Class 11 Biology Breathing and Exchange of Gases Human respiratory system

Human respiratory system

  • A pair of external nostrils opens out above the upper lips, which leads to a nasal chamber through the nasal passage, and the nasal chamber opens into nasopharynx, which is a portion of pharynx.
  • Nasopharynx opens through glottis of the larynx region into the trachea.
  • Larynx is a cartilaginous box which helps in sound production and hence called the sound box.
  • During swallowing, glottis can be covered by a thin elastic cartilaginous flap called
  • Trachea is a straight tube extending up to the mid-thoracic cavity, which divides at the level of 5th thoracic vertebra into a right and left primary bronchi.
  • Each bronchi undergoes repeated divisions to form the secondary and tertiary bronchi and bronchioles ending up in very thin terminal bronchioles.
  • The tracheae, primary, secondary and tertiary bronchi, and initial bronchioles are supported by incomplete cartilaginous rings.
  • Each terminal bronchiole gives rise to a number of very thin, irregular walled and vascularised bag-like structures called alveoli.
  • The branching network of bronchi, bronchioles and alveoli comprise the lungs.
  • Two lungs are covered by a double layered pleura, with pleural fluid between them.
  • The outer pleural membrane is in close contact with the thoracic lining, whereas the inner pleural membrane is in contact with the lung surface.
  • The external nostrils up to the terminal bronchioles constitute the conducting part, whereas the alveoli and their ducts form the respiratory part.
  • The conducting part transports the atmospheric air to the alveoli, clears it from foreign particles, whereas exchange part is the site of actual diffusion of O2 and CO2 between blood and atmospheric air.
  • The lungs are situated in the thoracic chamber, formed dorsally by the vertebral column, ventrally by the sternum, laterally by the ribs and on the lower side by the dome-shaped diaphragm.
  • Respiration involves the following steps:
  • Breathing or pulmonary ventilation by which atmospheric air is drawn in and CO2 rich alveolar air is released out.
  • Diffusion of gases (O2 and CO2) across alveolar membrane.
  • Transport of gases by the blood.
  • Diffusion of O2 and CO2 between blood and tissues.
  • Utilisation of O2 by the cells for catabolic reactions and resultant release of CO


Fig. Human respiratory system

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