Maarg Manthan · Topic 3.9

Respiratory System: External and Internal Respiration in Humans, Animals, Plants and Insects

Science & Technology › Human Physiology – Organ Systems · Topic 3.9

The respiratory system brings oxygen into the body and removes carbon dioxide. In humans, air enters through the nose, passes through the pharynx, larynx and trachea, and divides into two bronchi that branch into smaller bronchioles ending in tiny air sacs called alveoli, where gases are exchanged with the blood. Breathing is driven by the diaphragm and intercostal muscles. External respiration is the exchange of gases between the lungs and the blood, and internal respiration is the exchange between blood and tissues. Oxygen is carried mostly by haemoglobin in red cells, and carbon dioxide is carried mainly as bicarbonate. Animals have gills, tracheae, skin or lungs for gas exchange, and plants exchange gases through stomata and lenticels.

5 min read · UPSC Prelims · Science & Technology · Human Physiology – Organ Systems

At a Glance

Gas exchange site Alveoli
Breathing muscles Diaphragm and intercostals
Oxygen carrier Haemoglobin
Insects Tracheae and spiracles
Fishes Gills
Exam link UPSC Prelims, Science & Technology: Human Physiology – Organ Systems

Where Does It Fit?

This is the ninth topic of the macro on human physiology and organ systems. It connects breathing to the cellular respiration of the first macro and prepares for the transport system. Previous topic: Endocrine System: Hypothalamus, Pineal Body, Pituitary Gland, Thyroid, Adrenal Glands, Reproductive Glands. Next topic: Transport System: Blood Vascular System and Lymphatic System.

How Does Breathing Work in Humans?

Part	Function
Nose and nasal cavity	Filter, warm and moisten air with hairs and mucus
Pharynx and larynx	Pass air, with the voice box in the larynx and the epiglottis that covers the windpipe during swallowing
Trachea and bronchi	The windpipe with C-shaped cartilage rings splits into two bronchi, which enter the lungs
Bronchioles and alveoli	Air sacs with thin walls surrounded by capillaries. Hundreds of millions of alveoli give a very large surface for gas exchange
Diaphragm and ribs	Inhalation: the diaphragm contracts and flattens and the ribs move up and out, so the chest volume rises and air enters. Exhalation: they relax and air is pushed out
Lungs and pleura	The right lung has three lobes and the left has two, and a double membrane, the pleura, covers them

How Are Gases Carried?

Oxygen: about 97 per cent is carried by haemoglobin as oxyhaemoglobin, and the rest is dissolved in plasma.
Carbon dioxide: about 70 per cent is carried as bicarbonate, 20 to 25 per cent bound to haemoglobin and a small part dissolved.
Control of breathing: the respiratory centre in the brainstem sets the rhythm, and it responds mainly to carbon dioxide levels in the blood.
Rate and volumes: a resting adult breathes about 12 to 20 times a minute, and the tidal volume is about 500 millilitres per breath.
Carbon monoxide: binds to haemoglobin much more strongly than oxygen does, which makes it poisonous.
High altitude: lower oxygen causes faster breathing and more red blood cells in the long run.

How Do Other Organisms Respire?

Amoeba and sponges: gases diffuse across the cell surface or body wall.
Earthworm: breathes through moist skin.
Insects: a network of air tubes called tracheae opens to the outside through spiracles and delivers air directly to the tissues.
Fishes: gills take dissolved oxygen from water in a counter-current exchange.
Frogs: use lungs, skin and the lining of the mouth, and tadpoles use gills.
Birds: have lungs with air sacs, which give a one-way flow of air through the lungs.
Plants: exchange gases through stomata in leaves, lenticels in stems and the surface of roots, with no special breathing organ. Mangrove plants have pneumatophores.

What Else Should You Remember and Why Does It Matter?

Disorders: asthma, chronic bronchitis and emphysema, which are included in chronic obstructive pulmonary disease, pneumonia, tuberculosis and lung cancer.
Occupational diseases: silicosis from silica dust, asbestosis from asbestos fibres and black lung in coal miners.
Air pollution link: fine particles of PM2.5 reach the alveoli and enter the blood, and India has the National Clean Air Programme, as in the study of air quality.
Smoking: damages the airways and is the leading cause of lung disease.
Respiration and breathing: breathing moves air, while cellular respiration releases energy in cells, as in Cellular Respiration and Energy Metabolism (ATP Production).
Artificial respiration: methods such as CPR, which is chest compression with rescue breaths, and ventilators help when breathing stops, and oxygen concentrators were widely used during COVID-19.
Exam pattern: matching animals with their respiratory organs and questions on the diaphragm movement.
Respiratory system in summary: the respiratory system of a human also helps in speech, smell and keeping the body cool, and the nose warms and cleans the air before it reaches the lungs.
Lung capacity: vital capacity, the largest volume of air that can be breathed out after a deep breath, is about 3.5 to 4.5 litres in adults, and some air always remains in the lungs, called residual volume.
Hiccup and cough: a hiccup is a sudden contraction of the diaphragm, and a cough is a protective reflex that clears the airway.

As an analytical point, large internal surfaces with thin walls and a steady blood flow make gas exchange efficient. Air pollution and smoking damage that surface and reduce the supply of oxygen.

Why Is This Topic Important?

Facts Organs of animals asked
Mechanics Diaphragm and volume
Link Pollution and disease

What Came Next?

The next topic covers the blood vascular and lymphatic transport systems. Next topic: Transport System: Blood Vascular System and Lymphatic System.

Key People and Terms

Alveolus Air sac of the lung
Diaphragm Breathing muscle under the lungs
Spiracle Opening of insect air tube
Gills Aquatic breathing organs
Pleura Membrane around lungs
Pneumatophore Breathing root of mangroves

Exam Corner

Points to Remember

Gas exchange in alveoli.
Insects: tracheae; fishes: gills.
Inhalation: diaphragm flattens.
Right lung 3 lobes; left 2.
CO binds haemoglobin strongly.

Do Not Confuse With

Breathing and respiration Gas exchange versus energy release
Trachea and bronchi Windpipe versus its two branches
Gills and tracheae Fishes versus insects

Memory Hook

Nose, pharynx, larynx, trachea, bronchi, alveoli gas; diaphragm flat to breathe in, gills for fish, tracheae for the insect class.

Mains Angle

Questions link respiration with pollution and health. These points add depth.

Human Airways and exchange
Others Gills, tracheae
Health Air pollution

A Question You May Face

An original practice question, not a past paper question.

“Describe the mechanism of breathing in humans and compare the respiratory organs of different groups of organisms.”

How to Answer

Tract Parts.
Mechanism Diaphragm.
Other organisms Gills, tracheae.
Health Pollution.

GS Relevance

Prelims: parts of the human respiratory tract, mechanics of breathing, respiratory organs in animals, plants, disorders. GS Paper 3: science and technology, health.

Frequently Asked Questions

Where does gas exchange occur in the lungs?

Gas exchange occurs in the alveoli, which are tiny thin-walled air sacs at the ends of the bronchioles, surrounded by capillaries. Oxygen diffuses from the air into the blood and carbon dioxide diffuses from the blood into the air, and the huge number of alveoli gives a very large surface area for exchange.

How does the diaphragm help in breathing?

During inhalation the diaphragm contracts and flattens and the rib muscles lift the ribs, which increases the volume of the chest cavity and lowers pressure, so air flows in. During exhalation the diaphragm relaxes and rises, the chest volume falls and air is pushed out.

How is oxygen transported in the blood?

About 97 per cent of oxygen is carried by haemoglobin in red blood cells as oxyhaemoglobin, and the rest is dissolved in plasma. Haemoglobin releases oxygen in tissues where the oxygen level is low, and carbon monoxide is dangerous because it binds haemoglobin much more strongly than oxygen does.

How do insects breathe?

Insects breathe through a network of air tubes called tracheae, which open to the outside through small holes called spiracles along the sides of the body. The tracheae branch into very fine tubes that carry air directly to the tissues, so insects do not use blood to carry oxygen.

How do plants exchange gases?

Plants exchange oxygen and carbon dioxide mainly through stomata on leaves and lenticels on woody stems, and through the surface of roots. They have no special breathing organs, and mangrove plants growing in waterlogged mud have special breathing roots called pneumatophores.

PYQ Practice — Statement Analysis

1 Gas exchange in the lungs takes place in the alveoli.

True

Thin-walled air sacs.

2 Insects breathe through gills.

False

They use tracheae.

3 The diaphragm contracts during inhalation.