A closed circulatory system consists of a heart that pumps blood through blood vessels, distributing nutrients and oxygen, and retrieving waste products discarded from the tissues. Many invertebrates have open circulatory systems; however, the earthworm has the more efficient closed circulatory system.
The pressure in the blood vessels of an earthworm's closed circulatory system is high due to its 5 "hearts" pumping blood into the vessels and blood vessel contractions sustaining that pressure. In an open circulatory system, hemolymph, a nutrient-filled fluid akin to blood, leaves the blood vessels to ooze slowly into the body cavities to bathe the cells. An earthworm can sustain higher activity levels since higher blood pressure in a closed system leads to faster circulation of blood and elevates metabolic levels.
In an open circulatory system, cells must be close to body cavities so that nutrients, wastes and gases may effectively transfer to and from the hemolymph. Consequently, open circulatory systems are ideal only for certain body types. If an animal such as an earthworm has an atypical shape or size, a closed circulatory system proves more efficient because its countless smaller blood vessels can lie in close proximity to every cell of the body.
Directed Blood Flow
A closed circulatory system allows the earthworm to push blood along the blood vessels, diverting it to certain body parts as needed. An earthworm escaping from a bird looking for its next meal will reroute any available blood to relevant areas, such as the muscles controlling locomotion. Furthermore, after consuming a meal, the earthworm will shunt its reserve blood supply to aid digestion.