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The heart is the central organ of the circulatory system, and pumps blood into the blood vessels which carry it to all the body parts. All these events occur with each heartbeat. The average heart rate in a normal person is 72 beats per minute, so one can imagine the speed at which blood flows through the body.
Gas exchange: Respiratory organs: - essentially a system that provides oxygen to the organism. Usually, the circulatory system (if present) then delivers this to the cells, but oxygen needs to be put into the circulatory system to begin with. Thus, respiratory organs have developed. - Many different kinds: - skin - gills - trachea - lungs