Describe external structure of heart. 

Heart is the mesodermally derived organ. It is situated in the thoracic cavity, in between the two lungs, slightly tilted to the left.

It has the size of a clenched fist. It is protected by a double walled membranous bag, pericardium.

It is enclosed with pericardial fluid.

Human heart has four chamber, two relatively small upper chambers are called atria and two, longer lower chambers called ventricles.

Ventricles are larger than atriums but contain same amount of blood.

Two atrium and two ventricles are separated from outside by clear cross septum. It is called cardiac septum.

Ventricles possess inter ventricle septum and lateral inter ventricle septum. That contains arteries which supply blood to heart.