Heart atrium (cardiac) muscle bundles (mammal), coloured scanning electron micrograph (SEM). The atrium (also known as the auricle) is one of the four chambers of the heart. Surrounding the atrial chamber is a thin-walled cardiac muscle. Shown here are cardiac muscle bundles (pectinate muscles) that make up the atrium cardiac muscle. The atrium receives blood as it returns to the heart to complete the circulatory cycle (the ventricle pumps blood out of the heart to start a new cycle). Magnification: x44 when shortest axis printed at 25 millimetres.
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