You have met the body's systems one at a time: breathing, the heart and blood, nerves, digestion and removing wastes. In a living body none of them works alone. Exercise shows it best.
Key idea
When you run, working muscles need more oxygen and sugar and make more carbon dioxide and heat. The nervous system senses the change and speeds up breathing and the heart rate, so the blood delivers more oxygen. The skin sweats to shed heat.
Why it works
Your cells release energy by respiration: sugar + oxygen → carbon dioxide + water + energy. Running muscles need much more energy each second, so they need more oxygen and sugar. The digestive system supplied the sugar, the respiratory system brings in oxygen, the circulatory system carries both, and the nervous system adjusts the pace of everything. Keeping conditions steady this way is called homeostasis.
Worked example: Read the data
A student's heart rate is 70 beats per minute at rest and 140 after running. By how many times did it increase?
- 140 ÷ 70 = 2.
Answer: It doubled (2 times).
Common mistake
Students think each organ system has its own separate job and does not affect the others. During exercise, the muscular, respiratory, circulatory and nervous systems change together, and the skin joins in to cool you. Ask of any change: which other systems must respond?