2.2 Gas exchange

• All living organisms exchange gases with their environment. Gases are echanged across

  respiratory surfaces. A respiratory surface must be:

   Thin (short diffusion pathway)

   Permeable to gases

   Moist

   Have a large surface area
• Gases are always exchanged by diffusion.
• Single celled organisms such as Amoeba have a

  large surface area to volume ratio and therefore gas exchange across the cell surface

  membrane is sufficient.
• Huge organisms such as an elephant cannot rely on diffusion alone, as they have a smaller surface area to volume ratio. They must have a ventilation

  mechanism and sometimes a circulatory system with specialised blood pigments to

  ensure that respiratory gases are exchanged with the body tissue rapidly.
• The Amoeba has a large surface area

  and a short diffusion pathway which allows

  oxygen to diffuse throughout the organism

  quickly enough to accommodate its respiratory

  needs.
• Large multicellular organisms cannot

  rely on diffusion alone; they are adapted with

  specialised respiratory surfaces, circulatory

  systems and blood pigments to facilitate the

  transport of gases.
• Amoeba -

  the membrane of the single cell is thin so diffusion distances into the cell are

  short. Gaseous exchange by diffusion across the cell surface

  is rapid enough to supply oxygen for respiration and to

  remove carbon dioxide.
• Flatworm -

  Flatworms are aquatic, and being flat, have a much larger

  surface area than spherical organisms. Their large surface

  area to volume ratio has overcome the problem of size

  increase as no part of the body is far from the surface and

  so diffusion paths are short.
• Earthworm -

  • terrestrial organism.
  • cylindrical, so its surface area to volume ratio is smaller than a flatworm’s.
  • Its skin is the respiratory surface, which it keeps moist by secreting mucus. It has a
  • low oxygen requirement because it is slow moving and has a low metabolic rate.
  • Enough oxygen diffuses across the skin surface to reach the blood capillaries beneath.
• Metabolic rate – The rate of

  energy expenditure by the body.
• Oxygen dissolves in the

  moisture on the earthworm’s

  surface before diffusing into

  capillaries.
• Bony fish have a specialised gas exchange surface – the gills.
• Gills have a large surface area due to gill

  filaments. Gill filaments are a specialised

  respiratory area. Water is a dense

  medium with relatively low oxygen

  content. This means that water must be

  forced over the gill filaments.
• The

  density of the water prevents the gills

  from collapsing (maintaining the large

  surface area).
• In bony fish gas exchange: Water is forced over the

  gills by a ventilating mechanism. Flow of

  water is one way – unidirectional.
• Ventilation in a bony fish allows

  water to be passed continuously

  across the gills even when the

  fish is resting. Ventilation is

  achieved by pressure changes in

  the buccal (mouth) and opercular

  (gill) cavities.
• Stage 1 of the ventilation mechanism in bony fish

  • The mouth opens and the floor of the buccal cavity is lowered.
  • Volume of the buccal cavity increases and pressure decreases. The operculum remains closed.
  • Water is pulled into the buccal cavity from the outside due to the change in pressure.
• Stage 2 of the ventilation mechanism in bony fish

  • The mouth closes and the buccal cavity contracts, raising the floor of the buccal cavity.
  • Water is forced across the gills.
• Stage 3 of the ventilation mechanism in bony fish

  • Pressure in the gill cavity increases and forces the operculum (gill slit) open.
  • Water leaves via the operculum.
• Bony fish gills have an extensive network of capillaries to allow efficient diffusion of oxygen.

  The blood pigment haemoglobin and a circulatory system carry oxygen throughout the

  fish.
• Gill filaments have gill plates or lamellae. Water flows between the gill plates (lamellae)

  in the opposite direction to the blood flow in the gill capillaries.
• Counter current flow (bony fish) increases

  the efficiency of diffusion by

  maintaining a steep

  concentration gradient across

  the whole gill filament. Blood

  always meets water with

  relatively high oxygen content.
• Parallel flow - Water flows across the filament (through

  the gill plates) in the same direction as

  blood flow in the gill capillaries. Occurs in cartilaginous fish
• Parallel flow

  • The oxygen concentration gradient is not maintained. Equilibrium is reached at 50% between the water and the blood.
  • Diffusion of oxygen from the water to the blood does not occur across the entire gill plate
  • Rate of diffusion is lower and decreases as equilibrium is reached.
  • Less oxygen is absorbed into the blood.
• Counter-current flow

  • A steep oxygen concentration gradient is maintained allowing diffusion of oxygen across entire gill plate.
  • Rate of diffusion is high.
  • A greater amount of oxygen is absorbed into the blood. The percentage oxygen saturation will be higher.
• The respiratory surface in amphibians, reptiles and birds share the following characteristics:

   Large surface area

   Moist surface

   Short diffusion pathway (thin walls).

   Circulatory system with blood pigments to carry oxygen

   Internal lungs to minimise water loss (not in amphibians).

   Ventilation mechanism
• Gas exchange in insects occurs

  through paired holes, called

  spiracles, running along the side of

  the body. The spiracles lead into a system of branched, chitin lined air-

  tubes called tracheae.
• To reduce water loss insects have evolved a rigid waterproof

  exoskeleton which is covered by a cuticle.
• Insects have a relatively small surface area to

  volume ratio and cannot use their body surface to exchange gases by diffusion.
• Spiracles

  • can open and close like valves; this allows gaseous exchange to take place and reduces water loss.
• The end of gas exchange in insects: The spiracles lead into tracheae which lead to tracheoles. Gas exchange takes place at the end of the tracheoles. Oxygen

  passes directly to the cells; this is very

  rapid.
• Compression and expansion of the abdomen ventilates the tracheal system in insects. Ventilation

  carries the respiratory medium (air) to the respiratory surface at the end of the

  tracheoles. Spiracles open and close to allow air in and out of the tracheal system.
• When the abdomen is expanded the thorax spiracles

  are open and the abdominal spiracles are closed; air enters the tracheal system through

  the thorax spiracles. As the abdomen is compressed the thorax spiracles close and the

  abdominal ones open; air leaves the tracheal system via the abdominal spiracles.
• The

  expansion and compression of the abdomen ventilates the tracheal system – air is drawn

  in via the spiracles in the thorax and expelled via the spiracles in the abdomen.
• The lungs are enclosed within an airtight compartment called the thorax.
• The trachea transports air to the

  bronchi. The bronchi transport air

  to the bronchioles and the

  bronchioles transport air to the

  alveoli.
• The alveoli are the

  respiratory surface and the site

  of gaseous exchange by diffusion.
• The lungs are not muscular and need a ventilation mechanism.
• Ventilation is a mechanism

  which moves the respiratory medium (air) to and from the respiratory surface.
• Mammals ventilate their lungs by negative pressure

  breathing, forcing air down into the lungs.