exchange surfaces

• Specialised exchange surfaces are needed as the size of the organism increases and the surface area to volume ratio decreases
• In multicellular organisms, the distance substances need to cross is 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 include:

  • Large surface area, such as root hair cells or folded membranes
  • Thin to ensure a short distance for substances to cross
  • Good blood supply/ventilation to maintain a steep gradient
• Mammalian gaseous exchange system:

  • Lungs are a pair of structures with a large surface area located in the chest cavity
  • Lungs can inflate and are protected by the rib cage
  • Lubricating substance prevents friction between rib cage and lungs
  • External and internal intercostal muscles contract to raise and lower the ribcage
  • Diaphragm separates the lungs from the abdomen area
  • Air enters through the nose, trachea, bronchi, and bronchioles
  • Gaseous exchange occurs in the walls of alveoli, tiny sacs filled with air
• Structures and functions of mammalian gaseous exchange system:

  • Cartilage supports the trachea and bronchi, preventing lung collapse
  • Ciliated epithelium moves mucus to prevent lung infection
  • Goblet cells secrete mucus to trap bacteria and dust
  • Smooth muscle constricts the airway to control airflow
  • Elastic fibres stretch and recoil to control airflow
• Ventilation:

  • Flow of air in and out of alveoli
  • Composed of inspiration and expiration stages
  • External intercostal muscles contract during inspiration, internal relax
  • Diaphragm contracts and flattens during inspiration
  • Internal intercostal muscles contract during expiration, external relax
  • Diaphragm relaxes and rises during expiration
• Spirometer:

  • Device used to measure lung volume
  • Vital capacity is the maximum volume of air that can be inhaled or exhaled
  • Tidal volume is the volume of air breathed in and out at rest
  • Breathing rate is the number of breaths per minute
  • Residual volume is the volume of air always present in the lungs
• Ventilation and gas exchange in bony fish and insects:

  • Fish have a small surface area to volume ratio and impermeable skin, requiring a specialised exchange surface
  • Bony fish have gills with gill filaments and lamellae for gas exchange
  • Blood and water flow across the lamellae in a counter current direction
  • Insects use spiracles and trachea/tracheoles for direct gas transport to tissues
  • Spiracles can be opened and closed to avoid excessive water loss