What is the study of heart its functions and diseases called?

The study of heart its function and diseases is called Cardiology.

Cardiology is a branch of medicine that deals with the diagnosis and treatment of diseases of the heart and blood vessels. It is concerned with the prevention, diagnosis, and treatment of congenital heart defects, coronary artery disease, heart failure, arrhythmias, valvular heart disease, and other heart-related conditions. Cardiologists are medical doctors who have received additional training in the field of cardiology. They are able to perform a variety of tests, such as electrocardiograms, stress tests and echocardiograms to diagnose heart problems. They can also prescribe medications, perform procedures and surgeries, and provide lifestyle counseling to help manage heart disease.