The UPSC mains GS-3 paper syllabus has various subjects like Economics, Science and Technology. Science and technology in UPSC is mostly concerned with current Affairs. but it is necessary to understand the basics of science. Human Body consists of some of the most complex systems. Respiratory Systems is a very advanced in humans. It involes various stages of distribution of oxygen molecules from the air which is taken in and collection the carbon di oxide from the cell and exhaling it back from the nostrils.


  • What is Respiratory system
  • Aerobic respiration and Anerobic Respiration
  • Parts of respiratory System
  • Functions of respiratory system
  • Respiration in Humans


The human respiratory system consists of a group of organs and tissues that help us to breathe. Lungs are the primary organs of the respiratory system which help in the exchange of gases. The other main parts of this system include a series of airways for air passages, blood vessels and the muscles that facilitate breathing.


Aerobic respiration

When oxidation of food takes place in the presence of oxygen, it is called aerobic respiration. At the end of aerobic respiration, 36 energy of ATP is released and 55 to 60% of energy is utilised and rest of the energy is dissipated as heat.

Anaerobic respiration

Anaerobic respiration is the type of respiration through which cells can break down sugars to generate energy in the absence of oxygen. This is in contrast to the highly efficient process of aerobic respiration, which relies on oxygen to produce energy.



The nose has a couple of exterior nostrils, which are divided by a framework of cartilaginous structure termed as the septum. This is the structure that evenly separates the right nostril from the left nostril. Tiny hair follicles that cover the interior lining of nostrils acts as the body’s first line of defence against foreign pathogens.They also provide additional humidity for inhaled air. It provides air for respiration, serves the sense of smell, conditions the air by filtering, warming, and moistening it, and cleans itself of foreign debris extracted from inhalations.


The area of the throat containing the vocal cords and used for breathing, swallowing, and talking. Also called voice box.Two cartilaginous chords lay the framework for the larynx. They are situated at the point of joining the pharynx and trachea. It is also termed as Adam’s apple or the voice box. It is the portion which rises and falls during swallowing of food particles. It generates sound as air passes through the hollow in the middle.


Pharynx, cone-shaped passageway leading from the oral and nasal cavities in the head to the esophagus and larynx. The pharynx chamber serves both respiratory and digestive functions. Thick fibres of muscle and connective tissue attach the pharynx to the base of the skull and surrounding structures.The nasal chambers open up into a wide hollow space termed as the pharynx. It is a common path for both air and food. It functions by preventing the entry of food particles into the windpipe. The epiglottis is an elastic cartilage, which serves as a switch between the larynx and the oesophagus by allowing the passage of air into the airway to the lungs, and food in the gastrointestinal tract.


The trachea or the windpipe rises below the larynx and moves down to the neck.  The trachea extends further down into the breastbone and splits into two bronchi, one for each lung. The walls of trachea comprise C-shaped cartilaginous rings which give hardness to the trachea and maintain it by completely expanding.


The trachea splits into two tubes termed as bronchi, which enter each lung individually. The bronchi divide into secondary, tertiary, and to bronchioles, which is again further divided into small air-sacs called the alveoli. The alveoli are minute sacs of air with thin walls and single-celled manner. It enables the exchange of oxygen and carbon dioxide molecules into or away from the bloodstream.


Lungs are the chief centres of the respiration in humans and other vertebrates. They are located in the thoracic cavity of the chest near the backbone and on either side of the heart. They are the pair of large, spongy organs, mainly involved in the exchange of gases between the blood and the air. Compared to the left lung, the right lung is quite bigger and heavier.

Respiratory Tract – 

Outside the lungs

  • External nostrils– For the intake of air.
  • Nasal chamber– which is lined with hair and mucus to filter the air and remove dust and dirt.
  • Pharynx– It is a passage behind the nasal chamber and serves as the common passageway for both air and food.
  • Larynx– Also known as the soundbox as it helps in the generation of sound and thus helps us in communicating.
  • Epiglottis– It is a flap-like structure that covers the glottis and prevents the entry of food into the windpipe.
  • Trachea– It is a long tube passing through the mid-thoracic cavity.

Inside Lungs –

  • Bronchi– The trachea divides into left and right bronchi.
  • Bronchioles– Each bronchus is further divided into finer channels known as bronchioles.
  • Alveoli– The bronchioles end up into the balloon-like structures known as the alveoli.


Inhalation and Exhalation

The respiratory system helps in breathing, known as pulmonary ventilation. The air inhaled through the nose moves through the pharynx, larynx, and trachea into the lungs. The air is exhaled back through the same pathway. Changes in the volume and pressure in the lungs aid in pulmonary ventilation.

Exchange of Gases between Lungs and Bloodstream

Inside the lungs, the oxygen is exchanged for carbon dioxide waste through millions of microscopic sacs called alveoli. The inhaled oxygen diffuses into the pulmonary capillaries, binds to haemoglobin and is pumped through the bloodstream. The carbon dioxide from the blood diffuses into the alveoli and is expelled through exhalation.

Exchange of Gases between Bloodstream and Body Tissues

The blood carries the oxygen from the lungs around the body and releases the oxygen when it reaches the capillaries. The oxygen is diffused through the capillary walls into the body tissues. The carbon dioxide also diffuses into the blood and is carried back to the lungs for release.

The vibration of the Vocal Cords

While speaking, the muscles in the larynx move the arytenoid cartilage. These cartilages push the vocal cords together. During exhalation, when the air passes through the vocal cords, it makes them vibrate and creates sound.

Olfaction or Smelling

During inhalation, when the air enters the nasal cavities, some chemicals present in the air bind to it and activate the receptors of the nervous system on the cilia. The signals are sent to the olfactory bulbs via the brain.


Respiration is the process of taking in oxygen and giving out carbon dioxide. There are two types of respiration processes in humans:

  • External Respiration, that involves the inhalation and exhalation of gases.
  • Internal Respiration, that involves the exchange of gases between blood and body cells.



The nervous system is the major controlling, regulatory, and communicating system in the body.



Cells provide structure for the body, take in nutrients from food, convert those nutrients into energy, and carry out specialized functions.


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