Cardiac muscle
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Cardiac muscle is one of three types of vertebrate muscle tissues, with the other two being skeletal muscle and smooth muscle. It is an involuntary, striated muscle that constitutes the main tissue of the wall of the heart. The cardiac muscle (myocardium) forms a thick middle layer between the outer layer of the heart wall and the inner layer, with blood supplied via the coronary circulation. It is composed of individual cardiac muscle cells joined by intercalated discs, and encased by collagen fibers and other substances that form the extracellular matrix.
Image: Glanzstreifen
Image: 414c Cardiacmuscle
Differently oriented cardiac muscle fibers.
Intercalated discs are part of the cardiac muscle cell sarcolemma and they contain gap junctions and desmosomes.
Atrium (heart)
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The atrium is one of the two upper chambers in the heart that receives blood from the circulatory system. The blood in the atria is pumped into the heart ventricles through the atrioventricular mitral and tricuspid heart valves.
Right heart anatomy, right ventricle seen on right of illustration
Left atrial appendage shown at upper right
CT scan of the chest showing a thrombus in the left atrial appendage (left: axial plane, right: coronal plane)