The heart and circulation.

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Two loops

Blood brings every cell oxygen and food and takes its waste away, but only while it flows. The heart is a double pump: two sides, each with an atrium that gathers the returning blood and a ventricle that pushes it out. Valves between the chambers open only one way, so the blood does not flow back.

Show the two loops is On: the heart small in the middle, and the blood leaving it along two routes. Follow the dark blood leaving the right side, and the bright red blood leaving the left.

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Arteries and veins

Vessels that carry blood away from the heart are arteries, and vessels that bring it back are veins. Between them, inside the organs, are the capillaries: tubes so thin that blood cells pass through in single file, and oxygen and food cross their walls into the cells. The names follow the direction of flow, not the oxygen.

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Litres a minute

On every beat each ventricle pushes out an amount of blood called the stroke volume. The blood the heart pumps each minute, the cardiac output, is the heart rate times the stroke volume. Both sides pump the same amount, or blood would pile up in one of the loops.

$\text{cardiac output} = \text{heart rate} \times \text{stroke volume}$$\text{cardiac output} =$$\text{heart rate} \times \text{stroke volume}$⁠

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Once round

An adult has about five litres of blood. If the heart pumps almost five litres a minute, how long does it take all the blood to pass through it once?

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An athlete’s heart

Endurance training, such as long runs or swimming, enlarges and strengthens the left ventricle. An endurance athlete’s resting pulse can be 50 or lower, and the body still gets the blood it needs. How?

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In exercise

During exercise the muscles need much more oxygen, and the heart raises its output. Nerves and hormones speed the heart up and also make it squeeze harder, and working muscles push more blood back to it. Heart rate is yours, and Heart is fixed at Typical adult. Read the output at the foot of the chart.

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Time to fill

Every beat has two parts: the squeeze, when the ventricles push the blood out, and the relaxed part, when they fill again. The chart shows both on a time line. What happens to them when the heart beats very fast?