The need for a circulatory system

• All organisms need to exchange materials with their environments
• Small animals with large surface area to volume ratios (or relatively inactive animals like jellyfish) can rely on diffusion alone to exchange oxygen, carbon dioxide and nutrients with their environment
• Larger animals have smaller surface area to volume ratios, so diffusion alone is insufficient for exchange of materials between cells further from the surface of the organism with the environment
• Circulatory systems are systems which carry around fluids containing materials needed by the organism, as well as waste materials that need to be removed

Open & closed systems

• Circulatory systems are either described as being open or closed
• In a closed circulatory system, blood is pumped around the body and is always contained within a network of blood vessels
  • All vertebrates and many invertebrates have closed circulatory systems
• In an open circulatory system, blood is not contained within blood vessels but is pumped directly into body cavities
  • Organisms such as arthropods and molluscs have open circulatory systems.
• Humans have a closed double circulatory system: in one complete circuit of the body blood passes through the heart (the pump) twice
• The right side of the heart pumps blood deoxygenated blood to the lungs for gas exchange; this is the pulmonary circulatory system
• Blood then returns to the left side of the heart, so that oxygenated blood can be pumped efficiently (at high pressure) around the body; this is the systemic circulatory system

The double circulatory system in mammals

Main circulatory system structures table