Summary Physiology; ventilation and gas exchange review

21/03/19

Dr Rachel O’Callaghan

Ventilation and Gas Exchange
Water Vs. Air Breathers

Different strategies:

1. O2 content in water is lower than air
a. Water breathers must ventilate more
b. Higher energy cost
2. Water is more dense and viscous than air
a. More energy required to move over surface
b. Higher energy cost
• Most water-breathing animals use unidirectional ventilated gills

Air Breathers

1. Energy cost is not a big concern
a. O2 availability is high
b. Density of medium is low
2. Evaporation across surface is a problem
a. Internal gaseous exchange surfaces limit/recover evaporation
b. Lungs are favourable
• Most air-breathing animals use tidally ventilated lungs

Respiratory Strategies

• Sponges
o Water-breather
o Circulate medium through body
o Water in through ostia
o Choanocyte (flagellated cells)
o Water circulates in central cavity
▪ Exits via spongocoel
• Crustaceans
o Water-breather
o Small species – no gills, just diffusion over body
o Large species – gills located under carapace
▪ Gill bailer propels water to mouth
o Water entry point is different for species:
▪ Shrimp – back and side edges of carapace
▪ Crayfish and lobster – base of legs
▪ Crabs – base of claw
• Elasmobranchs – sharks, skates, rays
o Gills – open mouth, increases volume, decreases pressure
▪ Water sucked in through mouth and spiracles
o Close mouth, muscles contract to decrease volume and increase pressure
▪ Water moves out of gills