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A city needs many systems to stay healthy and grow. A system is a group of parts that work together to get a job done. No one part does everything by itself. The transportation system of a city has roads, buses, trucks, and cars to move people and supplies. The water system has drains, pipes, and pumps to move water through the city. The garbage system has trucks and dumps to keep the city clean. Many different systems meet the needs of a city's people. Like the people in a city, all of your cells have needs that must be met. How do they get food and oxygen? How do wastes get carried away? Like a city, your body has many systems to take care of it's cells. The body has a transportation system that moves food and oxygen to each cell. It then takes away cell wastes. This system is the circulatory system. It includes wastes. This system is the circulatory system. It includes the heart, blood, and blood vessels. The blood vessels are tubes that carry blood through the body. As with all systems, each part of the circulatory system has structures to help get the job done.
Arteries, capillaries, and veins
Blood vessels are like roads for blood cells. The three kinds of blood vessels are arteries, capillaries, and veins. Most arteries carry blood with lots of oxygen. Arteries are blood vessels that carry blood away from the heart to other parts of the body. Arteries have thick, muscular walls. These walls stretch as the heart pushes blood into them. Arteries branch many times into smaller and smaller tubes. The smallest arteries branch to become capillaries. A capillary is the smallest kind of blood vessel. Side by side, ten capillaries would be barely as thick as one hair! Some capillaries re so narrow that red blood cells must flow through them in a single-file line. Capillary walls are only one cell thick. Gases can pass through these thin walls. Oxygen and glucose sugar move from the blood cells in the capillaries to your body's cells. Carbon dioxide and other wastes move from cells to the blood in the capillaries. Capillaries join together to form your smallest veins. Veins are blood vessels that transport blood toward the heart. Tiny veins join many times to form larger veins. Unlike arteries and capillaries, veins have valves. Valves are flaps that act like doors to keep blood flowing in only one direction. Valves open to let blood flow to the heart. Valves close if the blood starts flowing away from the heart.
Part 2 coming soon!