacteria.

  Distribution: trachea, bronchi, large bronchioles
• What is the function of ciliated epithelium and Which parts of the gas exchange system contain it?

  Cilia move mucus out the airways.

  Distribution: trachea, bronchi, bronchioles
• What is the function of squamous epithelium and Which parts of the gas exchange system contain it?

  squamous epithelium are thin to give a short diffusion distance for gaseous exchange. They provide a large surface area.

  Distribution: alveoli and blood capillaries
• What is the function of the capillaries and Which parts of the gaseous exchange system contain it?

  capillaries provide a large surface area for exchange between blood and alveolar air.

  Distribution: trachea, bronchi, bronchioles, and alveoli
• What are the properties of efficient gas exchange systems?

  • moist
  • well-ventilated
  • good blood supply
  • warm
  • large surface area
  • thin barrier
  • permeable
• How does a large surface area make for an efficient gas exchange system in mammals?

  This creates a larger surface area across which molecules can diffuse (or be transported).
• How does having a thin barrier make for an efficient gas exchange system in mammals?

  There are 2 narrow layers of cells between the air and blood: 1 layer of squamous epithelia of the alveoli and 1 layer of squamous endothelia of the capillary. Being thin minimises the distance a molecule needs to diffuse.
• How is being permeable make for an efficient gas exchange system in mammals?

  respiratory gases (oxygen and carbon dioxide) can diffuse through the phospholipid bilayer that makes up the plasma membrane of squamous epithelial cells as they are small, uncharged, non-polar molecules.
• How is being moist make for an efficient gas exchange system in mammals?

  Fluid lining the alveoli allows gases to dissolve and diffuse into the body. This fluid contains surfactant secreted by specialised cells in alveoli, which prevents squamous epithelial cells of the alveoli sticking together.
• How is being well-ventilated make for an efficient gas exchange system in mammals?

  For gas exchange surfaces, the organ (e.g. lungs or gills) needs to be well-ventilated so that a steep concentration gradient is maintained. It allows efficient delivery of gases at the exchange surface.
• How does having a good blood supply make for an efficient gas exchange system in mammals?

  The alveoli are surrounded by a dense network of capillaries. This is so that whatever needs to be diffused in can be quickly transported to where it is needed and a steep concentration gradient can be maintained.
• How does being warm make for an efficient gas exchange system in mammals?

  Mammals are endotherms (maintains a constant temperature) and body heat warms air, increasing the kinetic energy of molecules and the rate of diffusion.
• What happens in the alveoli and in the capillaries to maintain a steep concentration gradient for diffusion of oxygen and carbon dioxide at the gas exchange surface?

  • alveoli: air is constantly being replenished by ventilation
  • capillaries: movement of blood transports gases to and from gas exchange surfaces
• Outline the process of inspiration in the lungs.

  • inspiration is an active process because muscles contract to make this happen and so ATP is used in the process
  • external intercostal muscles and diaphragm contract
  • ribcage moves up and out, while the diaphragm flattens
  • volume of chest cavity increases, so the pressure in the lungs decreases
  • air pressure in the lungs drops below atmospheric air pressure
  • air flows from the higher pressure in the atmosphere to the lower pressure in the lungs
  • elastic fibres stretch to accommodate the larger volume of air
• Outline the process of expiration in the lungs.

  • expiration is a passive process (for relaxed exhalation) because the muscles just relax to allow this process to occur
  • external intercostal muscles and diaphragm relax
  • ribcage moves down and inwards, while the diaphragm returns back to its domed-shaped position
  • volume of chest cavity decreases, so pressure in the lungs increases
  • air pressure in the lungs rises above atmospheric air pressure
  • air flows from the higher pressure in the lungs to the lower pressure in the atmosphere
  • elastic fibres recoil to help force air out of the alveoli
• Define operculum and How is it useful for gas exchange in fish?

  Operculum is a bony flap that protects the gills, which contain the gas exchange surface in bony fish.
• What is the buccal cavity?

  The space inside the mouth above the gill arches.
• What are gill arches?

  The bony structures that support gill filaments found between the buccal cavity and operculum.
• What are gill filaments?

  Gill filaments are thin projections from the gill arches that contain blood vessels that branch into the lamellae.
• What are gill lamellae (gill plates)?

  They are plates on the surface of gill filaments that are the site of gas exchange and contain capillaries in which blood flows counter-current to water flow.
• How are the gills in bony fish useful for gas exchange?

  • gills are made of a series of gill arches on each side, each of which support multiple gill filaments
  • gill filaments have plates projecting from the upper and lower surfaces, called gill lamellae
  • gill lamellae are one or more cells thick and have large number of capillaries within them, close to the surface
• How is being having a large surface area related to having an efficient gas exchange system in bony fish?

  presence of long, thin gill filaments with protruding plates (gill lamellae) increases surface area for gas exchange.
• How is having a thin barrier related to having an efficient gas exchange system in bony fish?

  there are 2 narrow layers of cells between water and blood: 1 layer of squamous epithelia of gill lamellae and 1 layer of squamous endothelia of the capillary
• How is being permeable related to having an efficient gas exchange system in bony fish?

  respiratory gases (oxygen and carbon dioxide) can diffuse through phospholipid bilayer that makes up the plasma membrane of squamous epithelial cells as they are small, uncharged, non-polar molecules
• How is being moist related to having an efficient gas exchange system in bony fish?

  living in water, there is no difficulty in keeping exchange surfaces moist. Gases are already dissolved in water and diffuse into the body.
• How is being well-ventilated related to having an efficient gas exchange system in bony fish?

  fish have a ventilation mechanism that constantly pushes water over their gills. the counter-current mechanism helps to maintain a steep concentration gradient allowing efficient delivery of gases at the exchange surface.
• How is having a good blood supply related to having an efficient gas exchange system in bony fish?

  gill lamellae contain a network of capillaries in which blood flows counter current to water flow.
• How is being warm related to having an efficient gas exchange system in bony fish?

  water provides a stable environment, as its high specific heat capacity prevents large changes in temperature.
• How is a concentration gradient maintained across an exchange surface in bony fish?

  large concentration gradient is maintained across the exchange surface by ensuring the oxygenated blood is constantly replaced with deoxygenated blood, as well as having a nearly continuous flow of water over the gills. they also have a counter-current exchange mechanism to help maintain the concentration gradient.
• What happens in the gill lamellae and in the capillaries to maintain a steep concentration gradient for diffusion of oxygen and carbon dioxide at the gas exchange surface?

  The water flow over the gill lamellae is constantly replenished by ventilation. In capillaries, the counter current movement of blood transports gases to and from gas exchange surfaces.
• How does ventilation occur in bony fish?

  ventilation in bony fish occurs by gill irrigation.
• How does the counter-current mechanism in bony fish work?

  Blood in the lamellae of the gills and the water passing over the gills show counter-current flow, which means they flow in opposite directions. Water flows from the front edge of the gill filament to the rear edge, whereas the blood inside flows from the rear edge to the front edge. This counter-current flow allows a concentration gradient to be maintained across the whole of the lamella, and so almost all of the dissolved oxygen to be extracted from the water.