How Your Heart Works

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.

How your 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.

Did you know?

  • The heart is a cone-shaped structure which lies in the middle of the chest, slightly to the left, behind the breastbone and between the lungs.
  • The heart of a healthy 70 kg adult pumps about 7,200 litres of blood daily at a rate of approximately 5 litres per minute.
  • The heart is enclosed in a sac called the pericardium. The wall of the heart is made up of three layers known as the epicardium (outer layer), myocardium (middle layer), and endocardium (inner layer) respectively. The epicardium and endocardium are thin protective layers. In contrast, the myocardium is a thick, muscular layer which provides the strength for the heart to function as a pump.
  • The average heart rate is roughly 72 beats per minute, which translates into approximately 100,000 beats per day. This figure may, however, differ widely among people of different age groups. For instance, the heart rate may be as high as 120 beats per minute in infants, and as low as 60 beats per minute in adults.
  • The average adult human heart is about the size of a clenched fist and weighs about 300 g.
  • The characteristic “lub-dub” of a heartbeat is caused by the movement of the heart valves during each cardiac cycle. In the first phase, which is known as the systole, the tricuspid andmitral valves close, producing the “lub” sound. In the second phase (the diastole), the pulmonary and aortic valves close, resulting in the “dub” sound.