It's the 2 valves that you have in your heart that push it in and out. For example, get an old plastic bottle(a small one) and putit a basin full of water, squeeze it and the water goes out, release it and water goes back in; that's how it works except instead of a bottle there is the heart, and instead of your hand squeezing it there are 'pumps'(or valves).
I hope that answers your question without too much confusion.
The arteries carry blood from the heart and the veins carry blood to the heart.
veins
The aorta is the largest artery in the body. An artery transports blood away from the heart, whereas a vein transports blood toward the heart. The aorta begins at the left ventricle of the heart, and nearly all the blood pumped by the heart passes through it. The aorta is about as thick as a garden hose. The aorta loops over the heart before moving downward through the trunk of the body, in a "candy cane" shape. Many other arteries branch off from the aorta to supply blood to all the parts of the body.
Deoxygenated blood (from the body) enters the right atrium of the heart through the VENA CAVA; whereas, oxygenated blood (from the lungs) enters the left atrium of the heart through the PULMONARY VEIN.
the aorta Aorta, the largest artery in the human body.
Your heart contracts, forcing your blood out your aorta, and through your body.
Aorta
The left ventricle is the heart chamber that pumps the blood through the aorta to the rest of the body.
The aorta is an artery, starting near the heart, that pumps freshly oxygenated blood through the body.
The heart pushes blood to your body through the arteries. The blood returns through the veins. The large, flexible artery into which the heart pumps is the aorta.
The aorta is the large artery that carries blood from the left side of the heart. From there, blood is transported through the systemic circulation.
Oxygenated blood enters the heart in the pulmonary vein. It passes through the left ventricle, before exiting in the aorta.
The aorta is the largest artery in the body and consequently is the largest leaving the heart. The other artery to leave the heart is the pulmonary artery (incidentally the only artery that doesn't carry oxygenated blood).