The circulatory system is your body's delivery network. The heart pumps blood through a closed system of blood vessels -- arteries, capillaries, and veins -- to deliver oxygen and nutrients to every cell and carry away waste products like carbon dioxide. The heart is actually two pumps in one: the right side sends blood to the lungs to pick up oxygen (pulmonary circulation), and the left side sends oxygen-rich blood to the rest of the body (systemic circulation). Blood makes a complete loop through your body in about one minute, and your heart beats roughly 100,000 times per day to keep this cycle going.
Have students find their pulse at the wrist or neck and count heartbeats for 15 seconds, then multiply by 4 to get their resting heart rate. Compare resting rates to rates after jumping jacks. Use a two-loop diagram showing pulmonary and systemic circulation as separate circuits connected by the heart. Trace the journey of a single red blood cell through the entire circuit. Use rubber tubing of different diameters to model arteries (thick, elastic), capillaries (tiny), and veins (thinner, with valves).
You've learned that your heart pumps blood and your lungs add oxygen to it. Now you're ready to see the full picture: how the circulatory system works as an integrated delivery and waste-removal network.
Your heart is a muscular pump about the size of your fist, located in the center of your chest (slightly tilted to the left). It has four chambers: two upper chambers called atria (receiving rooms) and two lower chambers called ventricles (pumping rooms). A wall of muscle called the septum divides the heart into left and right halves, and these halves function as two separate pumps working in sync.
The right side of the heart handles pulmonary circulation -- the loop between the heart and lungs. Blood that has already circulated through the body (now low in oxygen and loaded with carbon dioxide) enters the right atrium, drops into the right ventricle, and gets pumped to the lungs through the pulmonary arteries. In the lungs, it drops off carbon dioxide and picks up fresh oxygen. Then it returns to the heart through the pulmonary veins.
The left side handles systemic circulation -- the loop between the heart and everything else. Freshly oxygenated blood from the lungs enters the left atrium, drops into the left ventricle (the heart's strongest chamber), and gets pumped out through the aorta -- the body's largest artery. From there, blood travels through progressively smaller arteries until it reaches capillaries, microscopic vessels with walls only one cell thick. In the capillaries, oxygen and nutrients pass out to cells, and carbon dioxide and waste pass in. The now-deoxygenated blood collects in small veins, which merge into larger veins, eventually returning to the right atrium through the two largest veins (superior and inferior vena cava). And the cycle starts again.
Three types of blood vessels form this network. Arteries carry blood away from the heart and have thick, elastic walls to handle the pressure of each heartbeat. Capillaries are tiny, thin-walled vessels where the actual exchange of gases and nutrients happens. Veins carry blood back toward the heart and have thinner walls plus one-way valves that prevent blood from flowing backward -- important because the blood in veins is under lower pressure.
Your heart beats about 100,000 times per day, pumping roughly 2,000 gallons of blood. During exercise, it beats faster and harder to deliver more oxygen to working muscles. You can feel this directly by checking your pulse -- each pulse beat corresponds to one heartbeat pushing blood through your arteries.