9.2.1 Respiration in Humans

• We breathe in (inhale) and breathe out (exhale) air continuously.
• This is to obtain oxygen and release carbon dioxide.

The Breathing Process

• The process of inhaling and exhaling air is called breathing .
• Our body has a specific system for this — the respiratory system .
• Air enters through nostrils → nasal passages → lungs through the windpipe .
• In the lungs, the windpipe divides into branches that end in small balloon-like sacs called alveoli .
• Tiny hair and mucus in the nostrils trap dust and dirt — this is why we breathe through the nose, not the mouth.

• Breathing = process of inhaling and exhaling air.
• Respiratory system = body system for breathing and gas exchange.
• Pathway of air: nostrils → nasal passages → windpipe → lungs → alveoli.
• Nostrils filter dust and dirt using tiny hair and mucus.

🏥 Science and Society — COVID-19 and Respiratory Health

While most dust is filtered by the nostrils, tiny infectious particles can sometimes reach the lungs.
During the COVID-19 pandemic, the SARS-CoV-2 virus affected the respiratory system.
It led to breathing difficulties and serious lung problems in many people.

🔬 Activity 9.2 — Let Us Make a Model of Breathing

Materials needed

Wide transparent plastic bottle with a lid (bottom removed).
Y-shaped hollow tube.
Two deflated balloons and rubber bands.
Clay and a thin rubber sheet.

Procedure

Remove the bottom of the bottle. Make a hole in the lid.
Fix two deflated balloons to the forked end of the Y-tube. Secure with rubber bands to make airtight.
Insert the straight end of the tube through the lid. Seal with clay to make airtight.
Attach a thin rubber sheet tightly to the open base using a rubber band.
Pull the rubber sheet downward from the centre — observe the balloons.
Release the rubber sheet upward — observe the balloons again.

Observation

When rubber sheet is pulled down: the balloons inflate.
When rubber sheet is released up: the balloons deflate.

Explanation

The balloons represent the lungs. The rubber sheet represents the diaphragm.
When the diaphragm moves down (inhalation): chest space increases and air enters the lungs.
When the diaphragm moves up (exhalation): chest space decreases and air is pushed out.
The ribs also move up and outward during inhalation, and down and inward during exhalation.

→ Diaphragm movement drives breathing.

◆ Summary

• Balloons = lungs
• Rubber sheet = diaphragm
• Pull down = inhale (inflate)
• Release up = exhale (deflate)

Exhaled Air

• We inhale air from our surroundings and exhale air with more carbon dioxide.
• Inhaled air has nearly 21% oxygen and 0.04% carbon dioxide.
• Exhaled air has nearly 16–17% oxygen and 4–5% carbon dioxide .
• Not all oxygen is used up — exhaled air still contains some oxygen.

• Exhaled air has more CO₂ and less O₂ than inhaled air.
• Lime water turns milky when it reacts with CO₂ — this is the test for carbon dioxide.

🔬 Activity 9.3 — Let Us Explore What We Breathe Out

Note

To be demonstrated by the teacher.

Materials needed

Two test tubes A and B with equal amounts of freshly prepared lime water.
A syringe or pichkari.
A straw.

Procedure

In test tube A, pass air using a syringe or pichkari (this is the air we inhale).
In test tube B, repeatedly blow exhaled air through the mouth into lime water using a straw.
Observe any changes in the colour of lime water in both test tubes.

Observation

Test tube A: lime water does not change colour.
Test tube B: lime water turns milky (or cloudy).

Explanation

Lime water turns milky when it reacts with carbon dioxide.
Test tube B turned milky — exhaled air contains more carbon dioxide than inhaled air.
Test tube A did not change — the air we inhale has very little carbon dioxide (~0.04%).

→ Exhaled air has more CO₂.

◆ Summary

• Tube A = no change (inhaled air)
• Tube B = milky (exhaled air)
• Lime water + CO₂ = turns milky
• Exhaled air has more CO₂

🏥 Science and Society — Breathing Practices for a Healthy Life

Breathing exercises have been practised across different cultures in India and worldwide for centuries.
Pranayama is well known for improving respiratory health, mind relaxation, and concentration.
In Ladakh, people practise Tummo breathing — a technique that improves lung function and keeps the body warm in cold weather.
Deep breathing techniques promote well-being. Some traditions combine deep breathing with chanting for relaxation and mental clarity.

Gas Exchange in the Alveoli

• Through breathing, fresh air fills the alveoli in the lungs.
• Alveoli have thin walls surrounded by fine tubes (blood vessels) containing blood.
• Oxygen from the alveoli passes into the blood.
• Blood carries carbon dioxide from the body to the alveoli — it is released into the air.
• Blood transports oxygen to all parts of the body.

• Alveoli = tiny balloon-like sacs where gas exchange happens.
• Oxygen from alveoli enters the blood — blood carries it to the whole body.
• Carbon dioxide from the body enters the alveoli — breathed out during exhalation.

• When we eat food, the body breaks it down into simple substances like glucose (sugar).
• Oxygen helps break down glucose to release energy — this is respiration .
• Breathing is a physical process — moving air in and out of the body.
• Respiration is a chemical process — using oxygen to break down glucose inside the body.

The Circulatory System

• The body has a unique system for transporting nutrients, oxygen, and other substances — the circulatory system .
• It includes the heart, blood, and blood vessels .
• The heart pumps blood through blood vessels to all parts of the body.
• Blood delivers nutrients and oxygen, and carries away waste products.

🏥 Science and Society — Smoking Is Harmful

Smoking is extremely harmful to health. It damages the lungs and increases the risk of lung cancer and other respiratory illnesses.
It leads to persistent coughing and frequent infections.
Smoking also releases toxic chemicals into the air — non-smokers who breathe this experience passive smoking.
This is especially dangerous for children, pregnant women, and the elderly.