Breathing

• Oxygen (O2) is utilised by organisms to indirectly break down nutrient molecules like glucose and derive energy
• Carbon dioxide (CO2) is released during catabolic reactions
• O2 has to be continuously provided to cells and CO2 produced by cells have to be released out
• Exchange of O2 from the atmosphere with CO2 produced by the cells


  Called breathing, commonly known as respiration
• Respiratory organs and the mechanism of breathing are described in the following sections
• Mechanisms of breathing vary among different groups of animals depending mainly on their habitats and levels of organisation
• Animals exchanging O2 with CO2 by simple diffusion


  • Sponges
  • Coelenterates
  • Flatworms
• Animals using moist cuticle for gas exchange

  • Earthworms
• Animals using tracheal tubes for gas exchange

  • Insects
• Animals using gills for gas exchange

  • Most aquatic arthropods and molluscs
• Animals using lungs for gas exchange

  • Terrestrial forms
• Vertebrates and their respiratory organs

  • Fishes - gills
  • Reptiles, birds, mammals - lungs
  • Amphibians - moist skin and lungs
• Mammals have a well-developed respiratory system
• Human respiratory system includes external nostrils, nasal chamber, pharynx, larynx, trachea, bronchi, bronchioles, and alveoli
• Larynx helps in sound production and is called the sound box
• Epiglottis prevents the entry of food into the larynx during swallowing
• Trachea divides into right and left primary bronchi
• Each bronchi undergoes repeated divisions to form secondary and tertiary bronchi and bronchioles
• Alveoli are thin, irregular-walled, vascularised bag-like structures in the lungs
• Lungs are covered by a double-layered pleura with pleural fluid between them to reduce friction
• Parts of the human respiratory system

  • Diaphragm
  • Epiglottis
  • Larynx
  • Trachea
  • Cut end of rib
  • Pleural membranes
  • Alveoli
  • Pleural fluid
  • Bronchiole
• Figure 17.1 shows a diagrammatic view of the human respiratory system with a sectional view of the left lung
• Event

  2015-16(19/01/2015)
• The conducting part of the respiratory system transports atmospheric air to the alveoli, clears it from foreign particles, humidifies it, and brings it to body temperature
• The exchange part of the respiratory system is the site of actual diffusion of O2 and CO2 between blood and atmospheric air
• The lungs are situated in the thoracic chamber which is anatomically an air-tight chamber
• Respiration involves breathing, diffusion of gases across the alveolar membrane, transport of gases by the blood, diffusion of O2 and CO2 between blood and tissues, and utilisation of O2 by cells for catabolic reactions
• Breathing involves two stages: inspiration where atmospheric air is drawn in, and expiration where alveolar air is released out
• Inspiration occurs when the intra-pulmonary pressure is less than the atmospheric pressure, and expiration occurs when the intra-pulmonary pressure is higher than the atmospheric pressure
• Inspiration is initiated by the contraction of the diaphragm and external inter-costal muscles, leading to an increase in thoracic and pulmonary volume
• Relaxation of the diaphragm and inter-costal muscles leads to expiration by reducing thoracic and pulmonary volume
• A healthy human breathes 12-20 times per minute
• Process of breathing

  Intra-pulmonary pressure increases slightly above atmospheric pressure causing the expulsion of air from the lungs, i.e., expiration
• Ability to increase the strength of inspiration and expiration with the help of additional muscles in the abdomen
• An average healthy human breathes 12-16 times per minute
• Volume of air involved in breathing movements can be estimated by using a spirometer which helps in clinical assessment of pulmonary functions
• Respiratory Volumes and Capacities

  • Tidal Volume (TV)
  • Inspiratory Reserve Volume (IRV)
  • Expiratory Reserve Volume (ERV)
  • Residual Volume (RV)
  • Inspiratory Capacity (IC)
  • Expiratory Capacity (EC)
  • Functional Residual Capacity (FRC)
  • Vital Capacity (VC)
  • Total Lung Capacity
• Tidal Volume (TV)


  Volume of air inspired or expired during a normal respiration, approx. 500 mL
• Inspiratory Reserve Volume (IRV)

  Additional volume of air a person can inspire by a forcible inspiration, averages 2500 mL to 3000 mL
• Expiratory Reserve Volume (ERV)


  Additional volume of air a person can expire by a forcible expiration, averages 1000 mL to 1100 mL