One-way valves control the flow of blood through these four chambers. Smaller arteries branch off from the aorta and carry blood to organs and tissue. The time between the beginning of one heartbeat to the beginning of the next is called the cardiac cycle . The heart muscle contracts when these electrical impulses run across it. If the natural pacemaker ceases to function properly, an artificial pacemaker can be used to regulate the heartbeat.
Blood vessels Blood vessels are the series of arteries and veins which carry blood through the body.
Arteries carry oxygen-rich blood from the heart to the various parts of the body. Veins carry blood from the organs and tissues to the heart where it is pumped to the lungs for oxygen to be added. It is here that carbon dioxide and waste products are exchanged for oxygen and nutrients. The blood then starts its return journey to the heart via a series of progressively larger veins. Coronary arteries , located on the outside of the heart, are important because they supply the heart muscle with oxygen.