Human heart consists of two pumps:

Deoxygenated blood is received by the right side of the heart from the body that is then pumped to the lungs for oxygenation.

This oxygenated blood from the lungs is then received by the left side which pumps it to the body via the aorta.

The deoxygenated blood is again received by the right atrium via superior vena cava and inferior vena cava.

The human heart is a conical structure that is surrounded by two layers of the pericardium. The space between pericardium is filled with pericardial fluid.

There are two grooves visible externally longitudinal and transverse. This groove shows the position of internal division into chambers.

There are four chambers in a mammalian heart. Two atria and two ventricles.

Coronary artery and veins are present in the groove.

Superior and inferior vena cava enters the right atrium.

Four pulmonary veins enter the left atrium.

From the center portion of the heart, the aorta arises along with the pulmonary artery and its branches.

A muscular septum divides the heart into four chambers.

The upper two chambers are called atria. They have a thin wall.

The lower two chambers are called the ventricle. They have thicker walls as they are acting as a pump.

The chambers are lined by a layer of endocardium.

Walls of the heart is formed by myocardium.

The Wall of the left ventricle is thicker than the right ventricle as it needs high pressure to pump blood around the body.

The sequence of one heartbeat is called the cardiac cycle. It consists of two phases:

Systoleà contraction of heart muscles.

Diastoleàrelaxation of the heart muscle.

During atrial diastole, blood is filled in both atria.

The atrioventricular valve is closed.

During atrial systole the atria contracts.

This creates pressure inside the atria.

Thus the atrioventricular valve opens and blood flows from atria to ventricles.

When the ventricle is filled with blood this distends the heart wall.

This causes contraction of the ventricle and initiates ventricular systole.

The atrioventricular valve shuts and the blood is pushed into the aorta and pulmonary vessels.

The closure of the atrioventricular valve produces the first heart sound LUB.

During ventricular diastole, the ventricle relaxes. The pressure inside the ventricles falls.

The blood is filled inside the aorta and pulmonary vessels. This causes an increase in pressure in the 

vessels.

This causes closure of aortic and pulmonary valves which produces second heart sound DUB.

Cardiac muscles are myogenic that is the impulse for contraction is generated with the cardiac tissue. Thus, it does not depend upon the electrical stimuli from nerves.

At the top of the atrium, a patch of tissue is presently called a sinoatrial node or SA node. The cells in SA node contracts creates an action potential. The potential travels down from the SA node. This causes contraction in the atrium.

Below the SA node at the interventricular septum, another tissue is presently called the AV node. The impulse generated at the SA node reaches the AV node. The impulse is delayed here for a while. This ensures that the ventricular contraction begins after atrial contraction ends.

From the AV node, the action potential travels along the septum and the wall of the ventricle through a bundle of His and Purkinje fibers.

This causes the contraction of ventricles.

When this cycle ends the contraction of the SA node is delayed for a while. This is the phase of diastole.