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The heart in normal sinus rhythm
The heart is a muscular pump which delivers blood, containing oxygen, to the body.
It is divided into two upper chambers, or "atria", which collect blood returning via the great veins, and two lower chambers or "ventricles", which pump blood out through the aorta (main artery) and the lungs. Normally, the heart beats in a regular, organised way, at a rate of 60-100 beats per minute.
The heartbeat is driven by the “sinus node”, a clump of specialised cells, which emit electrical impulses and is situated in the atria. The sinus node is sometimes referred to as the heart's natural pacemaker. These electrical impulses spread through the atria and then into the ventricles via a connecting cable (the "AV node").
The sinus node controls the timing of the heart, according to the needs of the body. An example of this is during exercise, when the heart rate speeds up. When the heart is beating normally like this, we refer to it as "sinus rhythm", or "normal sinus rhythm".