My Body — Class 3 Science (Samacheer Kalvi)
TN State Board (Samacheer Kalvi) Class 3 Science. Five organ systems and how to look after them.
1. Five systems working together
The body is organised into systems, each with a job.
| System | Job |
|---|---|
| Digestive | Breaks food down and takes in nutrients |
| Respiratory | Takes in oxygen and gives out carbon dioxide |
| Circulatory | Carries blood to every part of the body |
| Skeletal | Supports the body and protects the organs |
| Nervous | Controls everything and carries messages |
2. The path of food
Food travels a long way, and digestion happens along the whole route, not in one place.
Mouth → food pipe → stomach → small intestine → large intestine → waste leaves the body.
The mouth starts digestion: chewing breaks food up and saliva begins to soften it. The stomach churns the food with acid. The small intestine, which is six to seven metres long, is where most of the nutrients are actually absorbed. The large intestine takes back water.
So digestion does not happen only in the stomach. The stomach is one stage of several, and the small intestine does the most.
3. The path of air
Nose → windpipe → lungs.
The nose filters and warms the air. The windpipe carries it down. The lungs take in oxygen and pass out carbon dioxide.
The food pipe and the windpipe both start at the back of the mouth, and it matters that they do not get mixed up. A small flap called the epiglottis covers the windpipe whenever you swallow. When you talk and eat at once, that flap can be caught out, which is why you cough.
4. The heart and blood
The heart is a pump about the size of your fist, and it beats between 60 and 100 times a minute without ever resting.
It pushes blood through blood vessels to every part of the body. Blood carries oxygen from the lungs and digested food from the intestine to the cells that need them.
Put a hand on the left of your chest and you can feel it working.
5. Bones
The skeleton gives the body its shape and protects the soft parts inside.
| Bone | What it protects or does |
|---|---|
| Skull | Protects the brain |
| Backbone | Keeps the body upright |
| Ribs | Protect the heart and lungs |
| Femur, the thigh bone | Longest bone in the body |
Joints such as the elbow and knee let bones move against each other, and muscles attached to the bones pull them.
6. The brain and nerves
The brain sits inside the skull and is the control centre. Nerves are thread-like carriers that take messages to and from it, running through the spinal cord inside the backbone.
The brain does far more than thinking and studying. It controls breathing, heartbeat, digestion, movement and memory, and it never stops, even in sleep.
Touch something hot and your hand pulls away before you have decided to move it. The message reaches the brain and comes back in a fraction of a second.
7. Caring for the body
- Eat a balanced diet, so every system gets what it needs.
- Exercise daily, which strengthens the heart, the muscles and the bones.
- Sit and stand straight, because poor posture bends the backbone over time.
8. Worked examples
Example 1. Where is most of the food actually absorbed?
In the small intestine. The mouth and stomach prepare the food, but the small intestine, six to seven metres of it, does most of the absorbing.
Example 2. What keeps food out of the windpipe?
A flap called the epiglottis covers the windpipe whenever you swallow.
Example 3. Which bones protect the brain, and which protect the heart and lungs?
The skull protects the brain. The ribs, in twelve pairs, protect the heart and lungs.
Example 4. Name two things the brain controls that you never think about.
Breathing and heartbeat. Digestion is another.
9. Practice
- Name the five organ systems.
- Write the path of food in order.
- Write the path of air in order.
- About how many times a minute does the heart beat?
- Name the longest bone in the body.
- What is the job of the nerves?
- Give three ways to care for your body.
10. Answers
- Digestive, respiratory, circulatory, skeletal and nervous.
- Mouth, food pipe, stomach, small intestine, large intestine, then waste out.
- Nose, windpipe, lungs.
- Between 60 and 100 times.
- The femur, or thigh bone.
- They carry messages between the brain and the rest of the body.
- A balanced diet, daily exercise and good posture.
11. Summary
- Five systems: digestive, respiratory, circulatory, skeletal and nervous.
- Food goes mouth, food pipe, stomach, small intestine, large intestine.
- Digestion begins in the mouth and mostly happens in the small intestine, not the stomach.
- Air goes nose, windpipe, lungs; the epiglottis keeps food out of the windpipe.
- The heart is a fist-sized pump beating 60 to 100 times a minute.
- The skull protects the brain, the ribs protect the heart and lungs, the femur is longest.
- The brain controls breathing, heartbeat and digestion as well as thought.
