- Heart.

The human heart is a myogenic muscular organ,which pumps blood throughout the blood vessels by repeated, rhythmic contractions.

The heart is principally composed of cardiac muscle and connective tissue. Cardiac muscle is an involuntary striated muscle tissue found only in this organ and responsible for the ability of the heart to pump blood.

Heart rate:- is the number of heartbeats per unit of time, typically expressed as beats per minute (bpm). 

Heart Structure

The heart is made of four chambers, and blood flows through them in order:
 right atria>> right ventricle>> left atria>> left ventricle.

Between the right and left sides, though, the blood goes through the lungs to exchange carbon dioxide it collected in the body for oxygen it can carry to the body.

Blood enters the right atria from the venous system, and exits the left ventricle through the aorta into the arterial system.

Blood flows from the atrium to the ventricle during a heart beat based on the synchronous contraction of the heart muscle. Between each atrium and ventricle, there is a valve.

Valves are also found where the heart and aorta, and heart and pulmonary artery meet.

 Heart valves:

1. Tricuspid valve - separates the right atrium and ventricle.
2. Pulmonary valve - separates the right side of the heart from the pulmonary circulation.
3. Mitral valve - separates the left atrium and ventricle.
4. Aortic valve - separates the left side of the heart and the aorta.

The wall of the human heart is composed of three layers: (1,2)

1. The outer layer is called the epicardium, or visceral pericardium since it is also the inner wall of the pericardium.
2. The middle layer is called the myocardium and is composed of cardiac muscle which contracts.
3. The inner layer is called the endocardium and is in contact with the blood that the heart pumps.Also, it merges with the inner lining (endothelium) of blood vessels and covers heart valves.

*References:

1. Pratiyogita Darpan (April 2009). Competition Science Vision. Pratiyogita Darpan. pp. 55–. Retrieved 7 June 2012. 
2. "Heart". MedicaLook. Medicalook.com. Retrieved 2010-05-03