Circulatory system

By Guy and Ethan

What is the circulatory system?

The circulatory system (otherwise known as the cardiovascular system), is an organ system were blood circulates/ transports throughout the body. There are 2 types of circulatory systems open and closed. An open circulatory system pumps blood from the heart into an open cavity, where all tissues and organs get bathed in the blood at a very low pressure. A closed circulatory system, on the other hand, uses a series of vessels and arteries to carry blood to all extremities at a relatively high pressure so that every part of the body receives about the same amount of oxygenated blood.

What does it do for the animal?

Transports/ circulates oxygen, carbon dioxide, hormones, and blood cells to and from cells in the body to provide nourishment and help fight of diseases, maintain body temperature, and maintain homeostasis.

Cells, Tissues, and Organs within the system

Blood Cells, Blood Vessels, Heart and Heart Valves

Circulatory system within the 9 Animal Phylums


  • Circulatory System is not present
  • Complexity: Cellular level of organization
  • Representative animal: Sponge

Sponges are basically cells that have grouped together to form a body. In this regard, they are not so much one living being as a large community of cells that have formed in a genral body shape. As such, they do not have any true tissue, nor do they have a body cavity, and because they don't have organs they have no need for blood. Any nutrients the cells need can be filtered from water that flow through the porous body.


  • Circulatory System is not present
  • Complexity: Tissue level of organization
  • Representative animal: Jellyfish

Jellyfish are animals that are made up of a tissue covering, nerve cells, and water. They do not have a heart, despite the common misconception that the mass in the center of their body is a heart; it is actually a stomach. Because jellyfish are 95% water, they don't need any blood as they can filter nutrients out of the water as well as eating other, smaller creatures. They also don't have any organs or muscles that require blood, as they rely on nerve cells to detect surroundings and induce convulsions for movement.


  • Circulatory System is not present
  • Complexity: Organ level of organization
  • Representative animal: Marine Flatworm

Flatworms, unlike the previous few phylums, do have organs that require constant nutrients, despite having neither a respiratory system or circulatory system. Flatworms overcome this issue by being, well, flat; because they are so flat, they can draw nutrients out of the water they live in to support their bodies.


  • Circulatory System is not present
  • Complexity: Organ level of organization
  • Representative animal: Roundworms

Roundworms of the Nematoda phylum are almost exclusively parasitic, which is how they survive despite not having any blood circulation of their own; they suck blood, and all the nutrients it carries, out of the host organism that it is infecting.


  • Circulatory System present (closed) but in some (leeches) for example are partly (open)

Most annelids have a pair of coelomata (body cavities) in each segment, separated from other segments by septa or wall for dividing a cavity or structure into smaller ones, and from each other by vertical mesenteries (fold of membranous tissue that arises from the posterior wall of the peritoneal cavity and attaches to the intestinal tract) . Each septum forms a sandwich with connective tissue (biological tissue that supports, connects, or separates different types of tissues and organs in the body) in the middle and mesothelium (membrane that serves as a lining) from the preceding and following segments on either side. The mesothelium may also form radial and circular muscles on the septa, and circular muscles around the blood vessels and gut. Parts of the mesothelium, especially on the outside of the gut, may also form chloragogen cells that perform similar functions to the livers of vertebrates: producing and storing glycogen and fat, while producing hemoglobin and breaking down proteins; and turning nitrogenous waste products into ammonia and urea to be excreted


  • Circulatory System is present (open)

Molluscs are coelomates (main body cavity that positioned inside the body to surround and contain the digestive tract and other organs), their coeloms are reduced to fairly small spaces enclosing the heart and gonads. The main body cavity is a hemocoel through which blood and coelomic fluid circulate and which encloses most of the other internal organs. These haemocoelic spaces act as an efficient hydrostatic (fluids that are at rest, or when the flow velocity at each point is constant over time) skeleton. The blood contains the respiratory pigment hemocyanin as an oxygen-carrier. The heart consists of one or more pairs of atria auricles, which receive oxygenated blood from the gills and pump it to the ventricle, which pumps it into the aorta (main artery), which is fairly short and opens into the hemocoel.


  • Circulatory system is present (open)
Species in the arthropod phylum are similar to species from the mollusk system. The circulatory system helps give oxygen to the entire body and disposes of carbon dioxide, hemolymph (blood)

and go to the lacunas (cavities) draining and irrigating tissues. Insects do not have respiratory pigments because their blood does not carry any oxygen but all have a heart.


  • Circulatory system is present (Closed)
They have cilia circulating the fluids through each arm. Echinoderms have a network of fluid-filled canals that function in gas exchange, feeding and in movement. The network contains a central ring and areas which contain the tube feet which stretch along the body or arms. The tube feet poke through holes in the skeleton and can be extended or contracted. They do not have a true heart. While starfish contain most of this but have well structured system of tubes that works in place of the circulatory system.


  • Circulatory system is present (closed)