STEVE GSCHMEISSNER / SCIENCE PHOTO LIBRARY STEVE GSCHMEISSNER / SCIENCE PHOTO LIBRARY
Dissected heart. Light micrograph of a section through a whole heart, showing the chambers and some blood. The heart is a hollow muscular organ that pumps blood around the body. The blood (red) arrives from the body, and is then pumped between the heart and the lungs, and back to the heart before being pumped around the body again. The heart has four chambers, the two most powerful being the lower chambers, the ventricles, which have the thick walls (purple) seen here. The smaller upper chambers, called atria (not clearly seen), pump blood from one side of the heart to the other. Inside the heart are structures called valves, which regulate blood flow between the chambers. Magnification: x33 when printed 10 centimetres tall.
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