Respiration is the process by which nutrients are turned into usable energy. Inside the body, oxygen breaks down glucose to release energy, and carbon dioxide and water are formed.

What does the diaphragm do in breathing?

The diaphragm is a dome-shaped muscle below the lungs. When it moves down, the chest gets more space and air rushes in; when it moves up, the space shrinks and air is pushed out. So it helps in inhalation and exhalation.

How do frogs breathe?

As tadpoles they breathe with gills. Adult frogs use lungs on land, and when they are in water they exchange gases through their skin.

Respiration in Animals | Class 7 Science

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Breathing and the respiratory system

We can live for days without food and a day or two without water, but usually only a few minutes without breathing. Breathing means inhaling air into the lungs and exhaling it out. It happens through the respiratory system.

The respiratory system: nostrils, windpipe dividing into two branches, lungs with fine tubes ending in alveoli, the rib cage and the diaphragm — The path of air

Air enters through the two nostrils into the nasal passages, where tiny hair and mucus trap dust and dirt; that is why breathing through the nose is better. The air goes down the windpipe, which divides into two branches, one for each lung. Inside the lungs the branches divide again and again, ending in tiny balloon-like sacs, the alveoli. The rib cage protects the lungs.

How air moves in and out

A bottle model in which pulling a rubber sheet down fills two balloons, and the chest with ribs and diaphragm during breathing in and out — Model and real chest

In Activity 9.2, a bottle has two balloons on a Y-shaped tube and a rubber sheet over its base. Pulling the sheet down makes the balloons inflate; letting it go up makes them deflate. The balloons stand for the lungs and the sheet for the diaphragm, the dome-shaped muscle under the lungs. When we inhale, the ribs move up and outwards and the diaphragm moves down, so the chest has more space and air flows in. When we exhale, the diaphragm moves up and the ribs move down and inwards, pushing air out.

What we breathe out

In Activity 9.3, room air pushed through lime water with a pichkari leaves it clear, but breath blown through a straw turns it milky. Lime water turns milky with carbon dioxide, so exhaled air has more carbon dioxide.

Lime water stays clear with room air and turns milky with breathed-out air — Carbon dioxide test

Gas	Inhaled air → exhaled air (nearly)
Oxygen	21% → 16–17%
Carbon dioxide	0.04% → 4–5%

So not all the oxygen we breathe in is used up.

Gas exchange and respiration

The alveoli have thin walls wrapped in fine blood vessels. Oxygen passes from the air in the alveoli into the blood, and carbon dioxide passes from the blood into the alveoli to be breathed out. The heart pumps the blood through blood vessels, the circulatory system, carrying oxygen and nutrients everywhere.

Oxygen moving into the blood and carbon dioxide moving out at an alveolus, with the respiration equation — Exchange and energy

Inside the body, oxygen breaks down glucose from digested food to release energy. This is respiration:

Glucose + oxygen → carbon dioxide + water + energy

The energy lets us walk, run, play and think. Breathing is a physical process; respiration is a chemical process.

Looking after our lungs

Smoking damages the lungs and raises the risk of lung cancer, and others nearby suffer from passive smoking. Some infections, such as the virus that caused COVID-19, attack the respiratory system. Breathing practices such as pranayama, and Tummo breathing in Ladakh, have long been used to improve breathing and relaxation.

How other animals breathe

Cards: lungs in many land animals, gills in fish, gills then lungs and skin in frogs, moist skin in earthworms — Many ways to breathe

Cows, goats, lions, elephants, birds, lizards and snakes all breathe with lungs, though their lungs differ in structure. Most aquatic animals, such as fish, have gills, rich in blood vessels, that take in oxygen dissolved in water. Frogs use gills as tadpoles, and as adults use lungs on land and their skin in water. Earthworms exchange gases through their moist skin. Each way suits the animal’s habitat.

Why it matters

Every movement and every thought needs energy, and that energy comes from respiration, which depends on breathing. Review how food is broken down in nutrition and digestion in animals. Answers to the textbook questions are in the NCERT solutions for Chapter 9; see all of Chapter 9 notes.

Respiration in animals