The human circulatory system, also called the blood vascular system consists of a muscular chambered heart, a network of closed branching blood vessels, and blood, the fluid which is circulated.
- The ==heart, the meso dermally derived organ, is situated in the thoracic cavity, in between the two lungs==, slightly tilted to the left.
- It has the size of a clenched fist.
- It is protected by a double-walled membranous bag, ==pericardium, enclosing the pericardial fluid.==
- Our heart has four chambers, two relatively small upper chambers called atria and two larger lower chambers called ventricles.
- A thin, muscular wall called the ==interatrial septum== separates the right and the left atria, whereas ==a thick-walled, interventricular septum==, separates the left and the right ventricles.
- The atrium and the ventricle of the same side are also separated by a thick fibrous tissue called the ==atrioventricular septum.==
- However, each of these septa is provided with an opening through which the two chambers of the same side are connected.
The opening between the right atrium and the right ventricle is guarded by a valve formed of three muscular flaps or cusps, ==the tricuspid valve, whereas a bicuspid or mitral valve guards== the opening between the left atrium and the left ventricle.
- The openings of the right and the left ventricles into the ==pulmonary artery and the aorta respectively are provided with the semilunar valves.==
- The valves in the heart allow the flow of blood only in one direction, i.e., from the atria to the ventricles and from the ventricles to the pulmonary artery or aorta.
- These valves prevent any backward flow.

The entire heart is made of ==cardiac muscles==.
- The walls of the ventricles are much thicker than that of the atria.
- A specialized ==cardiac musculature called the nodal tissue== is also distributed in the heart.
- A patch of this tissue is present in the right upper corner of the right atrium called the ==sino-atrial node (SAN).==
- Another mass of this tissue is seen in the lower left corner of the right atrium close to the atrioventricular septum called the ==atrioventricular node (AVN).==
- A bundle of ==nodal fibers, the atrioventricular bundle (AV bundle) continues from the AVN which passes through the atrioventricular septa to emerge on the top of the interventricular septum== and immediately divides into a right and left bundle.
- These branches give rise to minute fibers throughout the ventricular musculature of the respective sides and are called ==Purkinje fibers.==
- The nodal musculature has the ability to generate action potentials without any external stimuli, i.e., ==it is autoexcitable.==
- However, the number of ==action potentials that could be generated in a minute== varies at different parts of the nodal system.
The ==SAN can generate the maximum number of action potentials, i.e., 70-75 min–1==, and is responsible for initiating and maintaining the ==rhythmic contractile activity== of the heart.
- Therefore, it is called the ==pacemaker.==
Our heart normally beats 70-75 times in a minute (==average 72 beats min–1==).