Module 3.1.1- Exchange surfaces

• Two reasons why diffusion alone is enough for unicellular organisms

  metabolic activity is low

  SA:V is large
• Need for specialised exchange surfaces


  SA:V ratio- bigger the organism, smaller SA:V ratio,

  distance for substances to travel is longer.

  Metabolic Activity- higher, greater O2 demands and CO2 production, can't be exchanged fast enough due to low SA:V ratio.
• Exchange surface features


  large SA

  thin layers

  good blood supply- maintain steep concentration gradient

  ventilation- maintain steep concentration gradient
• Nasal cavity- human gaseous exchange system


  large SA with good blood supply

  hairy lining- secretes mucus to trap dust and bacteria

  moist surfaces- increases humidity of incoming air reducing evaporation from the exchange surfaces
• Trachea


  a wide tube reinforced by incomplete rings of strong, flexible cartilage

  its branches are lined with a ciliated epithelium with goblet cells
• Why are cartilage rings incomplete


  so food can move easily down the oesophagus behind the trachea
• Goblet cells


  secrete mucus onto lining of trachea, to trap dust and microorganisms which are then moved by cilia
• Bronchus

  In the chest cavity the trachea divides to form the left bronchus and vice versa. Also contains rings of cartilage.
• Bronchioles


  Bronchi divide further to form bronchioles with no cartilage, smooth muscle walls (constricts/relaxes) and a thin layer of flattened epithelium
• Alveoli structure


  elastin + collagen → stretch and elastic recoil of the lungs
• Alveoli adaptations

  layer of thin flattened epithelial cells → short diffusion pathwaylarge surface area surrounded by capillaries → good blood supplycovered in lung surfactant allowing alveoli to be inflated and speeds up the transport of gases + reduces the surface tension of fluid in alveoli good ventilation

  
• Diaphragm


  broad, domed sheet of muscle, which forms the floor of the thorax
• External intercostal muscles


  A muscle that raises the rib cage, decreasing pressure inside the chest cavity
• Internal intercostal muscles


  A muscle that lowers the rib cage during forced expiration
• Thorax is lined by the pleural membranes in which the space between them is filled with...


  ...a thin layer of lubricating fluid so the membranes slide easily over each other as you breathe.
• Inspiration (active)


  diaphragm lowers and contracts

  external intercostal muscles contracts

  ribs move upwards and outwards

  volume of thorax increases

  pressure decreases and is lower than atmospheric air

  air is drawn through to equalise pressure

  
• Expiration (passive)


  diaphragm moves up and relaxes

  external intercostal muscles relax

  ribs move downwards and inwards under gravity

  volume of thorax decreases

  pressure increases and is greater than atmospheric air

  air is drawn out to equalise pressure
• Peak flow meter


  a handheld device often used to test those with asthma to measure how quickly the patient can expel air
• Vitalographs


  a more sophisticated peak flow meter that produces a a graph of the amount the person breathes out and how quickly it is breathed out (forced expiratory volume)
• Spirometer


  An instrument for measuring the air entering and leaving the lungs.

  
• Tidal volume


  Volume of air that moves in and out of the lungs during a normal breath

  
• Vital capacity


  volume of air that can be breathed in when the strongest possible exhalation is followed by the deepest possible intake of breath.

  
• Inspiratory reserve volume


  The maximum volume of air you can breathe in over and above a normal inhalation

  
• Expiratory reserve volume


  Amount of air that can be forcefully exhaled after a normal tidal volume exhalation

  
• Residual volume


  The volume of air remaining in lungs after maximum exhalation. not measured directly

  
• Total lung capacity


  vital capacity + residual volume
• Ventilation rate equation


  tidal volume x breathing rate (per minute)
• Spiracles


  Small openings in the exoskeleton of insects that connect to internal cavities called hemocoels where respiratory gases are exchanged
• How can insects minimise water loss

  Spiracles kept closed by spiracle sphincters
• Trachea tubes from spiracles are lined by...


  ...spirals of chitin, which keep them open if they are bent or pressed

  chitin is relatively impermeable so little gaseous exchange takes place in trachea
• Tracheoles in insects


  trachea branches no chitin lining so freely permeable to gases vast amount of tracheoles means very large SA
• Tracheal fluid in insects


  The fluid found at the ends of the tracheoles in the tracheal system limits the penetration of air for diffusion when oxygen demands build-up, lactic acid build up in tissues moving water out of tracheoles by osmosis exposing more SA for gaseous exchange surface
• Mechanical ventilation of the tracheal system


  air is actively pumped into the system by muscular pumping movements of the thorax/abdomen. these movements change the volume of body and therefore pressure in trachea/tracheoles. Air is drawn in/out of the trachea or tracheoles to equalise pressure
• Collapsible enlarged trachea or air sacs


  act as air reservoirs which are used to increase the amount of air moved through the gas exchange system. usually inflated and deflated by the ventilating movements of the thorax and abdomen
• Gills adaptations


  large surface area good blood supply thin layers
• Operculum


  A protective flap that covers the gills of fishes maintaining a flow of water over the gills
• Gill structure


  gill filaments gill lamellae gill arch- supports the structure of the gills

  
• Gill filaments


  occur in large stacks (gill plates) and need a flow of water to keep them apart, exposing the large surface area needed for gaseous exchange
• Gill lamellae


  where gas exchange occurs, with counter current flow between blood and water with rich blood supply and large SA
• When the fish mouth is open...

  buccal cavity is lowered increases volume of buccal cavity pressure decreases water moves in opercular valve is closed opercular cavity expands, decreasing the pressure in opercular cavity

  buccal cavity moves up increasing pressure and here we are moving from open mouth to closed mouth...