Describe internal structure of heart in detail. 

Internally heart is divided into four chambers. In that there are two atriums and two ventricles.

Two atriums are separated by inter atrial septum.

Thick walled inter ventricular septum separates the right and left ventricles.

The atrium and ventricle of the same side are separated by atrio ventricular septum. Howeve

each of these septa are 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 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 allows 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 ventricles are much thicker than that 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 lower left corner of the right atrium close to the atrio ventricular septum called the atrio ventricular node ($AVN$).

A bundle of nodal fibres atrio through the atrio ventricular septa to emerge on the top of the inter ventricular 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 right and left bundle are known as bundle of $HIS.$

The nodal musculature has the ability to generate action potentials without any external stimuli. ie. it is autoexcitable.

However the number of action potentials that could be generated in a minute vary at different parts of the nodal system. The $SAN$ can generate the maximum number of action potentials. i.e. - $70-75 \mathrm{~min}^{-1}$ 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 $\min ^{-1}$ ).