10.1 - Types of Circulatory System

The Necessity for Transport Systems in Complex Multicellular Organisms

All living organisms require the exchange of substances such as nutrients, waste products and respiratory gases constantly in order to survive. The exchange of these substances usually occurs through diffusion. However, in large multicellular organisms, diffusion alone is not enough to make sure constant supply of oxygen and nutrients to the cells due to

• Their small total surface area to volume (TSA/V) ratio, and
• Great distance between the source of the substances and the cells.

In order to overcome these problems, they require a circulatory system to help with the distribution of nutrients and oxygen as well as the removal of waste products.

Types of Circulatory Systems in Multicellular Organisms

The circulatory system of large multicellular organisms can be categorized into two:

• Open circulatory system
• Closed circulatory system

Figure 1 - In an open circulatory system (left), hemolymph is pumped directly into the hemocoel which contains the body cells. In a closed circulatory system (right), the blood flows continuously within the heart and vessels. The exchange of substances occurs through the blood capillaries walls.

Figure 2 - Insects have an open circulatory system. One or more hearts pump the hemolymph into the hemocoel. When the heart contracts, hemolymph flows from the heart into the hemocoel. Here, chemical exchange occurs between the hemolymph and body cells. When the hearts relax, the hemolymph is drawn back into the hearts through pores called ostia (singular, ostium).

Figure 3 - A fish has a heart with one atrium and one ventricle. Blood that leaves the ventricle goes to the gill capillaries first for gaseous exchange. The gill capillaries carry oxygenated blood to the systemic capillaries. In systemic capillaries, oxygen diffuses in while carbon dioxide diffuses out of the body cells. Since the circulatory system of fish has only one circuit, it is called a single circulatory system.

Figure 4 - In amphibian such as frogs, they have a heart which consist of two atria and one ventricle. Deoxygenated blood enters the right atrium, while oxygenated blood goes into the left atrium. The blood then enters into a single ventricle. Although there is some mixing of oxygenated and deoxygenated blood , most of the oxygenated blood tends to remain on the left side of the ventricle, while the deoxygenated blood tends to remain on the right side. The ventricle then pumps the blood to the pulmocutaneous and systemic circulations. Since the blood flows in 2 separate circuits, the system is known as a double circulatory system.

Figure 5 - Humans have a heart that consist of two atria and two completely separate ventricles. In pulmonary circulation , deoxygenated blood in the right ventricle is pumped into the pulmonary arteries and transported to the lungs to be oxygenated. In the systemic circulation , blood is pumped from the heart to the rest of the body. Oxygenated blood flows into the left ventricle to be pumped into the systemic capillaries via aorta. Since it involves 2 separate circulations, this is called a complete double circulatory system.

Comparison between circulatory systems of insects, fish, amphibians and humans

• Similarities

  • Present in all multicellular organisms
  • Consist of a heart to pump blood throughout the body or hemolymph (in insects)
  • Function in transporting nutrients and waste products
  • The heart has valves to prevent backflow of blood

• Differences

  • Insects

    • Types of circulatory system: Open circulatory system
    • Separation of oxygenated and deoxygenated blood: -
    • Number of circulations: -
    • Number of heart cavities: Many

  • Fish

    • Types of circulatory system: Closed circulatory system
    • Separation of oxygenated and deoxygenated blood: -
    • Number of circulations: Single (blood flows through the heart once in a complete circulation)
    • Number of heart cavities: Two (one atrium and one ventricle)

  • Amphibians

    • Types of circulatory system: Closed circulatory system
    • Separation of oxygenated and deoxygenated blood: Incomplete (mixture of oxygenated and deoxygenated blood in the ventricle)
    • Number of circulations: Double (blood flows through the heart twice in a complete circulation)
    • Number of heart cavities: Three (two atria and one ventricle)

  • Humans

    • Types of circulatory system: Closed circulatory system
    • Separation of oxygenated and deoxygenated blood: Complete (no mixture of oxygenated and deoxygenated blood in the ventricle)
    • Number of circulations: Double
    • Number of heart cavities: Four (two atria and two ventricles)