Gas exchange

Single celled organisms

• substances can diffuse directly into or out of the organism across the cell surface membrane
• quick diffusion rate due to reletively highh surface area:volume ration.

Multicellular organisms

• diffusion is slow because
  • some cells are deep within the body 
  • low surface area:volume ratios
  • higher metabolic rate means they use up oxygen and glucose more quickly
• this is why specialised exchange surfaces are needed

Large surface area

• root hair cells

Thin

• some are only one cell thick
  • Alvioli

Good blood supply and/or ventilation

• increases efficiency
  • alveoli - surrounded by capillaries
  • Gills - contain a large network of blood vessels

For the volume

Width x depth x hight

for the surface area

number of sides x width of sides x length of sides 

                                            CYLINDRICAL MEASUMENTS

for the volume

V = π r 2 h

for the surface area

A = 2 π r h + 2 π r 2

O2 is inspired and then CO2 is expired

The lungs

• Insects have system of treacheal tubes throught their bodies
• spiricles open into the sides of these allowing air to enter
• the smallest of these tubes are called tracheoles 
  • they are in contact with the tissues
• when muscles are active, fluid is drawn into the tissue
• the fluid is then released when the muscle is at rest removing waste CO2
• tracheoles provide a large surface area
• some small insects can rely on diffusion to get gases in and out of tissues
• larger insects ventilate their tracheal system with rythemic body movements

• Because water contains less oxygen than air, diffusion of gases is slower

Gills:

• made up of numerous folds giving a large surface area for diffusion 
• located in the pharynx
• supported by a bony gill arch
• along each arch is a double row of gill lamellea 

Lamellea:

• out of water the lamelea stick together ad collapse meaning the fish suffocates
• there is a counter current flow of blood around the lamellea which means that gas exchange is always happening

Ventilation of the gills:

• to take in water:
  • the operculum closes the opening at the back of the pharynx

• • the floor of the mouth cavity is lowered
  • the volume inside increases causing the pressure to fall
  • water rushes into the mouth
• For water to pass out:
  • the mouth closes
  • the floor of the mouth cavity raises 
  • the volume inside decreases and so the pressure increases forcing water back over the gills
  • the operculum opens and water flows out

The counter current system: