The systems work together.

Step 1 of 6

One chain

In the earlier Lessons you met three systems one at a time: the digestive system takes food in, the breathing system takes oxygen in, and circulation shares both out. But none of them works alone. For a leg muscle to get oxygen takes a whole chain: the diaphragm, the air sacs, the haemoglobin, the heart, the arteries and the capillaries inside the muscle.

Peel back the layers is at Organs. With Light up a system you can light up each system on its own. Try a few, and think about which one reaches everywhere in the body.

Step 2 of 6

Blood goes where it is needed

In exercise the heart does not only pump more blood: the body also redirects it. Small muscles in the walls of the small arteries widen the vessels in the organs that are working and narrow them in organs that can wait. The chart shows each minute’s blood as one bar, split by organ, over the same bar at rest, dashed. Time since eating is fixed, and Exercise effort is yours.

Step 3 of 6

The muscles’ share

Peel back the layers is at Muscles. At half of full effort the heart pumps two and a half times as much as at rest, and most of the extra goes to the muscles. Work out what share of all the blood they get.

Step 4 of 6

Half the blood

At rest the muscles get about a fifth of the blood, though they are nearly half the body’s weight. As the effort grows, so does their share. Find the effort at which they get exactly half.

Step 5 of 6

After a meal

The digestive system can ask for blood too. Time since eating now stands at six hours, and the gut is almost at rest. Exercise effort is fixed at rest.

Step 6 of 6

All at once

Exercise effort stands at 75%: a fast run. Light up the systems one by one and compare them on the chart with the dashed bar at rest. Some systems step up their work, and some give it up until the exercise is over.