SCIENCE
The human heartbeat
How one of your hardest-working muscles keeps your blood pumping
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our heart began to beat when you were a four-week-old foetus in the womb. Over the course of the average lifetime, it will beat over 2 billion times. The heart is composed of four chambers separated into two sides. The right side receives deoxygenated blood from the body, and pumps it towards the lungs, where it picks up oxygen from the air you breathe. The oxygenated blood returns to the left side of the heart, where it is sent through the circulatory system, delivering oxygen and nutrients around the body.
The pumping action of the heart is coordinated by muscular contractions that are generated by electrical currents. These currents regularly trigger cardiac contractions known as systole. The upper chambers, or atria, which receive blood arriving at the heart, contract first. This forces blood to the lower, more muscular chambers, known as ventricles, which then contract to push blood out to the body. Following a brief stage where the heart tissue relaxes, known as diastole, the cycle begins again. The heart consists of four chambers, separated into two sides
The cardiac cycle A single heartbeat is a series of organised steps that maximise blood-pumping efficiency
Right atrium
Deoxygenated blood from the rest of the body enters the chamber via the superior and inferior vena cava.
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Atrial systole Left atrium
Oxygenated blood arrives from the lungs via the pulmonary vein and flows into this chamber.
Diastole
The cardiac muscle cells are relaxed, allowing blood to enter the ventricles freely.
The atria contract, decreasing in volume and squeezing blood through to the ventricles.
Blood enters the ventricles
The blood moves down into the ventricular chamber due to a difference in pressure.
Ventricular septum
A thick, muscular wall separates the two ventricular chambers of the heart.
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