Exchange surfaces and breathing

• When can diffusion alone be enough to supply the needs of a single-celled organism?

  • Metabolic activity is usually low - oxygen demands and carbon dioxide production is relatively low
  • Large surface area to volume ratio - unlike many-layered multicellular organisms, they don't have complex organ systems, with large distances between cells. Gas exchange by diffusion is fast enough to be sufficient.
• volume of a sphere


  $4/3 \pi r ^3$
• surface area of a sphere


  $4\pi r^2$
• features of a good exchange surface

  • Large Surface area - folded walls provide more space for molecules to pass through. e.g. root hair cells, villi
  • Thin, permeable, layers - reduces the diffusion distance making the process fast and efficient. e.g. alveoli, villi
  • Good blood supply - maintains a steep concentration gradient by constantly delivering and removing substances for rapid diffusion. e.g. alveoli, gills
  • Ventilation for gases - maintains a diffusion gradient e.g. alveoli, gills
• factors which affect the need for an exchange surface

  • size
  • surface area
  • metabolic activity
• Nasal cavity

  • large surface area with a good blood supply - warms the air to body temperature
  • hairy lining - secretes mucus to trap dust and bacteria, protecting lung tissue from irritation and infection
  • Moist surface - increases humidity of air entering the lungs to reduce evaporation from the exchange surface
• Trachea

  main airway carrying clean, warm, moist air down into the chest

  • supported by incomplete, C-shaped, rings of cartilage. Preventing collapse during inspiration, whilst still allowing food to pass through the oesophagus behind
  • lined with ciliated epithelium with goblet cells - goblet cells secrete mucus to trap dust and microorganisms, cilia beat and move the mucus away from the lungs. (goes into the throat and is swallowed and digested)
  • cigarette smoke stops these cilia beating
• Bronchus

  • divides the trachea to lead to the different sides of the lungs
  • similar in structure to the trachea, just smaller
• Bronchioles

  • smaller bronchioles have no cartilage rings
  • Bronchiole walls contain smooth muscle which contracts or relaxes to constrict or dilate, changing the amount of air reaching the lungs
  • lined with a thin layer of flattened epithelium making some gas exchange possible
• Alveoli

  • Contain elastic fibers allowing the alveoli to stretch as air is drawn in and return to resting size to squeeze air out
  • Large surface area
  • Thin layers - walls made of squamous epithelium reduce the diffusion distances
  • Good blood supply - Capillaries are also only one cell thick, lie close to alveoli walls, and are narrow to restrict red blood cell movement
  • Steep concentration gradient - constant flow of blood brings carbon dioxide and carries off oxygen, whilst good ventilation moves air in and out of the lungs
  • Coated in surfactant - reduces cohesive forces between water molecules preventing collapse. Dissolves oxygen
• Inspiration - an energy using process

  1. diaphragm contracts, flattening and lowering
  2. external intercostal muscles contract, moving the ribs upwards and outwards

  This increases the volume of the thorax, reducing the pressure below atmospheric, so air is drawn in - down a pressure gradient - to equalize the pressure.
• Expiration - a passive process

  1. diaphragm relaxes, returning to the resting dome shape
  2. external intercostal muscles relax, moving the ribs down and inwards under gravity
  3. alveoli elastic fibers return to their normal length

  this decreases the volume of the thorax, increasing the pressure above atmospheric, so air is moved out - down a pressure gradient - to equalize the pressure.
• Forced Expiration - an energy using process

  1. internal intercostal muscles contract, pulling the ribs down hard and fast
  2. abdominal muscles contract forcing the diaphragm up

  this rapidly increases the pressure in the lungs
• Peak flow meter


  measures the rate at which air can be expelled from the lungs often used by people with asthma
• Vitalographs

  measures the forced expiratory volume and speed on a graph
• Spirometer

  measures different aspects of the lung volume

  investigates breathing patterns

  • subject should be free of asthma
  • there should be no air leaks in the apparatus
  • mouthpiece should be sterilized
  • soda lime should be fresh and functioning

  shows a downwards trend because carbon dioxide is being absorbed/removed by soda lime reducing the volume of gas in the system over time
• Tidal volume


  Air moved in and out with each breath at rest - around 500cm^3
• Vital capacity 


  Maximum volume of air that can be moved by the lungs - deepest possible inhale and the strongest possible exhale - 2.5 - 5.0 dm^3

  • depends on: height, age, gender, exercise levels
• inspiratory reserve volume 


  maximum amount of air you can breath in a forced inhale above the normal

  • vital capacity - tidal volume - expiratory reserve volume
• Expiratory reserve volume 


  Maximum amount of air you can breath out in a forced exhale above the normal

  • vital capacity - tidal volume - inspiratory reserve volume
• Residual volume 


  volume of air left in your lungs after a forced exhale, this cannot be measured directly - usually 1.5 dm^3
• Total lung capacity


  the sum of vital capacity and the residual volume
• equation for ventilation rate

  ventilation rate (volume of air inhaled per minutes) = tidal volume x breathing rate (breaths per minute)
• Why do insects need a gaseous exchange systems

  • active - high oxygen requirements
  • relatively high metabolic demand
  • tough exoskeleton made of chitin is not permeable to gas
  • open circulatory system - usually no blood pigments to carry oxygen
• Insect gas exchange system - Tracheal system

  • Spiracles - small openings in the thorax and abdomen which allows air to enter and leave the system. kept closed during periods of low oxygen demand to minimize water loss
  • Tracheae lead way from the spiracles - reinforced by spirals of chitin (impermeable so little exchange can take place) , they carry air into the body.
  • branch into Tracheole (0.6-0.8 ym in length) - a single elongated cell with no chitin lining and therefore permeable to gases. Small and therefor can run between individual cells through an insects tissues
• tracheal fluid

  • fills the ends of tracheoles - which are present in high numbers, increasing the surface area for gas exchange - limiting the penetration of air.
  • can be withdrawn into the body fluid to increase the surface area of the tracheole wall exposed to air e.g. during times of lactic acid build up, the water potential of muscle cells lower causing osmosis to move the fluid into the cells.
• How do insects increase oxygen to meet demands e.g. when flying

  • Mechanical ventilation - muscular pumping of the thorax and abdomen can change the volume of the body influencing pressure gradients in the tracheae and tracheoles
  • Air sacs can act as reservoirs - they are usually inflated or deflated by the ventilating movements of the thorax and abdomen
• Discontinuous gas exchange

  spiracles have three states - closed, open, fluttering

  • closed - no movement of gases, carbon dioxide diffused into bodily fluids (process called buffering)
  • fluttering - provides new oxygen whilst minimizing water loss
  • open - wide for carbon dioxide to rapidly diffuse out, sometimes pumping movements of abdomen and thorax help maximize this

  it could protect the trachea from parasites and debris, or to reduce water loss
• Respiration in bony fish

  • preventing water loss is not an issue
  • have to overcome the viscosity of water and slower diffusion rate - movement of water in lung-like structures would use too much energy. instead, they move water in one direction
• Gills

  Two rows of gill filaments - slender branches of tissue present as stacks (gill plates) - known as primary lamellae attached to a bony gill arch. each filament is folded into a secondary lamellae - the main site of gas exchange - providing a large surface area

  • large surface area
  • good blood supply
  • thin layers
• the Operculum is a bony flap which protects the gills and maintains the flow of water
• Ventilation in fish

  use Buccal (mouth) pump ventilation rather than ram ventilation, which involves continual movement

  • Open - floor of the Buccal cavity is lowered increasing the volume. This decreases the pressure in the cavity moving water in down an pressure gradient. the opercular valve shuts expanding the opercular cavity
  • Closed - the pressure increases, moving water up from the buccal cavity and over the gills. the operculum opens and the buccal cavity constricts. this increases the pressure forcing water out over the gills. this happens steadily to maintain the flow of water
• effective gas exchange

  • tips of adjacent gill filaments overlap increasing the resistance to, and slowing down, the flow of water providing more time for gas exchange
  • Counter current flow - oxygen moves down a concentration gradient the entire time (otherwise down the gill length equilibrium is reached) the blood flows through the gill lamellae in the opposite direction of the water maintaining a concentration difference along the full length of the lamella allowing maximum oxygen uptake
• Skilled dissection 


  minimum damage to the specimen by only removing the tissues which obscure vision e.g. to view gills, the operculum must be removed