6.2.3 The Blood System: Double Circulation

The need for a circulatory system

• All organisms need to transport materials to where they are needed inside their tissues
  • Small organisms (or relatively inactive animals like jellyfish) can rely on diffusion alone to transport oxygen, carbon dioxide and nutrients around their bodies
  • Larger organisms have more layers of cells, so diffusion alone is insufficient for transport of materials between cells further from the exchange surface of the organism

• Circulatory systems are systems which transport fluids containing materials needed by the organism, as well as waste materials that need to be removed
  • Circulatory systems ensure that fluids containing these substances reach all of the cells in an organism quickly enough to supply their needs and remove waste

Humans have a closed, double circulatory system

• A closed circulatory system is one in which blood is contained within a network of blood vessels
  • As opposed to an open circulatory system in which the fluid fills the body cavity e.g. as in insects

• A double circulatory system passes through the heart twice for every one complete circuit of the body, with blood passing through two separate circuits known as pulmonary and systemic circulation
  • In the pulmonary circulatory system
    • The right side of the heart pumps deoxygenated blood to the lungs for gas exchange
    • Blood pressure is lower in the pulmonary system; this prevents damage to the lungs

  • In the systemic circulatory system
    • Oxygenated blood returns to the left side of the heart from the lungs
    • The left ventricle then pumps the oxygenated blood at high pressure around the body

The double circulatory system in mammals

Main Circulatory System Structures Table