Transport systems in multicellular animals

• Need for specialised transport systems:

  • Metabolic demands are high so diffusion over the long distances is not enough to supply the quantities needed.
  • Surface area to volume ratio gets smaller so diffusion distances get bigger and the amount of surface area available becomes smaller.
  • Molecules like hormones or enzymes are made in one place and need to be transported to another.
  • Food digested in one organ system, needs to be transported to every cell for respiration.
  • Waste products need to be removed.
• Circulatory system= carry gases like oxygen and carbon dioxide, nutrients, waste products and hormones around the body.

  • Have a liquid transport medium that circulates around the system.
  • They have vessels that carry the transport medium.
  • They have a pumping mechanism to move the fluid around the system.
• Mass transport system= when substances are transported in a mass of fluid with a mechanism for moving the fluid around the body.
• Open circulatory system


  • Very few vessels to contain the transport medium.
  • Pumped straight from the heart into the body cavity= haemocoel.
  • In the haemocoel the transport medium is under low pressure.
  • It comes into direct contact with the tissues and cells. This is where exchange takes place between transport medium and cells.
  • Tranport medium returns to the heart through an open-ended vessel.
• Open- ended circulatory systems

  • Found in invertebrate.
  • Insect blood is called haemolymph, it transports food and nitrogenous waste products and the cells involved in defence against disease.
  • The body cavity is split by a membrane and the heart extends along the length of the thorax and the abdomen of the insect.
  • The haemolymph circulates but steep diffusion gradients cant be maintained for efficient diffusion.
  • Amount of haemolymph flowing to a tissue cant be varied to meet changing demands.
• Closed circulatory system

  • Blood is enclosed in blood vessels and doesn't come directly into contact with cells.
  • The heart pumps blood around the body under pressure and relatively quickly, and the blood returns directly to the heart.
  • Substances leave and enter the blood by diffusion through the walls of the blood vessels.
  • Amount of blood flowing can be adjusted by widening or narrowing blood vessels.
  • Contain a blood pigment to carry respiratory gases.
  • Found in many animal phyla.
• Single closed circulatory system

  • Blood flows through the heart and is pumped out to travel around the body before returning to the heart.
  • Blood only travels once thought the heart for each complete circulation.
  • Blood passes through two sets of capillaries before it returns to the heart. First one it exchanges oxygen and carbon dioxide. The second one, substances are exchanged between the blood and the cells.
  • As blood passes through narrow vessels, blood pressure drops and returns to the heart slowly, this limits efficiency of exchange processes so activity levels are limited.
• Single closed circulatory system in fish

  • Have a relatively efficient single circulatory system, so they can be very active.
  • They have a countercurrent gaseous exchange mechanisms in gills that allows them to take a lot of oxygen from the water.
  • They don't need to maintain their own body temperature.
  • This reduces the metabolic demands.
• Double closed circulatory system

  • Blood is pumped from the heart to the lungs to pick up oxygen and unload carbon dioxide and then returns to the heart.
  • Blood flows through the heart and is pumped out to travel all around the body before returning to the heart.
  • Blood travels twice through the heart for each circuit of the body.