It is protected by a double walled membranous bag, pericardium, enclosing the pericardial fluid. Our heart has four chambers, two relatively,
l. Small upper chambers - Atria and
2. Two larger lower chambers - Ventricles.
A thin, muscular wall called the inter atrial septum separates the right and the left atria, whereas a thick-walled, the inter-ventricular 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 atrio-ventricular septum (cupid valves).
Blood passes through valve as it leaves each chambers of heart. These valves prevent any backward flow, thus limiting flow to one direction only. The 4 heart valves includes,
- Tricuspid valve - Opening of Right atrium and the right ventricle (formed of three muscular flaps or cusps)
- Bicuspid or Mitral - Opening of Left atrium and the Left ventricle.
- Pulmonary Valve - Located between right ventricle and the pulmonary artery.
- Aortic Valve - Located between left ventricle and aorta.
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 sino-atrial node (SAN). Another mass of this tissue is seen in the lower left corner of the right atrium close to the atrio-ventricular septum called the atrio-ventricular node (AVN).
A bundle of nodal fibres, AV bundle continues from the AVN which passes 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 thin (filaments) fibres throughout the ventricular musculature of the respective sides and are called purkinje fibres; that help in distributing impulses. Bundle of His is a collection of specialized heart muscle cells for electrical conduction. The nodal musculature has the ability to generate action potentials without any external stimuli.
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 (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).
