The heart, blood and staying healthy
Biology · Key Stage 2 · The School
A pump and a loop
The HEART is a muscle that pumps without ever resting. ARTERIES carry blood AWAY from the heart under pressure, which is why they have thick walls and why you can feel a pulse. VEINS bring it back at low pressure and have valves so it cannot run backwards. CAPILLARIES are one cell thick — thin enough for oxygen to pass straight through into the tissue.
What the blood is carrying
Oxygen from the lungs and nutrients from the small intestine go OUT to every cell; carbon dioxide and waste come BACK. That is why you breathe harder when you run: the muscles are using oxygen faster, so the heart beats faster to deliver it and to clear the carbon dioxide away.
What helps and what harms
Exercise strengthens the heart muscle, so it pushes more blood per beat and can beat more slowly at rest — a low resting pulse is a sign of fitness, not weakness. A diet high in saturated fat narrows arteries over years. Smoking damages the lungs and the vessels together, and alcohol affects the liver and the brain. None of this is instant, which is exactly what makes it easy to ignore.
Start in the right side of the heart, low on oxygen. It is pumped to the LUNGS, picks up oxygen and releases carbon dioxide, and comes back to the left side of the heart bright red. From there it is pumped out to the body — say to a muscle in your leg — where it hands over its oxygen. Now dark red, it returns by vein to the right side of the heart. One complete circuit, two trips through the heart.