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The human heart. The heart is a muscular organ located between the lungs in the centre of the chest (thorax), and is about the size of a fist. It pumps blood continuously around the body. An organism can lose consciousness within just a few seconds if the brain is deprived of blood.
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The human heart The heart is a muscular organ located between the lungs in the centre of the chest (thorax), and is about the size of a fist. It pumps blood continuously around the body. An organism can lose consciousness within just a few seconds if the brain is deprived of blood. In foetuses, the heart begins beating about 5–6 weeks after conception.
Cardiac muscle The heart mainly consists of cardiac muscle tissue, which like smooth muscle (but not skeletal muscle), contracts involuntarily. Cardiac muscle is made up of cells that are connected by cytoplasmic bridges. This enables electrical impulses to pass through the tissue. It contains large numbers of mitochondria and myoglobin molecules.