How the Circulatory System Works: A Guide for Grades 5–8

The circulatory system is the body's delivery service. It carries oxygen and nutrients to every cell and takes away waste such as carbon dioxide. It has three parts: the heart (the pump), the blood vessels (the roads) and the blood (the delivery trucks).

The heart

The heart is a muscle about the size of your fist. It has four chambers: two upper chambers called atria that receive blood, and two lower chambers called ventricles that pump it out. Valves between the chambers stop blood flowing backwards — the "lub-dub" sound of a heartbeat is the valves closing.

Blood vessels

• Arteries carry blood away from the heart. They have thick, muscular walls because the blood inside is under high pressure. The largest is the aorta.
• Veins carry blood back to the heart. Their walls are thinner, and many have valves to keep blood moving the right way.
• Capillaries are tiny vessels, often one cell thick, that connect arteries to veins. This is where oxygen and nutrients pass into the cells.

Memory trick: Arteries go Away from the heart.

Blood

• Red blood cells carry oxygen, using a protein called hemoglobin.
• White blood cells fight infection.
• Platelets help blood clot when you're cut.
• Plasma is the yellowish liquid that carries everything else.

The double loop

Blood actually makes two trips. The right side of the heart pumps blood to the lungs to pick up oxygen and drop off carbon dioxide. That oxygen-rich blood returns to the left side, which pumps it out to the rest of the body. This is why the circulatory and respiratory systems are always taught together.

Questions students often get wrong

• "Veins carry blue blood." Blood is always red — darker red when it has less oxygen. Veins only look blue through the skin.
• "Arteries always carry oxygen-rich blood." Almost — the pulmonary artery carries oxygen-poor blood from the heart to the lungs. Arteries are defined by direction (away from the heart), not by oxygen.
• "The heart is on the left side of the chest." It sits near the middle, tilted slightly to the left.

A quick experiment

Have students find their pulse at the wrist and count the beats in 15 seconds, then multiply by 4. Then do 30 jumping jacks and measure again. The rise shows the heart working harder to deliver more oxygen to working muscles.