The Circulatory System

by Karthik Gurumurthy

Your family might use a car to pick up groceries. A garbage truck takes your trash to the dump. An ambulance rushes medical professionals to the site of an emergency. Animals bodies also have ways to transport materials, such as oxygen, nutrients, hormone messages, and waste to different body parts.

Flatworms and other simple animals stretch and scrunch their bodies to move materials between neighboring cells. Larger animals have a circulatory system in which substances catch a ride to all body parts in a fluid called blood. Blood circulates (moves) through the body and acts as grocery getter, garbage picker-upper, and ambulance all in one. Special pumping organs, such as hearts and gills, keep the blood continually moving.

Some animals including insects and clams have “open” circulatory systems. Blood washes directly over their organs, passing fresh materials into the cells and  carrying away waste materials. Their pumping organs fill with blood and then push it back into the body by contracting (squeezing).

Other animals, including earthworms, sea stars, octopuses, and all vertebrates, have “closed” circulatory systems because the blood does not flow freely through the body.

Instead, it is channeled through a series of tubes called blood vessels. The heart pumps “fresh” blood full of oxygen and nutrients to the cells in blood vessels called “arteries”. “Old” blood carrying the cell’s waste materials, including carbon dioxide, travels back to the heart through blood vessels called veins.

The blood vessels closest to the pumping organs are the largest and strongest. As they extend theough the body, they branch into smaller and smaller blood vessels until they are thin capillaries. Materials pass between the blood and the cells through the thin capillary wells and the cell membranes.

Circulatory systems help to maintain homeostasis by changing the speed at which materials are moved through the body.