2. The mammalian gaseous exchange system

• HUMAN GASEOUS EXCHANGE SYSTEM

  • Mammals → relatively big = small SA: vol = large vol of cells + high metabolic rate (because they're active) = need lots of o2 for respiration = produce Co2 that needs to be removed (gas exchange = in lungs)
• NASAL CAVITY

  • Has a large SA with good blood supply = which warms air to body temp
  • Hairy lining = secretes mucus to trap bacteria = protecting lung tissue from irritation + infection
  • Moist surfaces = increase humidity of incoming air, reducing evaporation from exchange surfaces
  • After air has passed through nasal cavity --> when it reaches lungs its the same humidity + temp as air already in there
• TRACHEA

  • Main airway carrying clean, warm air
  • Its a wide tube supported by incomplete rings [c-shaped] a strong, flexible cartilage = stops trachea from collapsing
  • rings incomplete so food can easily move down oesophagus behind trachea
  • Trachea + its branches lined with = ciliated epithelium with goblet cells
  • Goblet cells: secrete mucus onto lining of trachea to trap things, that have escaped nose lining 
  • cilia = beat + move mucus along with any trapped dirt + microorganisms away from lungs
  • most goes into throat + is swallowed & digested
  • Effect of smoking = stops cilia beating
• BRONCHUS

  • In chest cavity trachea divides to form left bronchus + right bronchus
  • Similar in structure to trachea with = smaller rings [o-shaped] of cartilage, thinner walls
• BRONCHIOLES

  • Have no cartilage rings + walls contain smooth muscle
  • When smooth muscle contracts = bronchioles constrict [close up]
  • When smooth muscle relaxes = bronchioles dilate [open up]
  • Change amount of air reaching lungs [protection against harmful substances]
  • Lined with thin layers of flattened epithelium = making gaseous exchange possible

   
• ALVEOLI

  • Tiny air sacs = main gas exchange surfaces of body
  • Diameter = 200-300 micrometers
  • Consists of layer of thin flattened epithelial cells [squamous epithelial] + some collagen and elastic fibers
  • Elastic tissue = allow alveoli to stretch as airs drawn in
  • When they return to resting size = they help squeeze air out --> known as elastic recoil of lungs
• ALVEOLI 2

  • Large SA:
  • Millions alveoli per adult lung = large SA
  • If lung were simple balloon like structures = SA wouldn't be big enough
  • Thin layers:
  • Alveoli + capillaries that surround them have walls that are 1 epithelial cell thick = diffusion distance very short
  • Good blood supply:
  • Constant flow of blood through capillaries brings co2 + carries of o2 maintaining steep conc gradient for both co2 + o2 between air in alveoli and blood in capillaries
  • Good ventilation
  • Breathing moves air in + out alveoli helping maintain steep diffusion gradients for o2 & co2 between blood and air in lungs
• ALVEOLI 3

  • Inner surface of alveoli covered in thin layers of solution of water, salts, lung surfactant --> which makes it possible for alveoli to remain inflated [so alveoli walls don’t stick together]
  • O2 dissolves in water before diffusing into blood = but water can also evaporate into air in alveoli
  • Main adaptations of gas exchange system are to reduce water loss
• Ventilating the lungs

  • Ventilation: air being moved in + out lungs due to pressure changes in thorax [chest cavity] brought about by breathing movements
  • Ribcage: provides semi-rigid case within which pressure can be lowered with respect to air around it
  • Diaphragm: broad + domed sheet of muscle --> forms floor of thorax
  • External & internal intercostal muscles: found between ribs
  • Thorax: lined by pleural membrane which surrounds lungs
  • Space between them = pleural cavity, usually filled with thin layer of lubricating fluid so membranes slide over each other as you breathe
• INSPIRATION

  • Diaphragm = contracts = flattening + lowering
  • External intercostal muscles = contract
  • This causes ribs to move = upwards + outwards
  • Volume of thorax increases = pressure in thorax reduced
  • Air drawn in = equalising pressure in + out chest
• EXPIRATION

  • Diaphragm = relax = moving up + resting in dome shape
  • External intercostal muscles = relax
  • This causes ribs to move = downwards + inwards
  • Elastic fibers in alveoli return to normal length
  • Volume of thorax decreases = pressure in thorax increases
  • Air moves out = until pressure in + out equal again
• Active expiration

  • You can exhale forcibly using energy:
  • Internal intercostal muscles contract pulling ribs downward hard + fast
  • Abdominal muscles = contract forcing diaphragm up = to increase pressure in lungs rapidly

   

  • Passive expiration = internal intercostal muscles relax
  • Active expiration = internal intercostal muscles contract