gas exchange

• 3 factors that affect the need for an exchange surface/system

  • size: in smaller and single celled organisms exchange of substances can take place quickly and efficiently
  • surface area to volume ratio: small organisms have a small surface area and a small volume- bigger organisms- surface area increases
  • level of activity of an organism: the more active an organism, the more oxygen and nutrients needed and the more waste substances produced
• trachea

  • made out of cartilage
  • C shaped for flexibility when pressure is high - trachea will stretch out
• why alveoli are good at being an exchange surface

  • large surface area
  • good supply or good ventilation- maintain a gradient
  • thin layer- one cell thick
• maintaining structure of alveoli

  • decrease in pressure - tendency for lungs to collapse - cartilage help keep trachea and bronchi open
  • lung surfactant - phospholipid layer that coats surfaces of the lungs
• pleural cavity

  • each lung enclosed in double membrane known as pleural membrane
  • space between the two membranes is called the pleural cavity and filled with small amount of pleural fluid
  • fluid lubricates lungs and adheres to other walls of lungs to the thoracic (chest) cavity by water cohesion
• resting inspiration


  external intercostal muscles CONTRACT - lifting rib cage (active)
• resting expiration


  external intercostal muscles RELAX - rib cage falls (passive)
• rate of oxygen consumption


  difference between inspiratory volume 'A' and inspiratory volume 'B'
• tidal wave


  volume of air moving in and out in one breath
• residual volume


  volume of air left in lungs
• exchange ventilation process

  1. diaphragm contracts
  2. volume increases
  3. more space - pressure is lower
  4. air moves in through exchange system
  5. intercostal muscles (external) pull ribs outwards - inhalation
  6. intercostal muscles (internal) ribs bought inwards (exhalation)
• keeping airways clear

  • walls of trachea and bronchus contain goblet cells which secrete mucus made of mucin - traps microorganisms and debris
  • walls contain ciliated epithelial cells which are covered on the surface with cilia - beat regularly to move microorganisms and dust particles
• exchange (why do we breath)

  animals need to maintain a concentration gradient in their exchange surfaces
• mammals: closed circulatory system

  insects: open circulatory system
• insects: body fluid act as blood and tissue fluid

  mammals: blood found in blood vessels and tissue fluid surround cells
• tracheal system: made up of pipes called tracheae which branch into smaller pipes called tracheoles which contain tracheal fluid
• spiracles: pores that let air enter and leave the system
• insects need a greater supply of oxygen - they're more active

  • increase surface area
  • remove tracheal fluid to make more room
• ventilation is bony fish

  • bony fish exchange gases with water they live in through gills
  • bony fish have 5 pairs of gills which are found behind a bony plate called operculum
  • blood capillaries carry deoxygenated blood close to the surface of secondary lamallae so gas exchange takes place
  • each gill made up of 2 rows of gill filaments (primary lamallae) which are attached to a bony arch
• ventilation of bony fish (2)

  • concentration of oxygen in water is lower than in air so gills need to have a large surface area - achieved by filaments being thin and the fact their surface is folded into many secondary lamallae callled gill plates
• ventilation in fish steps

  1. fish opens mouth which lowers buccal cavity (mouth)
  2. this increases volume of buccal cavity and decreases pressure
  3. water sucked into cavity
  4. fish closes its mouth, raising floor of buccal cavity - decreases volume and increases pressure
  5. water forced out of cavity and across gill filaments
  6. increased pressure forces operculum to open so water leaves gills
• counter current flow

  • used to ensure maximum amount of oxygen absorbed from water
  • blood flows through gill arch and out along gill filaments(primary lamallae) and along gill plates (secondary lamallae).
  • water flows across gills at same time as blood flows through them (they flow in opposite direction)
  • ensures there's a steep concentration gradient between both liquids - ensures oxygen concentration is higher in water compared to blood
• fetal haemoglobin

  • RBC in fetal bloodstream contain special form of haemoglobin (fetal hb)
  • replaces with adult haemoglobin by 6 months after birth
  • contain alpha and gamma chains - gamma chains have a higher affinity for oxygen
• myoglobin

  • molecule with a similar structure to haemoglobin but with only one haem group
  • myoglobin has a very high affinity for oxygen, even at low partial pressures - means oxymyoglobin will only dissociate when oxygen levels are low. its found in muscle cells where it acts as an oxygen reserve
• altitude sickness

  • caused by acute exposure to low partial pressure of oxygen at high altitude
  • compensated by altitude acclimitisations
• haemocyanin - not found in humans

  • found in crabs, lobsters, snails , octopus
  • copper unit instead of haem/iron unit
  • causes blood to be blue
  • higher affinity than humans
• low affinity = high metabolic rate
• each haem group can combine with one oxygen molecule so one haemoglobin molecule can combine with a maximum of 4 oxygen molecules - oxyhaemoglobin. (quaternary)
• transport of oxygen

  • during gas exchange, in alveoli, oxygen is absorbed into bloodstream
  • oxygen molecules diffuse into RBC and become associated with haemoglobin they bind reversibly with haemoglobin to form oxyhaemoglobin.
  • haemoglobin has a high affinity for oxygen
• association and dissociation

  • ability of haemoglobin to associate and dissociate with oxygen depends on concentration of O2 in tissues - partial pressure of O2 for O2 tension
  • oxygen binds to haemoglobin when O2 is at high concentration and dissociates from haemoglobin when O2 is at a low concentration
  • partial pressure measured in kilopascals
  • oxygen sigmoid shaped
• bohr effect

  1. CO2 enters RBC and forms carbonic acid
  2. carbonic acid dissociates to release H+ ions
  3. H+ ions make cytoplasm of RBC more acidic
  4. PH change effects tertiary structure of haemoglobin this reduces affinity for O2
  5. haemoglobin cannot hold onto as much oxygen so oxygen is released from oxyhaemoglobin
• factors that effect affinity for oxygen

  CO2 + H2O -> H+ + HCO3- = catalysed by carbonic anhydrase

  -decreased affinity for O2 - bohr effect

  • CO2 effect -> haemoglobin less attracted to O2
• p(CO2)

  • low partial pressures: high affinity
  • high partial pressures: low affinity
• p(O2)

  • low partial pressures: low affinity
  • high partial pressures: high affinity
• factors affecting dissociation

  -blood temp:

  • increased blood temp
  • reduces haemoglobin affinity for O2
  • hence more O2 is delivered to warmed up tissue
• factors affecting dissociation

  -blood PH

  • lowering blood PH (makes blood acidic)
  • caused by presence of H+ ions from lactic acid or carbonic acid
  • reduces affinity of Hb for O2
  • more O2 delivered to acidic sites which are working harder
• factors affecting dissociation

  -CO2 concentration

  • higher CO2 concentration in tissue - less affinity of Hb for O2
  • so the harder the tissue is working the more O2 is released
• transporting carbon dioxide

  • 5% dissolved in plasma
  • 10% combines with haemoglobin to form carbaminohaemoglobin - 4 molecules of CO2 can bind to haemoglobin
  • vast majority (85%) transported in form of hydrogen carbonate ions (HCO3-)
• carbaminohaemoglobin

  • some of the CO2 entering erythrocytes escapes being broken down by carbonic anhydrase enzyme
  • this binds with haemoglobin - forms carbonaminohaemoglobin
• how haemoglobin loads and unloads oxygen in blood

  • oxygen loads onto haemoglobin at high partial pressure
  • in lungs haemoglobin has a high affinity for O2
  • tissues have a low partial pressure of oxygen as it has been used in respiration
  • in tissues haemoglobin have a lower affinity for oxygen