The heart, the lungs, the blood circulation and the blood :
The heart is a muscular organ located in the left-middle chest. The walls are composed primarily of cardiac muscle (myocardium). The heart-muscle is structured as striated muscle although it is controlled autonomously. It normally weighs between 250 and 350 kg, depending on the body size, and physical form. It has four chambers and its walls are built up in three layers. The three layers are:
The outer layer is called the pericardium, or visceral pericardium.
The middle layer is called the myocardium and is composed of the striated muscle.
The inner layer is called the endocardium and is in contact with the circulating blood. Also, it merges with the inner lining (endothelium) of blood vessels.
The outer layer is in constant contact with mucus, which protect the heart from frictional resistance with the other organs. The heart and mucus lies in a sac called pericardium which is also called the heart sac.
The heart has a septum that divides the left and the right side from each other. Both the left side and the right side have two chambers each, so that makes four chambers in the heart. These cambers are called left and right atrium and ventricle. These chambers are divided by atrioventricularvalves (AV valves). These AV valves are called:
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