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Open Circulatory Systems: Definition & Examples

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Instructor: Laura Enzor

Laura has a Master's degree in Biology and is working on her PhD in Biology. She specializes in teaching Human Physiology at USC.

Open circulatory systems allow the blood and interstitial fluids of an organism to mix. Explore examples and the definition of open circulatory systems, organisms with open circulatory systems and the benefits of an open circulatory system. Learn why some organisms have closed circulatory systems. Updated: 09/07/2021

What is an Open Circulatory System?

An open circulatory system describes a system where blood and interstitial fluid are allowed to mix in an organism. Interstitial fluid is just the fluid found between cells in the body.

So, what does this mean? Organisms that utilize an open circulatory system don't have 'true' blood since it is mixed with other fluids. The 'blood' in these organisms is defined as hemolymph, or the mixture between blood and interstitial fluid. You can remember that because 'heme' is Latin for 'blood,' and 'lymph' refers to liquid.

Humans have a closed circulatory system, which means that our blood and other fluids never mix. So, if humans don't have an open circulatory system, why should we care about it? Understanding how different organisms work can help us better understand why our bodies work the way they do!

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  • 0:02 What Is an Open…
  • 0:53 Organisms with Open Systems
  • 1:41 Benefits of Open Systems
  • 3:58 Closed Circulatory Systems
  • 4:49 Lesson Summary
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Organisms with Open Circulatory Systems

Two of the largest phyla in the world, Arthropoda and Mollusca, have an open circulatory system. Arthropods, members of the phylum Arthropoda, include most insects and sea organisms like crabs and lobsters. 'Arthro' is Greek for 'joint,' and 'pod' is Greek for 'leg,' so we combine this into 'jointed legs.' Think of what a grasshopper or a lobster look like; joint-legs is a great way to describe these animals!

Molluscs are members of the phylum Mollusca. They are typically bivalves, or organisms with two shells that are hinged together. Examples include clams and oysters. However, not all molluscs have an open circulatory system. Two major members of the mollusc family, octopus and squid, have a closed circulatory system.

Benefits of an Open Circulatory System

To explore the benefits of having an open circulatory system, we need to delve a little bit more into how it works. Because hemolymph flows freely in the body, this single fluid brings oxygen and nutrients to the organs and removes waste products.

Many insects have a tracheal system that brings oxygen into their bodies through a series of tubes. This means they have less energetic demand on their body since their oxygen system and circulatory system are separated. Think of how many times you breathe in a minute. The act of breathing brings oxygen into your body and gets rid of carbon dioxide. Now think about the number of cells in your body that have to carry those gases to your organs and muscles - that's a lot of work!

One cool factoid about insects with a tracheal system is that they have no need for respiratory pigments. The reason is because hemolymph isn't used to transport oxygen in these organisms, there isn't a need for them. In humans, we have two main respiratory pigments: hemoglobin and myoglobin. These pigments are what makes our blood and muscles red. So, if there aren't any respiratory pigments in insects, their hemolymph is colorless!

Another interesting aspect about organisms that utilize an open circulatory system is they can't create a blood pressure. In mammals, our blood pressure is due to the pumping of our heart. Remember, our blood is separated from our interstitial fluid. Organisms that use an open circulatory system don't have a 'true' heart. Instead, they utilize blood vessels that act as pumps to move hemolymph around the body.

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