Blow Hot, Blow Cold — Class 5 EVS (CBSE)
Based on the NCERT Looking Around Grade 5 textbook. Explore how your breath changes with your body, then solve the practice set.
1. Why this chapter matters
Have you noticed that when you blow on your hands, your breath feels warm in winter but cool on a mirror? And that your heartbeat changes when you run? This chapter explores the amazing human body — how our breath and heartbeat change with our activities, how doctors use a stethoscope to listen to our heart, and how our body maintains its temperature. It builds awareness of our own bodies and how they work.
2. Breath — warm and cold
Your breath can feel warm or cold depending on how you blow.
| How you blow | What it feels like | Why? |
|---|---|---|
| Open mouth, slow breath (like fogging a mirror) | Warm | Your breath is at body temperature (37°C). The slow stream does not mix with much cool air. |
| Pursed lips, fast breath (like blowing out a candle) | Cool | The fast stream mixes with cooler air, and evaporation cools your lips. |
Experiment
Blow on a mirror: Breathe slowly on a mirror with your mouth open. It becomes foggy. This is because your warm breath contains water vapour, which condenses on the cool mirror surface.
Blow on your hand: Now purse your lips and blow fast on your hand. It feels cool because the air is moving fast and evaporating moisture from your skin.
3. Heartbeat
Your heart is a muscular organ that pumps blood throughout your body. You can feel your heartbeat by placing your hand on your chest.
How heartbeat changes with activity
| Activity | Heartbeats per minute (approximately) |
|---|---|
| Sitting still (resting) | 70-80 |
| Walking slowly | 90-100 |
| Running fast | 120-150 |
| After exercise (cooling down) | Gradually returns to normal |
Why does the heart beat faster during exercise?
When you exercise, your muscles need more oxygen and energy. The heart pumps faster to deliver more oxygen-rich blood to the muscles.
4. The stethoscope
A stethoscope is a medical instrument used to listen to sounds inside the body — especially the heartbeat and breathing.
Parts of a stethoscope
| Part | Function |
|---|---|
| Chest piece (bell/diaphragm) | Placed on the body to pick up sounds |
| Tubing | Carries sound from chest piece to ears |
| Earpieces | Fit into the doctor's ears |
What the doctor hears
- Lub-dub, lub-dub: The sound of the heart valves closing.
- A normal heartbeat has a steady rhythm.
- If the rhythm is irregular, it may indicate a health problem.
Activity
Use a cardboard tube or a rolled-up paper tube. Place one end on a friend's chest (left side) and your ear on the other end. You will hear their heartbeat!
5. Respiration
Respiration is the process of breathing — taking in oxygen and giving out carbon dioxide.
| Action | What happens |
|---|---|
| Inhale (breathe in) | Your diaphragm moves down, chest expands, air rushes into the lungs |
| Exhale (breathe out) | Your diaphragm moves up, chest contracts, air leaves the lungs |
Counting breaths
A person at rest takes about 15-20 breaths per minute. During exercise, this can increase to 30-40 breaths per minute.
6. Body temperature
| Body temperature | Normal range |
|---|---|
| Normal | 36.5°C - 37.5°C |
| Fever | Above 37.5°C |
| Hypothermia | Below 35°C |
How body temperature is measured
| Method | Where thermometer is placed |
|---|---|
| Oral (mouth) | Under the tongue |
| Axillary (armpit) | Under the arm |
| Tympanic (ear) | Inside the ear canal |
| Temporal (forehead) | On the forehead (non-contact) |
7. Activity corner
Activity 1: Count your heartbeats for 15 seconds (place two fingers on your wrist or neck). Multiply by 4 to get beats per minute. Now run on the spot for 2 minutes and count again. What changed?
Activity 2: Count your breaths for one minute while sitting still. Then count again after running. Compare the numbers.
Activity 3: Use a cardboard tube to listen to a friend's heartbeat. Try before and after exercise. What difference do you notice?
8. Common mistakes
- Mistake: Thinking the heart is on the left side of the chest Fix: The heart is in the middle of the chest, slightly tilted to the left.
- Mistake: Confusing pulse (heartbeat felt at wrist) with heartbeat in the chest Fix: They are the same thing — the pulse is the heartbeat felt at the wrist or neck.
- Mistake: Believing the stethoscope amplifies sound Fix: The stethoscope does not amplify sound. It channels sound directly from the chest piece to the ears, blocking out external noise.
9. Key facts
- Breath feels warm when blown slowly (body temperature air).
- Breath feels cool when blown fast (mixes with cool air, evaporation).
- Resting heart rate: 70-80 beats per minute.
- Exercise increases heart rate and breathing rate.
- A stethoscope is used to listen to heart and breathing sounds.
- Normal body temperature: around 37°C.
- Respiration: inhale oxygen, exhale carbon dioxide.
10. Self-test
- Why does your breath feel warm when you blow slowly?
- What does a stethoscope do?
- What happens to your heart rate when you exercise?
- What is the normal body temperature of a healthy person?
- How does your breathing change during exercise?
11. Answer key
-
Why does your breath feel warm when you blow slowly? Answer: Because your breath is at body temperature (37°C). When you blow slowly, it does not mix with much cool air.
-
What does a stethoscope do? Answer: It lets doctors listen to sounds inside the body, especially the heartbeat and breathing.
-
What happens to your heart rate when you exercise? Answer: Your heart beats faster to pump more oxygen-rich blood to your muscles.
-
What is the normal body temperature of a healthy person? Answer: Around 36.5°C to 37.5°C (approximately 37°C).
-
How does your breathing change during exercise? Answer: Your breathing becomes faster and deeper to take in more oxygen and release more carbon dioxide.
12. Quick revision
- Slow breath = warm. Fast breath = cool.
- Heart pumps blood — heart rate increases with activity.
- Normal resting heart rate: 70-80 beats per minute.
- Stethoscope: listens to heart and lungs.
- Normal body temperature: ~37°C.
- Respiration: inhale O2, exhale CO2.
- Exercise affects heart rate, breathing rate, and body temperature.
- Listen to your body — it tells you when you are tired or unwell.
