General

• Why larger organisms need transport systems

  • SA:V ratio - low, and more layers of tissue
  • size - oxygen is used up on outer layers, inner layers do not receive any
  • metabolic activity - more active, more oxygen required for respiration for energy
• Why smaller organisms don't need transport systems

  • high SA:V ratio
  • diffusion can supply their oxygen and nutritional needs
  • more cells are at the surface
  • shorter diffusion distance to inner cells
  • fewer cells, lesser demand for oxygen and nutrients
• Components of circulatory systems

  • transport medium
  • vessels
  • pumping system
  • exchange surfaces
• Diffusion 


  • used by small organisms
  • net movement of atoms or molecules from an area of higher concentration to an area of lower concentration, down a concentration gradient
• Mass flow


  • used by large organisms
  • directed movement of substances in a mass of fluid over a large distance, driven by pressure
• mass flow is much faster than diffusion
• How does mass flow allow organisms to function?

  1. transports substances from exchange surfaces to tissues
  2. transports waste products of metabolism to excretory organs
  3. redistributes heat
  4. transports molecules for metabolism
  5. transports immune cells to sites of infection