Exchange and transport

• Need for specialised exchange surfaces

  • Arises as the size of the organism and its surface area to volume ratio decreases
  • In single-celled organisms, substances can easily enter the cell as the distance that needs to be crossed is short
  • In multicellular organisms, the distance is much larger due to a lower surface area to volume ratio
  • Multicellular organisms require specialised exchange surfaces for efficient gas exchange of carbon dioxide and oxygen
• Features of an efficient exchange surface


  • Large surface area (e.g. root hair cells or folded membranes)
  • Thin to ensure short distance for substances to cross
  • Good blood supply/ventilation to maintain a steep gradient (e.g. alveoli)
• Mammalian gaseous exchange system

  • The lungs are a pair of structures with a large surface area located in the chest cavity with the ability to inflate
  • The lungs are surrounded by the rib cage which serves to protect them
  • A lubricating substance is secreted to prevent the friction between rib cage and lungs during inflation and deflation
  • External and internal intercostal muscles between the ribs contract to raise and lower the ribcage respectively
  • A structure called the diaphragm separates the lungs from abdomen area
• Air flow into and out of the lungs

  1. Air enters through the nose, along the trachea, bronchi and bronchioles

  2. Gaseous exchange takes place in the walls of alveoli, which are tiny sacs filled with air
• Trachea, bronchi and bronchioles


  • Held open with the help of rings of cartilage, incomplete in the trachea to allow passage of food down the oesophagus behind the trachea
  • Composed of several layers including cartilage, glandular and connective tissue, elastic fibres, smooth muscle and blood vessels
  • Inner lining is an epithelial layer composed of ciliated epithelium and goblet cells
• Bronchioles


  • Narrower than the bronchi
  • Only the larger bronchioles contain cartilage
  • Wall is made of smooth muscle and elastic fibres
  • Smallest bronchioles have alveoli clusters at the ends
• Structures and functions of mammalian gaseous exchange system

  • Cartilage - supports trachea and bronchi, prevents lungs from collapsing
  • Ciliated epithelium - moves mucus along to prevent lung infection
  • Goblet cells - secrete mucus to trap bacteria and dust
  • Smooth muscle - contract to control airway diameter and air flow
  • Elastic fibres - stretch and recoil to control air flow
• Ventilation - inspiration and expiration


  1. Inspiration: External intercostal muscles contract, diaphragm contracts and flattens, increasing thorax volume and decreasing pressure, causing air to enter lungs

  2. Expiration: Internal intercostal muscles contract, diaphragm relaxes and rises, decreasing thorax volume and increasing pressure, forcing air out of lungs
• Spirometer


  • Device used to measure lung volume
  • Causes airtight chamber to move up and down as person breathes in and out, leaving a trace on a graph
• Lung volumes measured by spirometer

  • Vital capacity - maximum volume of air that can be inhaled or exhaled in a single breath
  • Tidal volume - volume of air breathed in and out at rest
  • Breathing rate - number of breaths per minute
  • Residual volume - volume of air always present in the lungs
  • Inspiratory reserve volume - additional volume that can be inhaled beyond tidal volume
  • Expiratory reserve volume - additional volume that can be exhaled beyond tidal volume
• Ventilation and gas exchange in bony fish

  • Fish have four pairs of gills, each with multiple projections called gill filaments and lamellae that participate in gas exchange
  • Blood and water flow across the lamellae in a counter-current direction
  • Ventilation maintains a continuous unidirectional water flow over the gills
• Ventilation and gas exchange in insects

  • Insects do not have a transport system, so oxygen is transported directly to tissues via spiracles (small openings of tubes called tracheae and tracheoles)
  • Tracheoles have a small amount of tracheal fluid that allows gases to dissolve and diffuse into cells
  • Spiracles can be opened and closed to avoid excessive water loss