The heart is a muscular pump which lies at the centre of the human cardiovascular system. The main function of the heart is to circulate blood throughout the body. Oxygen-rich blood flows out of the heart through the aorta, which then branches into smaller arteries carrying blood to all parts of the body. Conversely, oxygen-poor blood is transported back to the heart through a network of veins which culminate in two large veins known as the superior and inferior vena cava respectively.
The heart is divided vertically into two cavities by a muscular wall called the septum. The left cavity pumps blood throughout the body, while the right cavity pumps blood only to the lungs. Each cavity is in turn divided horizontally into two chambers, making a total of four chambers altogether. The two upper chambers are known as the atria, and the two lower chambers as the ventricles. The atria receive blood flowing back to the heart, while the ventricles hold blood that is to be pumped out of the heart.
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.