Describe the structure of the heart and explain the cardiac cycle 3.2

The heart is a pump made of four chambers, it is situated more to the left in the chest. it is a hollow muscular organ about the size of a fist. it beats constantly from birth to death and works harder than any other muscle in the body.
The heart is divided into two lobes, this is to stop oxygen rich blood from mixing with blood than contains carbon dioxide.
the heart and blood vessels comprise the cardio vascular system which circulates blood and oxygen around the body, the heart pumps around 5 litres of blood a minute and beats 100,000 times a day, it does this constantly and automatically.
Blood that contains CO2 returns to the heart after circulating around the body, the right side of the heart consists of the right atrium and right ventricle these collects and pumps the blood through the pulmonary arteries the lungs refresh the blood with a new supply of O2. Oxygen rich blood enters the left side of the heart the left atrium and left ventricle and is pumped through the aorta to the body, to supply the tissues with oxygen.
four valves in the heart keeps the blood moving around the body the correct way the tricusid valve, mistral valve pulmonary and aortic valve, they open only one way each valve opens and closes once per heart beat, the heart relaxes and contracts. the contraction is called systole and relaxed is diastole, in systole the right ventricle contracts and forces blood into the lungs and body the right ventricle contracts slightly before the left ventricle allowing the blood out of the atrium (LUB) into the ventricular then the artial ventricle node transmits an impulse to each ventricle making it contract. the ventricles then relax diastole and are filled with blood from the upper chambers (DUB), into the left and right atrium, the cycle then repeats.
the heart also receives blood from the coronary arteries these are on the surface of the heart and branch into smaller capillaries, the heart also has electrical wiring called the sinoatrial node (SA Node) to keep it beating, electrical impulses begin high in the right atrium and travel through specialized pathways to the ventricles telling the heart to pump this helps keep the heart beating in a normal regular rhythm this in turn keeps blood circulating, the continous exchange of oxygen rich blood and blood with carbon dioxide is what keeps us alive.