Every animal alive possesses a circulatory system that’s in charge of bringing nutrients to cells and removing wastes so they don’t cause disease. While they move fluids around the body, circulatory systems also help out with other tasks by

  • Delivering oxygen to cells and picking up carbon dioxide

  • Distributing hormones to cells

  • Maintaining body temperature by transporting heat

  • Transporting cells to fight infection

Animals have two types of circulatory systems:

  • In open circulatory systems, the animal’s heart pumps a bloodlike fluid called hemolymph through open-ended vessels into a chamber called the hemocoel, where it directly bathes the cells. In other words, the circulatory system’s fluid isn’t kept separate from the fluid around cells, which is called interstitial fluid. Muscle contractions push the hemolymph back toward the heart so it can be circulated throughout the animal again and again.

  • In closed circulatory systems, a network of closed tubes called vessels performs the transportation and prevents blood from coming into direct contact with the body’s cells. Three types of vessels move blood within closed systems:

    • Arteries carry blood away from the heart to the organs and tissues of the body. (You can think "a" for arteries and "a" for away to help you remember this.)

    • Veins carry blood from the organs and tissues back to the heart.

    • Capillaries are fine networks of vessels that connect arteries and veins within each tissue.

Animal hearts come in different sizes and shapes, but they have the same function: to pump fluid throughout the circulatory system. That fluid is either hemolymph or blood, depending on the type of circulatory system.

For questions 1–2, use the following terms to identify the type of circulatory system described in each situation.

a. Open circulatory system

b. Closed circulatory system

  1. In the grasshopper, the heart pumps fluid through an aorta that runs along the insect’s dorsal side. From there, fluid moves into chambers called sinuses where it comes into contact with body cells. Contractions of body muscles push the fluid back toward the heart.

  2. An octopus has three hearts. Two hearts pump fluid through vessels to the gills so that the fluid can receive oxygen and release wastes, and then they pump the blood through vessels to the third heart. The third heart pumps the oxygen-rich fluid through vessels to the rest of the body.

The following are the answers to the practice questions.

  1. The answer is a. Open circulatory system.

    The fluid flows into chambers and has direct contact with body cells.

  2. The answer is b. Closed circulatory system.

    The fluid is contained in vessels as it travels around the body.