oxygen diffusion - way for gases to get across plasma membranes 

• levels needed for metabolism can’t be obtained by diffusion over 0.5 mm
• respiratory organs increase surface area and decrease distance over which oxygen must move

gills - tissue that projects out into the water 

• has no support, would collapse w/o water
• loses lots of water to evaporation when exposed to air
• external gills - not enclosed within the body
  • must be constantly moved
  • easily damaged
  • branchial chambers - pumps water past nonmoving gills
• bony fish gills - found between mouth and opercular cavity
  • most efficient of all respiratory organs
  • operculum (gill cover) - moves to open/close opercular cavity
  • water moves in through mouth, leaves through operculum
  • ram ventilation - forces water over gills through body mov’t, not pumping
  • gill arches - 4 on each side of head, each contains 2 rows of gill filaments and lamellae
  • blood flows opposite to the water mov’t (coutercurrent)

air-breathing - different organs for terrestrial organisms 

• tracheae - network of air passages in insects
  • oxygen diffuses directly into different cells
  • spiracles - openings of tracheae, close to prevent water loss
• lung - saturates air w/ water vapor before gas exchange
  • air moves in/out through same passages
• amphibian respiration - less lung surface area than other vertebrates
  • positive pressure breathing - mouth pumps air into lungs
  • cutaneous respiration also used (sometimes more than normal respiration)
• reptile respiration - cannot breathe through skin (too dry, tough)
  • negative pressure breathing - rib cages, lungs expand through muscular contraction
• mammal respiration - higher metabolic rate than reptiles/amphibians
  • alveoli - tiny sacs in lungs, adds to surface area for gas diffusion
  • passage of air - mouth >> pharynx >> larynx (voice box) >> glottis (opening in vocal cords) >> trachea (windpipe) >> bronchi >> bronchioles >> alveoli
  • external respiration - between lungs, capillaries
  • internal respiration - between capillaries, tissues
• bird respiration - has most efficient respiratory system out of all terrestrial vertebrates
  • parabronchi - air vessels w/ unidirectional flow (like fish)
  • new/old air not mixed together like in other terrestrial animals
  • inspiration - inhaled air goes to posterior air sac, air in lungs goes to anterior air sac
  • expiration - air from anterior air sac exhaled, air from posterior air sac goes to lungs
  • cross-current flow - blood flows perpendicular to air flow, more efficient than mammals