The heart is a muscle that is responsible for pumping blood through out the body.

The heart has four different chambers through which blood is pumped.

The upper chambers are called the left and right atriums.

The lower chambers are called the left and right ventricles.

The circulatory system contains three vessels:

arteries, veins, & capillaries

What are arteries?◦ Arteries are the largest blood vessels and

they carry blood away from the heart

◦ These blood vessels have very tough, flexible, and thick walls because there is a lot of pressure on them due to the force of the flow of blood when the heart beats.

Veins are smaller than arteries and they are responsible for taking blood to the heart.

The walls of veins are smaller because there is less force on the blood heading back to the heart

What about capillaries? Capillaries are the smallest type of blood vessel.

In fact they are only one cell thick. They have the thinnest vessels and are in contact with every cell in the body.