Structure of the Heart
The human heart is a hollow, upside-down, pear-shaped shell. The inside of the heart is divided into four chambers, the left and right atria and the left and right ventricles, which periodically fill with blood and empty. The two atria form the curved top of the heart. The ventricles meet at the bottom of the heart to form a pointed base which points toward the left side of the chest. The left ventricle contracts most forcefully, so the heart beat is felt most strongly on the left side of the chest. A wall, called the septum, separates the right and left sides of the heart. A valve connects each atrium to the ventricle below it. The mitral or bicuspid valve connects the left atrium with the left ventricle. The tricuspid valve connects the right atrium with the right ventricle.
The Heart Wall
This consists of three layers: epicardium, myocardium, and endocardium. The epicardium is a thin outer layer which gives the surface of the heart a smooth, slippery texture. The endocardium is the smooth inner lining of the heart and is continuous with the large blood vessels to which the heart connects. The myocardium makes up the bulk of the heart and is responsible for its pumping action. The myocardium is made up of strong cardiac muscle fibres (which are cells) which are connected by electrical synapses (gap junctions) which allow muscle action potentials to spread from fibre to fibre.
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