How Your Heart Works

The heart is a muscular pump that lies at the centre of the human cardiovascular system. The primary 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, culminating in two large veins known as the superior and inferior vena cava.

The heart is divided into the left and right sides. The left cavity pumps blood to the rest of the body, while the right cavity pumps blood only to the lungs. Each cavity is also divided into two upper and lower chambers, making four chambers altogether. The two upper chambers are the atria, and the two lower chambers are the ventricles. The atria receive blood flowing back to the heart, while the ventricles hold the blood 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 returned to the heart) is pushed through the tricuspid valve into the right ventricle. In turn, the right ventricle contracts, forces open the pulmonary valve and causes blood to enter the pulmonary artery. The oxygen-poor blood is then transported via this artery to the lungs, where carbon dioxide is exchanged for oxygen through the walls of 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.

Fun Facts About Your Heart

• The heart is a cone-shaped structure 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 average adult heart is about the size of a clenched fist and weighs around 300g.
• The heart is enclosed in a sac called the pericardium. The wall of the heart is made up of three layers: the epicardium (outer layer), myocardium (middle layer) and endocardium (inner layer). The epicardium and endocardium are thin protective layers. In contrast, the myocardium is a thick and muscular layer, which gives the heart strength to function as a pump.
• The average heart rate is roughly 72 beats per minute, translating to approximately 100,000 beats per day, 35 million times a year. However, this figure may 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 characteristic “lub-dub” of a heartbeat is caused by the movement of the heart valves during each cardiac cycle. In the first phase (the systole), the tricuspid and mitral valves close, producing the “lub” sound. The pulmonary and aortic valves close in the second phase (the diastole), resulting in the “dub” sound.
• The heart can exert pressure strong enough to squirt blood up to 9 metres.
• Our system of blood vessels is over 96,500km long. That’s like going around the world 2 times!

For more information, visit healthy-heart.org.