Breathing and Exchange of Gases Notes for NEET, Download PDF

In this article, we are providing Breathing and Exchange of Gases notes which is an important chapter for NEET 2020

By Neha BagriUpdated : Aug 21, 2020 , 16:52 IST
Breathing and Exchange of Gases Notes for NEET, Download PDF

In this article, we are providing Breathing and Exchange of Gases notes which is an important chapter for NEET 2020. This is an important section to pay attention from the unit Human physiology as every year 2-3 questions are asked from this chapter. Let's begin with the brief introduction of Respiration. Moreover, you can download the Breathing and Exchange of Gases Notes PDF, we have shared at the end.

Breathing and Exchange of Gases Notes


Every living organism needs to obtain energy from the organic food either prepared by it (autotrophic organisms) or consumed by it (heterotrophic organisms). The process of oxidizing food to obtain energy by using oxygen and releasing carbon dioxide is called the respiration. It is a physicochemical process that includes the following:

  1. Breathing: It is the physical intake of oxygen through the process of inhalation and giving out of carbon dioxide through the process of exhalation.
  2. Cellular respiration: It is the chemical process of using inhaled oxygen to oxidize the food and release carbon dioxide.

Respiratory organs

There are the following categories of organs involved in respiration:

  1. Conducting parts: They clean, humidify and transport the air to the inside of the body.
  2. Exchanging parts: These allow the exchange of oxygen and carbon dioxide between the blood and air.

Human Respiratory System

The human respiratory system is the track that allows the air to move in from the nose to the lungs and also expel it in the same way. There are the following organs in the human respiratory system:

S.No.Upper respiratory tractLower respiratory tract
2Nasal cavityLungs

Upper Respiratory Tract:

  1. Nostrils: These are the openings through which the air enters and leaves the respiratory system.
  2. Nasal cavity: The nostrils lead to the nasal cavity which has hair cells and mucus. The air is cleaned, moistened and warmed and allowed to move further in the respiratory tract.
  3. Pharynx: It is a space that begins from the superior of the uvula and continues till the hyoid bone. It is shared by both the food coming from the buccal cavity and air coming from the nasal cavity. It is composed of ciliated epithelium that further traps the impurities in the air.
  4. Epiglottis: It is a muscular flap that covers the trachea while swallowing occurs to prevent the entry of food into the trachea.
  5. Larynx: The air moves through the larynx to the trachea. The vocal cords present in larynx vibrate with air to produce sounds.

Lower Respiratory Tract:

  1. Trachea: It is the windpipe that leads air into the lungs. It is lined by ciliated epithelium. On the outer surface, the C-shaped cartilaginous rings prevent the trachea from collapsing during breathing.
  2. Bronchi: The trachea branches into right and the left bronchus that enter into right and left lung respectively.
  3. Bronchioles: Each bronchus in the respective lung divide to form bronchioles.
  4. Alveoli: The bronchioles end up in sac-like structures called alveoli, which are supplied with blood vessels and act as the sites of exchange of gases.
  5. Lungs: Each lung is surrounded by the double-layered serous pleural membranes that have pleural fluid present between them. These provide protection to the lungs and reduce friction. The upper part is called apex and lower is called base. The Left lung is smaller and has two lobes. The Right lung is bigger with three lobes.
  6. Diaphragm: It is a muscular organ that separates the thoracic cavity from the abdominal cavity and plays an important role in the mechanism of breathing.

Mechanism of Breathing

Breathing process is a sum of inhalation of air from outside to the lungs and exhalation of the air from the lungs to the inside. These occur in the following manner:

  1. Inhalation:
  2. Diaphragm contracts and pushes itself towards the abdominal cavity. This increases the volume in the thoracic
  3. The external intercostal muscles contract and the ribs, as well as the sternum, move outwards.
  4. The increased thoracic cavity and outwardly moved bones provide essential volume to the lungs, due to which the pressure inside lungs decreases.
  5. Due to the pressure gradient, the atmospheric air is drawn in from outside (pressure is high) to the lungs (pressure is low)
  6. Exhalation:
  7. Diaphragm relaxes and moves towards the thoracic cavity, decreasing the volume.
  8. Inter-costal muscles also relax and the bones move back inward. This reduces the volume of the lungs.
  9. The pressure of the lungs become more than atmospheric, so the air is expelled out.

Respiratory Volumes

Tidal Volume (TV)

The volume of air inhaled or exhaled during normal breathing

500 mL

Inspiratory Reserve Volume (IRV)

The additional volume of air that can be inhaled by forceful inspiration

2500-3000 mL

Expiratory Reserve Volume (ERV)

The additional volume of air that can be exhaled by forceful exhalation

1000-1100 mL

Pulmonary Capacities

Residual Volume

The volume of air in lungs after forceful expiration

1100-1200 mL

Inspiratory Capacity (IC)


3000-3500 mL

Expiratory Capacity (EC)

EC = Tv + ERV

1500 mL

Vital Capacity (VC)

The maximum amount of air that be breathed in or out


Total Lung Capacity

The total volume of air in the lungs after forceful inspiration


Exchange of Gases

External exchange

Occurs between alveoli and blood

The partial pressure of oxygen (pO2) is more in alveoli than in blood, so it diffuses in the blood while pCO2 is more in blood than in alveoli, so it diffuses out of blood.

Internal Exchange

Occurs at the level of body tissues

In tissues, the pCO2 is more due to respiration, so CO2 moves in the blood and O2 moves in the tissues because pO2 is low in tissues.

Transport of Gases

Transport of Oxygen

98.5% of O2 is transported via haemoglobin as oxyhemoglobin. Rest is transported as dissolved in plasma. It is affected by the pH and partial pressure of the oxygen.

Transport of Carbon dioxide

5-7% through plasma; 10% through binding with haemoglobin; 85% as bicarbonate ions.

Disorders of the Respiratory System

  1. Asthma: Inflammation of bronchi and bronchioles resulting in difficulty of breathing and wheezing.
  2. Emphysema: Damage to the walls of alveoli affecting the transport of gases. The major cause is smoking.
  3. Occupational Disorders: These are caused due to exposure to fine dust, smoke, chemical fumes in industries. Silicosis and asbestosis are common occupational disorders caused due to continuous exposure to silica and asbestos dust at the workplace.

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Neha Bagri
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Neha Bagri