The heart is a pump whose walls are made of thick muscle.
They can squeeze (contract) to send blood rushing out.
There are also valves at the bottom of the large arteries that carry blood away from the heart: the aorta and the pulmonary artery.
These valves keep the blood from flowing backward into the heart once it has been pumped out.
The veins, in turn, unite with each other to form larger veins until the blood from the body is finally collected into the large veins that empty into the heart.
So the blood vessels of the body carry blood in a circle: moving away from the heart in arteries, traveling to various parts of the body in capillaries, and going back to the heart in veins. The human circulatory system is really a two-part system whose purpose is to bring oxygen-bearing blood to all the tissues of the body.
When the heart contracts it pushes the blood out into two major loops or cycles.
In the systemic loop, the blood circulates into the bodys systems, bringing oxygen to all its organs, structures and tissues and collecting carbon dioxide waste.
So there are actually four chambers (spaces) inside the heart.
Each top chamber is called an atrium (plural: atria). The atria are often referred to as holding chambers, while the ventricles are called pumping chambers.