IB DP Biology: SL复习笔记6.2.3 The Blood System: Double Circulation

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 Table1.-Closed-Double-Circulatory-System-Table