B3.1.2

• Why are specialised transport systems needed in animals?

  • high metabolic demands
  • small surface area to volume ratio
  • Biological molecules need transporting around the body
• Metabolic demands of multicellular organisms are too high to be sufficiently supplied by diffusion alone (high demand for oxygen and food and a greater amount of waste products are produced)
• The surface area to volume ratio gets smaller as organisms get bigger so diffusion distances increase and there is less surface area available to absorb or remove substances
• Biological molecules such as hormones and enzymes may be made in one place but needed in another.
• Food may be digested in one organ system but needs to be transported to every cell in the body for use in metabolic reactions
• waste products need to be removed from cells and transported to excretory organs.
• Characteristics of circulatory systems:

  • a liquid transport medium which circulates around the system
  • vessels that carry the transport medium
  • a pumping mechanism to move the fluid around the system
• A mass transport system is a transport system where substances are transported in a mass of fluid
• Circulatory systems can be…

  • open
  • closed
  • double
  • single
• Open circulatory systems are found mainly in invertebrate animals including most insects and some molluscs.
• Insect blood is called haemolymph , it does not carry oxygen or carbon dioxide as insects have a tracheal system but it does transport food and nitrogenous waste as well as defence cells.
• In open circulatory systems there are few vessels to contain the transport medium. It is pumped straight from the heart into the body cavity of the animal (haemocoel) at low pressure and directly contacts the tissues and cells to exchange substances before returning to the heart through an open ended vessel.
• Pathway of open circulatory system in insects:


  heart → haemocoel (body cavity) → cells → open ended vessel → back to heart
• Circulatory system of insects:

  1. Segmented heart contracts in a wave starting form the back of the abdomen and running all the way along to the thorax where the haemolymph is pushed into a single main artery.
  2. The artery opens up into the haemocoel (the body cavity)
  3. The haemolymph moves through the haemocoel at a low pressure directly contacting cells and tissues to exchange
  4. the haemolymph returns to the heart through a series of valves
• Disadvantages of open circulatory systems…

  • low pressure
  • steep diffusion gradient cannot be maintained
  • amount of haemolymph reaching a particular tissue cannot be controlled or varied.
• Closed circulatory systems are found in all vertebrates including fish and mammals.
• Closed circulatory systems transport respiratory gases (oxygen and carbon dioxide), nutrients (eg glucose), waste products and hormones.
• In a closed circulatory system blood is enclosed in blood vessels and does not come into direct contact with the body cells. The heart pumps blood around the system under pressure and relatively quickly with blood returning directly to the heart. Substances leave and enter by diffusion through the walls of the blood vessels.
• Most closed circulatory systems contain a blood pigment that carries respiratory gases.
• Advantages of a closed circulatory system…

  • blood can be pumped under pressure so it is faster
  • The amount of blood flowing to particular tissues can be adjusted by widening or narrowing the blood vessels.
• In single circulatory system blood only passes through the heart once for each complete circulation of the body.
• In a single closed circulatory system blood passes through two sets of capillaries (capillary beds) before returning to the heart:

  • firstly to exchange gases (oxygen and carbon dioxide)
  • secondly to exchange different substances with organ systems around the body
• Disadvantages of single closed circulatory system…

  • as a result of passing through two narrow sets of capillaries, blood pressure drops
  • blood returns to the heart quite slowly
  • limits efficiency of exchange (so activity levels of animals with single closed circulations tends to be low)
• Single closed circulatory systems are found in fish and annelid worms.
• Fish circulatory systems are the exception to the low efficiency rule of single closed circulatory systems and can be very active.
• Fish circulatory systems are sufficient because…

  • the countercurrent gas exchange system in the gills allows a large oxygen uptake from the water
  • Bodyweight supported by water, meaning less metabolic demand of muscle
  • Do not maintain their own body temperature (ectotherms), again reduces metabolic demands
• What circulatory system is this?

  Fish

  
• What is A?

  ventricle

  
• What is B?

  atrium

  
• What is C?

  heart

  
• What is d?

  Gills

  
• What is e?

  Body

  
• What is f?

  deoxygenated blood

  
• What is g?

  oxygenated blood

  
• What is h?

  deoxygenated blood

  
• What type of circulatory system is this?

  single closed

  
• Double closed circulatory systems are found in birds and mammals.
• In double closed circulatory systems blood passes through the heart twice for each complete circulation of the body.
• Double closed circulatory systems can be split into two circulations:

  • Pulmonary circulation
  • Systemic circulation
• Pulmonary circulation route:

  Heart → lungs → heart