Breathing gases

• Oxygen (O2)


  Utilized by organisms to indirectly break down nutrient molecules like glucose and to derive energy for performing various activities
• Carbon dioxide (CO2)


  Harmful gas released during catabolic reactions
• Exchange of O2 and CO2

  1. O2 from the atmosphere with CO2 produced by the cells

  2. This process is called breathing or respiration
• Breathing causes the chest to move up and down
• Respiratory organs and mechanism of breathing


  • Vary among different groups of animals depending mainly on their habitats and levels of organisation
• Lower invertebrates

  • Sponges, coelenterates, flatworms
  • Exchange O2 and CO2 by simple diffusion over their entire body surface
• Earthworms

  • Use their moist cuticle to exchange gases
• Insects

  • Have a network of tubes (tracheal tubes) to transport atmospheric air within the body
• Aquatic arthropods and molluscs

  • Use special vascularised structures called gills for the exchange of gases
• Terrestrial forms

  • Use vascularised bags called lungs for the exchange of gases
• Vertebrates

  • Fishes use gills, reptiles, birds and mammals respire through lungs, amphibians like frogs can respire through their moist skin
• Mammals have a well developed respiratory system
• Respiratory system in humans

  • Consists of external nostrils, nasal chamber, pharynx, larynx, trachea, bronchi, bronchioles and alveoli
  • The conducting part transports atmospheric air to the alveoli and the respiratory or exchange part is the site of actual diffusion of O2 and CO2 between blood and air
• Lungs

  • Situated in the thoracic chamber which is an air-tight chamber
  • Any change in the volume of the thoracic cavity will be reflected in the lung (pulmonary) cavity
• Respiration

  1. Breathing or pulmonary ventilation

  2. Diffusion of gases (O2 and CO2) across alveolar membrane

  3. Transport of gases by the blood

  4. Diffusion of O2 and CO2 between blood and tissues

  5. Utilisation of O2 by the cells for catabolic reactions and resultant release of CO2
• Mechanism of breathing

  1. Inspiration - Increase in thoracic volume decreases intra-pulmonary pressure causing air to move into the lungs

  2. Expiration - Decrease in thoracic volume increases intra-pulmonary pressure causing air to move out of the lungs
• Tidal Volume (TV)


  Volume of air inspired or expired during a normal respiration
• Inspiratory Reserve Volume (IRV)

  Additional volume of air a person can inspire by a forcible inspiration
• Expiratory Reserve Volume (ERV)

  Additional volume of air a person can expire by a forcible expiration
• Residual Volume (RV)


  Volume of air remaining in the lungs even after a forcible expiration
• Inspiratory Capacity (IC)


  Total volume of air a person can inspire after a normal expiration
• Expiratory Capacity (EC)

  Total volume of air a person can expire after a normal inspiration
• Functional Residual Capacity (FRC)

  Volume of air that will remain in the lungs after a normal expiration
• Vital Capacity (VC)

  Maximum volume of air a person can breathe in after a forced expiration or breathe out after a forced inspiration
• Total Lung Capacity

  Total volume of air accommodated in the lungs at the end of a forced inspiration
• Alveoli are the primary sites of exchange of gases
• Partial pressure

  Pressure contributed by an individual gas in a mixture of gases
• There is a concentration gradient for oxygen from alveoli to blood and blood to tissues, and for CO2 in the opposite direction
• The diffusion membrane is made up of the thin squamous epithelium of alveoli, the endothelium of alveolar capillaries and the basement substance in between them
• The total thickness of the diffusion membrane is much less than a millimetre, which is favourable for diffusion of O2 and CO2
• Gradient

  A difference in concentration or pressure that causes a substance to move from an area of high concentration or pressure to an area of low concentration or pressure
• The solubility of CO2 is 20-25 times higher than that of O2
• The diffusion membrane is made up of three major layers: the thin squamous epithelium of alveoli, the endothelium of alveolar capillaries, and the basement substance in between them
• The total thickness of the diffusion membrane is much less than a millimetre
• All the factors in our body are favourable for diffusion of O2 from alveoli to tissues and that of CO2 from tissues to alveoli
• About 97 per cent of O2 is transported by RBCs in the blood, and the remaining 3 per cent of O2 is carried in a dissolved state through the plasma
• Nearly 20-25 per cent of CO2 is transported by RBCs, whereas 70 per cent of it is carried as bicarbonate, and about 7 per cent of CO2 is carried in a dissolved state through plasma
• Haemoglobin

  A red coloured iron containing pigment present in the RBCs that can bind with O2 in a reversible manner to form oxyhaemoglobin
• Each haemoglobin molecule can carry a maximum of four molecules of O2
• Oxygen dissociation curve


  A sigmoid curve obtained when percentage saturation of haemoglobin with O2 is plotted against the partial pressure of O2 (pO2)