The heart has 3 layers of tissue:

• Pericardium – The thin outer layer that protects the heart
• Myocardium – a thick muscular middle layer that contracts and pumps out blood
• Endocardium – a thin inner lining

Inside the heart, there are four chambers- the upper two are right and left atria and lower two are right and left ventricles. The atria receive the blood that comes to the heart and the ventricles pump blood out from the heart. The right and left side of the heart is separated by a septum.

The heart also has 4 valves which allow unidirectional flow of blood every time the heart beats.

• the tricuspid valve is between the right atrium and right ventricle.
• The pulmonary valve is between the right ventricle and the pulmonary artery.
• The mitral valve is between the left atrium and left ventricle.
• The aortic valve is between the left ventricle and the aorta.

The electrical system of heart:

The pumping action of the heart is controlled by the heart’s electric system. As an electrical impulse moves through the heart muscles, the various chamber of heart coordinate and pump out blood.

An electrical stimulus is generated by the sinus node (also called the natural pacemaker) which is located in the right atria of the heart. This stimulus activates the atrial muscles. The electrical stimulus then travels down through the conduction pathways and causes the heart's ventricles to contract and pump out blood. The atria contract first, just a few milliseconds before the ventricles. This allows the blood from the atria to enter into the ventricles which is pumped out later.