Human gas Exchange

• Structure of Human Breathing System

  • Nostrils
  • Nasal cavity
  • Pharynx
  • Larynx
  • Trachea
  • Bronchi
  • Bronchioles
  • Lungs
  • Diaphragm muscles
  • External & internal intercostal muscles
• Nasal cavity


  • Lined with hairs that filter dust & germs in inhaled air
  • Mucus traps dust & germs and moistens the inhaled air
  • Blood capillaries warm the inhaled air
  • Olfactory cells allow us to smell the inhaled air (very sensitive)
• Glottis


  Narrow opening to larynx and trachea
• Epiglottis


  Flap of cartilage that closes over the larynx to prevent food from entering the trachea during swallowing
• Larynx (voice box)


  Contains vocal cords that vibrate when air passes over them to produce sound
• Trachea (wind pipe) & Bronchi

  • Supported by C-shaped cartilage rings that prevent them from collapsing during inspiration & swallowing
  • Inner lining consists of mucus-producing goblet cells and ciliated cells that sweep mucus containing foreign particles upwards to the pharynx
• Alveoli


  • Millions of tiny air sacs at the end of bronchioles provide a large surface area for gaseous exchange
  • Very thin wall provides a short distance for easy & fast diffusion of gases
  • Covered with a layer of water (mucus film) to dissolve gases for diffusion to occur
  • Richly supplied with blood capillaries for rapid transport of gases away from lungs
• Mechanism of breathing

  1. Breathing in / inhalation: air is brought into lungs

  2. Breathing out / exhalation: air is removed out of lungs
• Inspiration

  Diaphragm contracts & flattens<|>External intercostal muscles contract, intercostal muscles relax<|>Rib cage moves upwards & outwards<|>Volume of chest cavity increases<|>As atmosphere pressure is higher, air flows into lungs
• Expiration


  Diaphragm relaxes & becomes dome-shaped<|>External intercostal muscles relax, internal intercostal muscles contract<|>Rib cage moves downwards & inwards<|>Volume of chest cavity decreases<|>Air pressure inside lungs increases, forcing air out of lungs
• Exchange of gases at an alveolus

  1. Deoxygenated blood from heart to lungs has lower oxygen but higher carbon dioxide

  2. Oxygen dissolves in alveolar moisture, diffuses into blood plasma, then into red blood cells

  3. Carbon dioxide diffuses from blood into alveolus, removed during expiration
• Comparison of inhaled and exhaled air

  • Nitrogen: 79% in both
  • Oxygen: 21% in, 16% out (absorbed by body)
  • Carbon dioxide: 0.04% in, 4% out (produced by body)
  • Water vapour: Variable in, greater amount out (evaporates from alveoli)
  • Temperature: Same as atmosphere in, warmer out (air is warmed)
• During physical activity


  • Rate and depth of breathing increases (more oxygen needed, more carbon dioxide produced)
  • Anaerobic respiration produces lactic acid (oxygen debt)
  • After activity, breathing remains high to provide oxygen to break down lactic acid (pay off oxygen debt)