Supporting this is the , a collection of glands that secrete hormones. These chemical messengers regulate everything from our growth and metabolism to our sleep cycles and moods. The Shield: The Immune and Integumentary Systems
The heart beats about 100,000 times per day, pumping 2,000 gallons of blood through 60,000 miles of blood vessels—enough to circle the Earth twice. Blood delivers oxygen, nutrients, and immune cells while hauling away waste like carbon dioxide and lactic acid. The Human Body