...     Email link to a friend Printable version How The Heart Works Inside the right atrium of the heart sits a small bundle of muscle fibres and nerves. This is the sinus or sinoatrial node, which acts as the heart’s natural pacemaker. Each electrical impulse emitted by the sinus node sparks off a chain reaction within the heart that begins with the contraction of the atria. As the right atrium contracts, the blood in this chamber (i.e. oxygen-poor blood which has been returned to the heart) is pushed through the tricuspid valve into the right ventricle. The right ventricle in turn contracts, forcing open the pulmonary valve and causing blood to enter the pulmonary artery. The oxygen-poor blood is then transported via this artery to the lungs, where carbon dioxide in the blood is exchanged for oxygen through the walls of multitudinous microscopic blood vessels known as capillaries. After oxygenation, the now bright red, oxygen-rich blood flows from the lungs through the pulmonary veins back to the heart, entering through the left atrium. At the next contraction of the left atrium, the bicuspid/mitral valve is pushed open, allowing the blood to enter the left ventricle. The left ventricle then contracts, forcing open the aortic valve such that the oxygenated blood passes out of the heart into the aorta for distribution to the rest of the body.     Back to top