Breathing and Digestion
AdvancedAir travels through the windpipe and bronchi to the lungs for gas exchange; food passes through the stomach and intestines to become nutrients. These two systems work day and night to power the body.
- The air route: nose → windpipe → bronchi → lungs.
- The lungs are like sponges; gases are exchanged with the blood there.
- The food route: mouth → gullet → stomach → small intestine → large intestine.
文體小知識：說明文把身體的構造和功能講得清清楚楚。這一篇帶你跟隨空氣和食物，走進呼吸系統和消化系統一看究竟。
§Text Passage
1At the sports day, Ming finished the sixty-metre dash in one go. When he stopped, his chest rose and fell and he panted for a while. Two hard-working systems had been serving his body: the respiratory system brings oxygen in and sends carbon dioxide out, while the digestive system turns food into nutrients for the whole body.
2Air drawn in through the nose passes the throat and enters the windpipe. The windpipe is a tube held open by rings of cartilage. Lower down it splits into two bronchi, one leading to each lung. The lungs are like two soft sponges filled with countless tiny air sacs, where oxygen and carbon dioxide are exchanged with the blood.
3When we run, our muscles need more energy, so they need more oxygen. Breathing automatically becomes faster and deeper, sending plenty of oxygen into the blood and carrying away extra carbon dioxide. That is why we pant after running — a normal reaction as the body restocks oxygen, and it settles down after a rest.
4The journey of food begins in the mouth: teeth bite and chew the food, and saliva starts digesting starch. Swallowed food slides down the gullet into the stomach. The stomach is a muscular bag that squeezes food into a paste and adds gastric juice, beginning to break down the proteins.
5Next, the paste moves bit by bit into the small intestine, which is narrow, long and coiled. Its walls are covered with tiny villi, and most nutrients are absorbed there into the blood and carried all over the body. The leftovers pass into the large intestine, where water is absorbed; the rest leaves the body as faeces.
6The two systems work together: oxygen helps the body turn nutrients into energy for moving, learning and growing. Exercise often to train the heart and lungs; chew slowly at meals and rest a little after eating before running about. Take good care of these two tireless body managers!
1Summary 重點摘要
Air travels through the windpipe and bronchi to the lungs for gas exchange; food passes through the stomach and intestines to become nutrients. These two systems work day and night to power the body.
2Vocabulary cards Vocabulary
The system of windpipe, bronchi and lungs that brings oxygen in and sends carbon dioxide out.
The tube joining the throat to the lungs; rings of cartilage keep it open for air.
The two tubes branching from the windpipe, carrying air into the left and right lungs.
Spongy breathing organs where oxygen and carbon dioxide are exchanged with the blood.
The system of mouth, stomach and intestines that turns food into nutrients.
A muscular bag that grinds food into a paste and mixes it with gastric juice.
The long, coiled tube where most nutrients are absorbed into the blood.
The tube that absorbs water from leftover waste and forms faeces.
A gas making up about one fifth of air, needed to release energy in the body.
Tiny useful parts of digested food that the body uses to grow and move.
3Step by step Steps
- Step 1: Follow the air into the respiratory systemBreathing begins with an inhale. Air enters through the nose, where nose hairs and mucus filter out dust; it then passes the throat and slides into the windpipe. Rings of cartilage keep the windpipe open, like a hose with steel rings inside. In the chest the windpipe splits into two bronchi, one for each lung. Following this route, the air travels deep into the body, ready to exchange gases with the blood. Knowing the route helps us see how far each breath travels, and why the nose is our first filter.
- Step 2: See how the lungs exchange gasesThe lungs are the main characters of the respiratory system, one on each side, like two soft sponges. Inside are billions of tiny air sacs wrapped in fine blood vessels. When we inhale, oxygen passes through the thin walls of the sacs into the blood; carbon dioxide in the blood moves into the sacs and leaves with each breath out. This is gas exchange. The blood carries fresh oxygen to every cell, and only with oxygen can cells release energy to stay alive, so the lungs can never stop working, not even for a minute.
- Step 3: Understand why we pant after runningWhen we run, the leg muscles burn lots of energy and need more oxygen. The body automatically orders breathing to become faster and deeper, and the heart beats quicker, pushing oxygen into the blood and sweeping out extra carbon dioxide. So the chest heaves and we pant — not an illness, but the body restocking oxygen in time. After exercise, walk slowly and breathe deeply to let the breath settle naturally; do not sit down at once or gulp icy water. Treat your respiratory system kindly.
- Step 4: Follow the food into the digestive systemThe first stop of digestion is the mouth. Teeth cut, tear and grind the food, and saliva mixes in and starts digesting starch — so chew slowly. After swallowing, food travels down the gullet, pushed by wave after wave of muscle into the stomach. The stomach is an elastic muscular bag that stretches when full; it squeezes again and again, grinding the food into a paste, while gastric juice begins to break down the proteins. Food stays in the stomach for a while before passing bit by bit into the small intestine.
- Step 5: See how the small intestine absorbs nutrientsThe small intestine is the home ground of absorption. It is long and coiled, and its walls are lined with tiny villi that greatly enlarge the absorbing surface. Digestive juices from the liver and pancreas flow in too, breaking food down completely into absorbable nutrients. The nutrients pass through the villi into the blood and travel all over the body, to be used or stored by the cells. Though called small, the small intestine is over five metres long — the longest part of the digestive system and the most important for absorption.
- Step 6: The large intestine finishes, and we take careThe leftover waste finally enters the large intestine. The large intestine absorbs the extra water from the waste, which gradually firms up into faeces and leaves through the anus. To care for the digestive system, drink plenty of water and eat fruit and vegetables so bowel movements stay smooth; to care for the respiratory system, do aerobic exercise to train the heart and lungs, and stay away from second-hand smoke. Oxygen helps the body turn nutrients into energy, and the two systems work together day and night, giving us energy for school and play.
5Worked example Example
Example: After a 400-metre run in PE, Ming panted for minutes before settling down. Three hours after lunch, his tummy rumbled with hunger. Use the functions of the respiratory and digestive systems to explain why these two things happen.
- While running, the leg muscles keep contracting and burn lots of energy; releasing energy needs oxygen, so the respiratory system works harder.
- Breathing becomes faster and deeper; air floods through the windpipe and bronchi into the lungs, gas exchange speeds up, taking in more oxygen and letting out more carbon dioxide — hence the heaving chest and panting.
- After resting, the muscles need less energy, the demand for oxygen falls, breathing calms down and the panting stops.
- The lunch food was ground in the stomach and absorbed in the small intestine, and the nutrients travelled in the blood; three hours later the nutrients run low, blood sugar drops, and the brain sends a hunger signal reminding us to eat again.
- So panting is the respiratory system restocking oxygen, and hunger is the digestive system reminding us to refill nutrients — both systems are working properly.
- Answer: Panting after the 400-metre run happens because the muscles burned lots of energy; the respiratory system breathed faster and deeper, sending more oxygen through the windpipe and bronchi into the lungs and blood while removing carbon dioxide, and it settles after rest. Hunger three hours after lunch happens because the nutrients have been digested, absorbed and used up; the brain sends a hunger signal to remind us to eat. Both show that the respiratory and digestive systems are working normally.
6Common mistakes Mistakes
7Quick check Quick check
Q1After the windpipe, which two tubes does air enter?
Q2Which organ carries out gas exchange?
Q3Where are most nutrients absorbed?
Q4What does the stomach do with food?
Q5When we pant after running, what is the body restocking?
8Recap Recap
- The air route: nose → windpipe → bronchi → lungs.
- The lungs are like sponges; gases are exchanged with the blood there.
- The food route: mouth → gullet → stomach → small intestine → large intestine.
- The small intestine absorbs most nutrients; the large intestine absorbs water.
- Panting restocks oxygen; chewing slowly helps digestion.