Gas exchange

Question 1

How does inspiration occur in humans?

Answer 1

• Active
  -external intercostal muscles contract
  -internal intercostal muscles relax
  -diaphragm contracts and flattens and moves downwards
  -ribs move up and outwards
  -volume of thorax increased
• reduces pressure in lungs
• air pressure in atmosphere > air pressure in lungs
• so forced to inhale

Question 2

How does expiration occur in humans?

Answer 2

• largely passive
• external intercostal muscles relax
• internal intercostal muscles contract
  -diaphram relaxes and becomes domed
  -ribs move inwards and downwards
  -volume of thorax decreases
• increased pressure in lungs
  -atmospheric pressure < the pressure in the lungs
• so forced to exhale