Heart - the heart is the engine at the center of the body's circulation system.it starts working even before we are born -and from the on,it beats constantly,throughout our lives.made from a special kind of muscle not found anywhere else in the body ,the hardworking heart contracts and relaxes about 70times every minute.This rhythmic pumping pushes essential blood out to the body,then fills the heart up again,ready for the next beat. The average adult man's heart weights around 340g about the same as a can of soup.a newborn baby's heart is the size of the table tennis ball.                       Heart rate - the heart rate is the number of beats per minute that the heartakes.a person's average heart rate varies according to different factors,such as age,gender,and level of fitness.      Inside the heart - each side of the heart has a small upper space,called an atrium,and a large space below,called a ventricle.during each heartbeat,blood I'd pumped from atrium to ventricle,then out of the heart . Valves open and close to make sure that the blood only flows in one direction.                        How the heart works - although a heartbeat lasts just one second,it has three stages.the rate at which the heart beats is controlled by a pacemaker located in the wall of the right atrium,which sends electrical signals to all parts of the heart.     Vital supplies - to keep the heart beating,the cells need a constant supply of fuel and oxygen.this is delivered by the coronary blood supply - the heart's own network of blood vessels,which penetrate the heart's walls to reach the muscle.2.5 billion – the number of times the heart will beat in an average lifetime.                                 Chest cross-section - chest as if it had been sliced through horizontally.the left  lung is smaller than the right lung ,as it shares space with the heart.           Tight squeeze – the heart sits in the chest, surrounded by the ribcage and between the two lungs,which take up most of the space in the chest cavity.                 Mighty muscle – a healthy adults heart is about the size of a clenched fist.it sits in the thorax and in most people the tip points towards the left side of the body. Muscle structure – the cross section of heart muscle coloured pink shows that the structure is a network of interlocking fibres.tge oval discs are mitochondria,which supply the muscle cells with the energy they need .                                                       Heart strings – the heartstrings of the tricuspid valve,which sits between the right atrium and right ventricle.these tough cords stop the valves from turning inside out when the ventricles contract.