Major Body Systems
FoundationThe body is made up of systems: the respiratory system exchanges oxygen and carbon dioxide in the alveoli, the circulatory system carries oxygen and nutrients around the body through the heart and blood vessels, and the digestive system breaks food down into nutrients that are absorbed mostly in the small intestine. The three systems work together to keep the body running.
- The respiratory system is made up of the nose, windpipe, bronchi and lungs, and oxygen and carbon dioxide are exchanged in the alveoli.
- Breathing is driven by the contracting and relaxing of the diaphragm and rib muscles, and it quickens during exercise.
- The circulatory system is made up of the heart and vessels: arteries leave the heart, veins return to it, and capillaries exchange materials.
文體小知識：人體科普說明文把身體各個系統的結構和功能分點講清楚，善用比喻令抽象的知識變得容易理解。
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1We breathe every moment of our lives. Air enters through the nose, passes down the windpipe and the bronchi, and reaches the two lungs. Inside the lungs are hundreds of millions of alveoli, shaped like clusters of tiny balloons: oxygen passes through them into the blood, while carbon dioxide leaves the blood and is breathed out. Breathing is driven by the muscles between the ribs and by the diaphragm below the lungs — when they contract, air is drawn in; when they relax, air is pushed out. A resting adult breathes about sixteen to twenty times a minute, and breathing quickens clearly during exercise.
2The heart works like a pump that never stops. It sits in the chest, slightly to the left, about the size of a fist. Each time it contracts it pushes blood into the blood vessels; a resting adult's heart beats about sixty to one hundred times a minute. There are three kinds of blood vessels: arteries carry blood away from the heart, veins carry it back, and capillaries are so fine that oxygen and nutrients can pass through them into every cell. In the blood, red blood cells carry oxygen, white blood cells fight germs, and platelets help wounds stop bleeding.
3Food begins a long journey of digestion the moment it enters the mouth: the teeth grind it up, and saliva starts to break down starch. The food moves down the oesophagus by waves of muscle and enters the stomach, where gastric juices churn and break it down. Most nutrients are actually absorbed in the small intestine, whose walls are covered with tiny villi that soak up nutrients into the blood like a sponge. The remaining waste passes into the large intestine, where water is absorbed, and the residue leaves the body through the anus.
4Although each system has its own job, they work together all the time. The nutrients absorbed by the digestive system and the oxygen taken in by the respiratory system are both delivered to every cell by the circulatory system; the cells use them to release energy, giving us the power to study, to move and to grow. To stay healthy we should exercise regularly to keep the heart and lungs strong, eat a balanced diet for smooth digestion, breathe fresh air, and avoid breathing in harmful substances such as tobacco smoke.
1Summary 重點摘要
The body is made up of systems: the respiratory system exchanges oxygen and carbon dioxide in the alveoli, the circulatory system carries oxygen and nutrients around the body through the heart and blood vessels, and the digestive system breaks food down into nutrients that are absorbed mostly in the small intestine. The three systems work together to keep the body running.
2Vocabulary cards Vocabulary
The system that brings oxygen into the body and removes carbon dioxide, including the nose, windpipe and lungs.
The system made up of the heart and blood vessels that moves blood around the body.
The system that breaks food down into nutrients and absorbs them, from the mouth to the intestines.
The millions of tiny air sacs in the lungs where oxygen and carbon dioxide are exchanged with the blood.
The sheet of muscle below the lungs that contracts and relaxes to help us breathe.
The blood vessel that carries blood away from the heart.
The blood vessel that carries blood back to the heart.
The tiniest blood vessels linking arteries and veins, where oxygen and nutrients enter the cells.
The wave-like squeezing of the digestive tract that pushes food along.
3Step by step Steps
- Step 1: Follow the path of airExample: take a deep breath while running — the air enters through the nose, travels down the windpipe and the bronchi, and finally reaches the tiny air sacs called alveoli deep inside the lungs. In the alveoli, oxygen passes through their very thin walls into the blood, while carbon dioxide moves from the blood into the alveoli and leaves the body when we breathe out. This route is short and smooth so that gas exchange happens quickly enough to supply the working muscles.
- Step 2: Feel the diaphragm at workExample: rest your hands at your sides and breathe in slowly and deeply; you will feel your ribs and belly expand, because the diaphragm contracts and moves down, making the chest cavity larger. When you breathe out, the diaphragm relaxes and rises, the chest shrinks, and the air is pushed out. The diaphragm works like an automatic bellows, and we never have to remind ourselves to breathe.
- Step 3: Know the heart and three vesselsExample: press two fingers lightly on your wrist, count the pulse for fifteen seconds and multiply by four to estimate the heartbeats per minute. Count again after skipping rope for thirty seconds — the rate rises clearly, because working muscles need more oxygen and nutrients. The heart is a pump that never stops: arteries carry blood away from it, veins return blood to it, and tiny capillaries reach into every tissue to exchange materials.
- Step 4: Learn the three parts of bloodExample: when you graze your knee, red blood flows out at first and a scab forms within a few minutes. Red blood cells contain haemoglobin and carry oxygen to every part of the body; white blood cells act like soldiers, surrounding and destroying invading germs; platelets make the blood clot to seal the wound. Each part has its own duty, so the blood can transport, defend and stop bleeding at the same time.
- Step 5: Follow food on its digestive journeyExample: after a bite of bread is chewed and mixed with saliva, the starch begins to break down into sugar. The soft ball of food passes through the throat and down the narrow tube joining the mouth to the stomach, where it stays for about two to four hours and is churned by gastric juice. Most nutrients are then absorbed in the small intestine, while the remaining waste loses water in the large intestine and finally leaves the body. Every organ does its share so the body can make full use of the nutrients.
- Step 6: See how the systems cooperateExample: nutrients from breakfast are absorbed into the blood by the small intestine, and the oxygen of every breath enters the blood at the alveoli; both depend on the heart pumping blood so they can reach the cells in the legs and the whole body. Once the cells receive oxygen and nutrients, they release energy for running, climbing stairs and concentrating in class. If any one of the three systems stops, the whole body is affected, so they must cooperate day and night.
- Step 7: Build habits that protect the bodyExample: exercising three times a week for thirty minutes makes the heart and lungs stronger; skipping breakfast can lower blood sugar in the morning and make you dizzy or lose focus; breathing in second-hand smoke damages the alveoli and interferes with gas exchange. Once we understand how the body systems work, we can explain why these habits matter and remind our family to protect their health together.
4Diagrams Diagrams
5Worked example Example
Example: after running 400 metres in PE, Ming feels his heart beating fast, his breathing quick and sweat all over. Use what you have learned to explain why his body reacts this way.
- Running makes the leg muscles demand a great deal of energy, so the cells use more oxygen and nutrients and produce more carbon dioxide.
- The respiratory system therefore speeds up: the diaphragm and rib muscles contract and relax more often, the breathing rate rises, and the alveoli exchange more gases.
- The circulatory system speeds up too: the heart beats more times per minute, carrying oxygen to the muscles faster and bringing carbon dioxide back to the lungs to be removed.
- Releasing energy also produces heat, so body temperature rises; evaporating sweat helps the body cool down.
- After a few minutes of rest the body's need for energy returns to normal, and the heart rate and breathing gradually slow down.
- Answer: Running makes the muscles need more oxygen and nutrients and produce more carbon dioxide and heat, so breathing and heartbeat quicken to strengthen transport, and sweating helps cooling — the respiratory and circulatory systems answering the body's needs.
6Common mistakes Mistakes
7Quick check Quick check
Q1After air enters through the nose, which structures does it pass on its way to the lungs?
Q2Where in the body are oxygen and carbon dioxide exchanged?
Q3What is the difference between arteries, veins and capillaries?
Q4In which organ are most nutrients absorbed?
Q5What do red blood cells, white blood cells and platelets each do?
8Recap Recap
- The respiratory system is made up of the nose, windpipe, bronchi and lungs, and oxygen and carbon dioxide are exchanged in the alveoli.
- Breathing is driven by the contracting and relaxing of the diaphragm and rib muscles, and it quickens during exercise.
- The circulatory system is made up of the heart and vessels: arteries leave the heart, veins return to it, and capillaries exchange materials.
- In the blood, red cells carry oxygen, white cells fight germs and platelets stop bleeding; nutrients are absorbed mainly in the small intestine.
- The respiratory, circulatory and digestive systems work together; regular exercise, a balanced diet and staying away from tobacco keep us healthy.