Breathing and exchange of gases

• Respiration is the process of exchange of O2 from the atmosphere with CO2 produced by the cells, commonly known as breathing
• • Mammals have a well-developed respiratory system
• Different respiratory organs are used by various organisms for gas exchange:

  • Lower invertebrates like sponges, coelenterates, flatworms exchange O2 with CO2 by simple diffusion over their entire body surface
  • Earthworms use their moist cuticle and insects have a network of tubes to transport atmospheric air within the body
  • Gills are used by most aquatic arthropods and molluscs, whereas lungs are used by terrestrial forms
  • Fishes respire through gills, while reptiles, birds, and mammals respire through lungs
  • Frogs can respire through their moist skin
• The human respiratory system includes:

  • Nostrils leading to a nasal chamber through the nasal passage
  • Nasal chamber opening into nasopharynx, the common passage for food and air
  • Nasopharynx opening through glottis of the larynx region into the trachea
  • Larynx, a cartilaginous box for sound production, covered by epiglottis to prevent food entry
  • Trachea dividing into right and left primary bronchi, further branching into secondary and tertiary bronchi and bronchioles ending in alveoli
  • Lungs covered by a double-layered pleura with pleural fluid between them, reducing friction
• • Conducting part transporting atmospheric air to alveoli
  • Exchange part facilitating diffusion of O2 and CO2 between blood and atmospheric air
• Breathing involves two stages:

  • Inspiration: atmospheric air is drawn in
  • Expiration: alveolar air is released out

  Increase or decrease of pulmonary volume occurs during breathing:

  • Inspiration: diaphragm contraction increases thoracic chamber volume
  • Expiration: relaxation of diaphragm and inter-costal muscles reduces thoracic volume
• Respiratory volumes and capacities include:

  • Tidal Volume (TV): approx. 500 mL, volume of air inspired or expired during normal respiration
  • Inspiratory Reserve Volume (IRV): averages 2500 mL to 3000 mL, additional volume of air inspired by forcible inspiration
  • Expiratory Reserve Volume (ERV): averages 1000 mL to 1100 mL, additional volume of air expired by forcible expiration
  • Residual Volume (RV): averages 1100 mL to 1200 mL, volume of air remaining in the lungs after forcible expiration
• • Expiration Capacity (EC), Inspiratory Capacity (IC), Functional Residual Capacity (FRC), Vital Capacity (VC), and Total Lung Capacity are also important respiratory capacities
• Alveoli are the sites of gas exchange through simple diffusion of O2 and CO2

  Factors affecting the rate of diffusion include solubility of gases and thickness of membranes

  Partial pressure of gases, such as pO2 for oxygen and pCO2 for carbon dioxide, play a role in gas exchange

  Transport of gases by the blood is crucial:

  • O2 is transported by RBCs and in a dissolved state through plasma
  • CO2 is transported by RBCs as carbamino-haemoglobin and as bicarbonate, with a small percentage carried in a dissolved state through plasma
• Oxygen binds with haemoglobin to form oxyhaemoglobin, with each haemoglobin molecule capable of carrying up to four molecules of O2

  Factors affecting oxygen binding with haemoglobin include partial pressure of O2

  Carbon dioxide is carried by haemoglobin as carbamino-haemoglobin, with binding related to partial pressure of CO2

  RBCs contain carbonic anhydrase enzyme facilitating the reaction of CO2 and H2O at tissue and alveolar sites