In order for blood to travel through the circulatory system, it must be propelled by the heart. The heart is a hollow muscular organ, made up of thick walls of muscular tissue called the myocardium. Its interior is lined by a layer of cells (endocardium), and on the outside it is covered by a double membrane (pericardium).
The heart is divided by a septum into two parts that are not communicated, right and left. Each part has two cavities:
- Auricle: in the upper part, small, and with thin walls. They receive the blood from the veins.
- Ventricle: below the atrium, with thick walls. They expel blood through the arteries.
The atrium and ventricle on each side are communicated with each other by an atrio-ventricular valve that only allows the passage of blood from the atrium to the ventricle:
- Mitral or bicuspid valve: communicates the left atrium with the left ventricle.
- Tricuspid valve: communicates the right atrium with the right ventricle.