• The process of exchange of oxygen from atmosphere with carbon dioxide produced from body cells is called breathing.


  • The process of breathing accompanied by the break -down of food to produce energy


  • According to body structure and habitat mechanism of breathing vary in different animals


The respiratory system can be studied in two parts

  • Respiratory tract

  • Respiratory organs


  • It consists of the external nostrils, nasal cavities, larynx, trachea, bronchi, and bronchioles

  • The respiratory system starts with a pair of external nostrils that lead to a nasal chamber through the nasal passage

  • Nasal cavity filters warm and moisten the air

  • Nasal chamber opens into the pharynx. It is the common passage for food and air. Epiglottis prevents the entry of food into the larynx

  • Glottis can be covered by a thin elastic cartilangious flap called epiglottis

  • Glotis leads to a thin-walled straight tube called trachea through the larynx.

  • Larynx is a cartilangious box called sound box because it produces sound.


  • The trachea is a straight tube which divides into right and left primary bronchi.Each bronchus  further divides into secondary, tertiary bronchi and bronchioles ending up in terminal bronchioles. Each bronchiole is further divided into a number of small ducts called alveolar ducts that opens into blind end sacs called alveoli.

  • The branching network of bronchi, bronchioles and alveoli comprise the lungs.


  • There are two lungs.Each lung is enclosed by a double-walled pleural membrane. The outer layer is called the parietal layer and the inner layer is called the visceral layer. In  between the two layers is present a fluid called pleural fluid

  • Lungs are located in the thoracic chamber which is limited dorsally by the vertebral column, ventrally by the sternum, laterally by the ribs and on the lower side by the dome-shaped diaphragm

Based on function, the respiratory system is divided into conducting part and respiratory part


Lungs are located in the thoracic chamber which is limited dorsally by the vertebral column, ventrally by the sternum, laterally by the ribs and on the lower side by the dome-shaped diaphragm


Mechanism of breathing can be studied in two steps


  • Movement of fresh air into the lungs is called inspiration

  • Both the process is carried out by creating a pressure gradient between the lungs and the atmosphere

  • During inspiration, volume of the thoracic cavity is increased by the movements of sternum, ribs and diaphragm. The sternum, ribs move upwards, forwards and outwards by the contraction of external intercostal muscles.

  • These activities of sternum, ribs and diaphragm enlarge the thoracic cavity

  • Expansion of thoracic cavity results in the expansion of the lungs.Due to the increase in the volume of lung pressure of air inside the lungs decreases as compared to atmospheric pressure. Therefore, fresh atmospheric air which is at higher pressure rushes into the lungs through the respiratory tract.


  • The movement of foul air from lungs to an outside is called expiration

  • During expiration volume of the thoracic cavity is decreased by the inward and downward movements of the ribs and the sternum and by upward bulging of a diaphragm.

  • These movements are brought about by the simple relaxation of external intercoastal muscles

Respiratory Volumes and Capacities

Tidal Volume (TV) -

A volume of air breathed in and out during normal breathing = 500 mL

Inspiratory Reserve Volume

The volume of air that can be drawn in forcibly =3000ml

Expiratory Reserve Volume (ERV)-

A volume of air that can be forcibly expelled out after normal expiration =1000ml

Residual Volume (RV

Some air is always left in the lungs even after forcibly breathing out.This is called left over =1500 ml

Inspiratory Capacity (IC):

Total volume of air a person can breathe in after a normal expiration

This includes tidal volume and inspiratory reserve volume ( TV+IRV=3500 ml

Expiratory Capacity (EC)

The total volume of air a person can breathe out after a normal inspiration. This includes tidal volume and expiratory reserve volume (TV+ERV

Functional Residual Capacity (FRC):

Some air is always left in the lungs even after forcibly breathing out. This includes ERV+RV

Vital Capacity (VC)

The volume of air that can be taken in and expelled out by maximum inspiration and expiration

Total Lung Capacity

The maximum amount of air lungs can hold is total lung capacity


  • The primary site for exchange of gases takes place in alveoli

  • Exchange of gases also takes place between blood and tissues

  • Exchange of gases takes place by simple diffusion based on the pressure gradient

  • Partial pressure-It is the pressure contributed by an individual gas in a mixture of gases. It is represented as pO2 for oxygen and pCO2 for carbon dioxide


Diffusion of oxygen

Diffusion of carbon dioxide

  • pO2 in alveolar air=104 mm Hg

  • pO2 in venous blood=40 mm Hg

  • Oxygen diffuses from alveoli to venous blood

  • pCO2 in alveolar air=40 mm Hg

  • pCO2 in venous blood=45 mm Hg

  • Carbon dioxide diffuses from venous blood to alveoli

The diffusion membrane is made up of three major layers

  • Squamous epithelium of alveoli

  • Endothelium of alveolar capillaries

  • Basement substance in between

  • All the factors in our body are favourable for diffusion of O2 from alveoli to tissues and that of CO2 from tissues to alveoli


  • Transport of gases is through blood.

  • 97 per cent of O2 is transported by RBCs  and 3 per cent of O2 is transported through the plasma

  • 20-25 per cent of CO2 is transported by RBCs whereas 70 per cent is carried as bicarbonate


  • Haemoglobin is a red coloured iron-containing pigment present in the RBCs and they bind with oxygen to form oxyhaemoglobin. Each haemoglobin molecule can bind to four molecules of oxygen. This binding is affected by the partial pressure of O2, Partial pressure of CO2, hydrogen ion concentration and temperature

  • Oxygen binds to haemoglobin on the lung surface and is dissociated in the tissues


  • Carbon dioxide combines with haemoglobin to form carbamino-haemoglobin

  • When pCO2 is high and pO2 is low as in the tissues, more binding of carbon dioxide occurs whereas, when the pCO2 is low and pO2 is high as in the alveoli, dissociation of CO2 from carbamino-haemoglobin takes place

  • In  the tissue where the partial pressure of CO2 is high, CO2 diffuses into blood and forms HCO3– and H+

  • In the alveolar site where pCO2 is low, CO2 and H2O is formed

  • CO2 trapped as bicarbonate is transported from tissues to alveoli  where CO2 is released out


Respiratory rhythm centre

  • It is a group of neurons  present in the medulla region of the brain which is  responsible for regulation of respiration

Pneumotoxic centre

  • It is a group of neurons present in the pons region of the brain. n. It can moderate the functions of the respiratory rhythm centre.

Chemosensitive centre

  • This is an area situated adjacent to the respiratory rhythm centre which is highly sensitive to CO2 and hydrogen ions.



       It is an allergic attack of breathlessness associated with bronchial obstruction or spasm


        The air pollutants break down the alveoli of the linings  reducing the surface area for gas exchange

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