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a small body of modified cardiac muscle cells in the upper right corner of the wall of the right atrium that acts as a pacemaker, initiating and maintaining the heartbeat.
mass extinction event of 250 million years ago, in which about 90% of all species on Earth died...
the parts and processes of the body that recognise and fight pathogens, such as viruses and...
(also called a glacial age) a long period of low temperature on Earth, during which large sheets...
is the gradual increase in the average temperature of Earth's atmosphere and oceans.
a blood vessel that returns oxygenated blood from the lungs to the left atrium of the heart.