Describe the internal structure of the human heart in detail.

(N/A) Internally, the heart is divided into four chambers: two atria and two ventricles.
The two atria are separated by an interatrial septum.
$A$ thick-walled interventricular septum separates the right and left ventricles.
The atrium and ventricle of the same side are separated by an 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 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 semilunar valves.
The valves in the heart allow the flow of blood only in one direction, i.e., from 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 those of the atria.
$A$ specialised 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 sinoatrial 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 fibres continues from the $AVN$ 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 fibres throughout the ventricular musculature of the respective sides and are called Purkinje fibres.
These fibres, along with the right and left bundle, are known as the bundle of $HIS$.
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 $min^{-1}$).