Position
The heart lie in the throactic cavity in the space between the lungs called mediastinum. It lies (oblique) the bottom part of the heart points to the left side and presenting a base above and apex below.
Structure

The wall of the heart consists of three layers which are;
Pericardium
Myocardium
Endocardium

Pericardium- It is the outer layer of the heart and is made up of two sacs
The outer sac which consists of fibrous tissue and the inner of continues double layer of serus membrane. The outer fibrous continuous with tunica adventita of the great blood vessels above and is adherent to the diaphragm below.
Its inelastic fibrous nature prevents over distension of the heart. The outer layer of the serous membrane (parietal pericardium) lines the fibrous sac.
The inner layer (visceral pericardium) or epicardium which is continuous with the parietal pericardium is adherent to the heart muscle.

Myocardium – the myocardium is composed of specialised muscle which is found only in the heart. This muscle doesn’t work by voluntary control but is striated, like skeletal muscle. Each fibre cell has a nucleus and one or more branches. The ends of the cell and their branches are in very close contact with the ends and branches of adjacent cells. Microscopically these (joints) or intercalated discs are thicker, darker lines than striations.
Running through myocardium is also the network of specialised conducting fibres, responsible for transmitting the hearts electrical signals. The myocardium is thicker at the apex and thins out towards the base. This reflects the amount of work each chamber contributes to the pumping of the blood.
Endocardium- this lines the chamber and the valves of the heart. It is smooth, thin membrane that permits smooth flow of the blood inside the heart. It is composed of flattened epithelial cells and it is continues with the endothelium lining the blood vessels.

Interior of the Heart Structure

The heart is divided by the septum into a right and left side. The partition (septum) consists of myocardium covered by endocardium. Each side is divided by an atrioventricular valve into upper atrium and the ventricle below. The atrioventricular valves are formed by double folds of endocardium and strengthened by fibrous tissue. The right atrioventricular valve (tricuspid valve) has three flaps or cusps and the left atrioventricular valve (mitral valve) has two cusps. Flow of the blood in the heart is one way. Blood enters the heart via atria and passes into ventricle below. The valves between the atria and ventricles open and closes passively according to changes in the pressure in the chambers. They open when the pressure in the atria is greater than ventricles. During the ventricular systole (contractions) the pressure in the ventricle rises above that in the atria and the valves snap shut to prevent the backflow of the blood. The valves are preventing from opening upwards into atria by tendinous cords called chordae tendineae which extend from the inferior surface of the cusps to little projections of myocardium into the ventricles, covered with endothelium named papillary muscle.

The flow of the blood through the heart
The two major veins if the body superior and inferior venae empty the blood into the right atrium. The blood then passes via atrioventricular valve into the right ventricle and then from ventricle is pumped out into the pulmonary artery (the only artery in the body that carries deoxygenated blood). The pulmonary artery is guarded by pulmonary valve which consists of three semilunar cusps. The valve prevents the backflow of the blood into the right ventricle while the ventricular muscle relaxes. When leaving the heart, the pulmonary artery divides into two, left and right pulmonary arteries which carry blood to the lungs where the exchange of gases is carried out; Oxygen is absorbed and carbon dioxide is excreted through