Heart

The heart is a muscular pump that is responsible for sending the blood around the body.

The heart is a complex organ, with 4 chambers each responsible for a different job. The right side pumps blood into the lungs to be oxygenated (infused with oxygen). The left side pumps the oxygenated blood out to the organs and tissues of the body, supplying them with vital gases, nutrients, hormones etc.

In a nutshell, the heart works as follows:
Right side: First the oxygen-depleted blood enters the heart through two large veins, the inferior and superior vena cava and flows into the right atrium. From the right atrium, it passes through the tricuspid valve into the right ventricle. The blood is then pumped through the pulmonary valve and into the lungs.
Once in the lungs, gaseous exchange takes place (carbon dioxide is removed and oxygen is added). The blood is then returned via the pulmonary vein to the left side of the heart.
Left side: The pulmonary vein empties oxygen-rich blood, from the lungs, into the left atrium. From here, the blood flows from your into your left ventricle through the open mitral valve and finally it is pumped through the aortic valve into the aorta – the blood vessel that feeds all of the other parts of your body.
The cycle repeats almost without fail through your entire life.