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
- In the pulmonary circulatory system
The double circulatory system in mammals
Main Circulatory System Structures Table
