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
Organisms that respire aerobically require oxygen to release energy from the breakdown of glucose and other organic substances, but oxygen is not the only substance that needs to be transported around an organism by a circulatory system.
Make sure you study any circulatory diagrams in the exam carefully to distinguish between single and double circulatory systems and to discern between pulmonary and systemic circulation.