Is an organ involved in circulatory, respiratory, and endocrine systems.

This is a muscular organ of cardiac muscle that provides blood throughout the body.

Cardiac muscle is an involuntary muscle that contracts on its own.

The heart beats (contracts) around 70 times a minute.

The heart itself is enclosed in a sac referred to as the pericardium

Two aspects of the pericardium include:

1.)   Fibrous pericardium
2.)   Serous pericardium

The heart is made up of 4 cavities or chambers

1.)   Right Atrium
2.)   Right Ventricle
3.)   Left Atrium
4.)   Right Atrium

The “Right side” of the heart deals with incoming blood from the body that is considered de-oxygenated or without oxygen. 

Conversily this states that the blood is filled with carbon dioxide. 

This system is referred to as Pulmonary Circulation.

Pulmonary Circulation

The blood enters the right atrium and enters the right ventricle through a valve called the tricuspid valve.  

It then is pumped from the right ventricle to the pulmonary arteries through the pulmonary valve. 

Within the lungs- carbon dioxide is released and oxygen enters the blood stream. 

The blood is pumped back to the heart by the pulmonary veins and enters the left atrium. 

It then enters the left ventricle via the mitral valve. 

Blood leaves the heart through the last valve via the aortic valve and is pumped throughout the body.

Coranary circulation is the vessles that supply the heart itself with blood

When blood leaves the heart it passes through arteries.

From arteries blood enters arterioles and finally into capillaries.

These are the thinner aspects of the blood vessels. Capillaries allow blood or nutrients to get into the tissues and cells.

Carbon dioxide and other wastes are collected into venules which flows into veins.

The blood is then returned to the heart.

Electrical system
How does the heart beat at a constant rate?   This answer lies within the specific cells of the heart.

There is a specific area in heart called the sinoatrial node (SA node) that regulate heartbeat.

The cells involved are called “pacemaker” cells. These cells send an electrical pulse throughout the heart that tells it to contract. 

The impulse travels the follwing route:

First, the impulse begins in the SA node
Second, It travels from the SA node to the right and left atria within miliseconds. 
Then the impulse travels to the AV node
Next to the Bundle of His
Then to the Right and Left bundle branches
Finally to the Purkinje fibers to the Right and Left ventricles.

If the SA node has lost function either temporarily or permanently then cells from the atrioventricualar node (AV node) will then become the “pacemaker” cells.