Capillaries – transfer blood between arteries and veins
within each tissue.
Consists of many invertebrates (including most molluscs and all arthropods)
Fluid is pumped among the tissues and there is no distinction
between circulatory and interstitial fluids. Instead this fluid is
called hemolymph and has the functions of both blood and
The vertebrate circulatory system is often called the cardiovascular system
Arteries don’t always carry oxygen-rich blood, and veins do not always carry
Capillaries are the places where exchange occurs. There is not any
The key differences between the open and closed system is the vessels in
an open circulatory system do not form an enclosed circuit from the heart,
through the body, and back to the heart, and the circulatory fluid is not
distinct from interstitial fluid, as is the blood in a closed circulatory system.
– Vertebrate cardiovascular systems reflect evolution
As aquatic vertebrates became adapted for terrestrial living, breathing
capability went from gills to lungs, which also lead to the closed circulatory