What Is a Heart Attack?<br > How the heart works<br > You don t need a science degree - or even an O-level<br > in biology - to understand the mechanics of a heart<br > attack. It really is very simple.<br > First， a word about the circulation of the blood. The<br > heart is a pump which sends blood on its way through<br > the body. Blood goes to the lungs to pick up oxygen<br > from the air we breathe. This bright red oxygenated<br > blood is then conveyed round the body in the arteries<br > and returns to the heart， with the oxygen used up， via<br > the veins as dark red， de-oxygenated blood. Then the<br > cycle starts all over again.<br > Figure 1 is a drawing of the four chambers that make<br > up the human heart， showing the direction of the blood<br > flow.<br > The walls of these chambers are composed of a<br > special kind of muscle which does not tire. Each side<br > of the heart， left and right， has a thin-walled chamber<br > called an atrium， which lies over a much bigger and<br > stronger chamber called a ventricle. Each atrium is<br > separated from its corresponding ventricle， which en-<br > sures that the blood flows in one direction only. The<br >i right atrium is separated from the right ventricle by the<br > tricuspid valve; while the left atrium is separated from<br > the left ventricle by what is called the mitral valve，<br > because it looks like a bishop s mitre. Notice also that<br >