Breathing and Exchange of Gases

Respiratory Organs

What happens when our body does not get enough oxygen? We feel breathless, tired, uneasy and can even become unconscious in extreme cases! Why do you think this happens? This is because oxygen is extremely important for producing energy. The respiratory organs are responsible for this process. Let’s study about these essential organs in our body.

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Different Respiratory Organs

Different animals have different respiratory organs based on their habitat and body organization. Invertebrates like sponges and flatworms breathe through simple diffusion over their body surface. Earthworms use their cuticle while insects use a network of tubes. Most aquatic animals use specialized structures called ‘gills’ whereas most terrestrial creatures use structures called ‘lungs’.

Did you know that frogs can breathe through their moist skin too? Among vertebrate animals, fishes breathe through gills; while birds, reptiles, and mammals use lungs to breathe. Mammals respire using a well developed respiratory system. Let’s learn about the human respiratory organs in more detail.

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Human Respiratory System

The human respiratory system has the following parts:

  • We take in oxygen from the atmosphere with our nostrils. The nostrils open into the nasal chamber through a nasal passage. This chamber then opens into the pharynx, a part of which is the common passageway for air and food.
  • The pharynx opens into the trachea through the larynx. The larynx is a box-like structure made of cartilage that helps to produce sound and is, therefore, called the ‘sound box’. A cartilaginous structure called epiglottis prevents the entry of food into the larynx. Trachea, on the other hand, is a tube that divides at the 5th thoracic vertebra into the left and right primary bronchi.
  • Each bronchus divides multiple times to give rise to secondary and tertiary bronchi which ultimately gives rise to thin, terminal structures called bronchioles.
  • The terminal bronchioles then give rise to a number of vascular bag-like structures called alveoli which have very thin walls.

Respiratory Organs

Parts of the human respiratory system.

This network consisting of bronchi, bronchioles, and alveoli forms the lung. Let’s learn in more detail about the lungs.


Humans have two lungs (right and left) that are covered by a double-layered covering pleuraThe layers contain pleural fluid in the middle that helps to reduce friction on the lung surface. The inner pleural membrane is in close contact with the lung whereas the outer membrane is in contact with the thoracic lining.

Respiratory Organs

Parts of the human lung. (Source: Wikimedia Commons)

In our body, the lungs are situated in an air-tight chamber called the thoracic chamber. This chamber consists of the vertebral column at the back, sternum in the front, ribs on the sides and diaphragm on the lower side. This setup of the lungs is such that any change in the volume of the thoracic cavity will affect the lung cavity. This is very important for respiration.

Functions of the Respiratory Organs

In terms of function, the respiratory system has the following two parts:

1) Conducting part – This is the part from the nostrils to the terminal bronchioles. The conducting part has the following functions –

  • Transport atmospheric air to the alveoli.
  • Clear the air from foreign particles.
  • Humidify the air.
  • Bring the air to body temperature.

2) Respiratory/Exchange part – This part includes the alveoli and their ducts. Here, actual diffusion of oxygen and carbon dioxide between blood and atmospheric air happens.

Steps Involved in Respiration

  • Breathing in of atmospheric air and breathing out of CO2 produced in the alveoli.
  • Diffusion of O2 and CO2 across the membrane of alveoli.
  • Transport of gases across the body by the blood.
  • Exchange of O2 and CObetween blood and tissues.
  • Cellular respiration which involves the use of O2 by cells for catabolic reactions and release of CO2.

Solved Example For You

Q: The deficiency of which of the following factors causes ‘Hypoxia’?

  1. O2
  2. Water
  3. CO2
  4. Vitamins

Solution: The answer is ‘a’. Hypoxia is a condition where the body or a part of the body does not get enough oxygen supply.

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