Exchange surfaces

• A single celled organism has short diffusion distance and a high SA:V ratio so has no need for specialised exchange systems
• As an organism gets bigger, SA: v ratio decreases

  more exchange has to take place to meet needs and metabolic activity
• Features of a good exchange surface

  • thin barrier to reduce diffusion distance
  • good blood supply - to maintain concentration gradients
  • large SA
• • air passes through then nasal/oral cavity
  • hairs in the cavity trap dust particles and pathogens
  • cavity warms and moistens the air before it enters the lungs
  • air then travels down trachea -> bronchi -> bronchioles -> alveoli
• one adaptation of the trachea:

  • walls contain cartilage
  • prevents walls collapsing due to low pressure in inspiration
  • flexible to allow movement
  • forms a c shape (incomplete rings)
  • absence of cartilage in region near esophagus allows food to pass easily
• another adaptation of the trachea:

  • walls lined with ciliated epithelia and goblet cells
  • cilia and goblet cells secrete and waft mucus containing pathogens to throat
  • to be swallowed and digested by stomach enzymes
• Each bronchus contains cartilage, smooth muscle, ciliated epithelium and goblet cells
• The walls of larger bronchioles are supported by cartilage and contain smooth muscle

  when smooth muscle relaxes, the bronchioles widen allowing more air to pass into the deeper parts of the lungs
• Alveoli:

  • sites of gas exchange
  • internal walls covered with thin layer of moisture
  • elastic fibres between them stretch and recoil during breathing
  • covered with extensive blood capillaries
• How the alveoli are adapted for efficient gas exchange:

  • large SA
  • walls of capillary and alveolus are one cell thick
  • walls consist of squamous cells - thin
  • narrow diameter of the capillary- RBCs close to the capillary wall decreasing diffusion distance
  • moist walls- helps to dissolve the gases
  • extensive capillary network (rich blood supply)- steep conc. for o2 and co2
• How is a steep o2 and co2 conc. gradient maintained in the lungs?

  • maintained by ventilation which ensures
  • conc. o2 in air of alveolus > conc. o2 in blood
  • conc. co2 in air of alveolus < conc. co2 in blood
• Bronchioles contain ciliated epithelium, smooth muscle and elastic tissue
• Inhalation - the external intercostal muscles contract

  • pulls ribs upwards and outwards
  • diaphragm contracts causing it to flatten
  • increases volume of thorax and lungs
  • reducing air pressure in lungs
  • air pressure in the lungs < atmospheric pressure
  • so air is drawn into the alveoli and elastic fibres stretch

  
• Exhalation - the internal intercostal muscles relax

  • pulls ribs downwards and inwards
  • diaphragm relaxes to domed shape
  • reducing the volume of the thorax and the lungs
  • air pressure in lungs > atmospheric pressure,
  • air is pushed out of the alveoli so elastic fibres recoil

  
• Inhalation is an active process as it involves muscle contraction
• Exhalation is a passive process unless exhaled strongly e.g: intense exercise, illness
• lungs are surrounded by plural membranes

  • with plural fluid between them
  • act as a lubricant as the lung volume changes
• dm x 1000 = cm
• Process of spirometry:

  • o2 is inhaled by the patient and the spirometer lid drops down
  • Exhaling back into the spirometer causes the lid to rise
  • Movements of the lid are recorded by a data logger and show the volume of air ventilated by the patients lungs
  • Co2 rich air is exhaled by the patient and absorbed by the soda-lime
• Precautions of spirometry:

  • clean /sterilised mouth piece
  • soda-lime must be fresh
  • make sure water chamber isn't overfilled
  • check for gas leaks
  • patient is healthy
• vital capacity depends on:

  • size of a person
  • age and gender
  • level of regular exercise
• Tidal volume - volume of air we breathe in/out each breath at rest (0.5 dm^3)
• Vital capacity - maximum volume of air that can be inhaled/exhaled in a single breath (2.5-5.0dm^3)
• Residual volume - volume of air always present in the lungs
• Expiratory/inspiratory reserve volume - additional volume of air that can be exhaled/inhaled on top of tidal volume

  • when tidal volume is exceeded during exercise
• How is o2 uptake measured using a spirometer?

  • how is this measured by using a spirometer:co2 absorbed by soda-lime in exhalationvolume of air drops in spirometer over timesame molecules of o2 are used as co2 made in respiration this drop in volume is assumed to be amount of o2 taken in
  • (amount of co2 measured in soda-lime = amount of o2 taken in)
• How is o2 uptake measured using a spirometer?

  • co2 absorbed by soda-lime in exhalation
  • volume of air drops in spirometer over time
  • same amount of o2 molecules is inhaled as co2 is made in respiration
  • this drop in volume is assumed to be amount of o2 taken in
  • (amount of co2 measured in soda-lime = amount of o2 taken in)
• How to measure o2 uptake from a spirometer trace?

  • take two points on the graph same trough or peak and measure difference in volume between them
  • measure length of time between them
  • volume(y) / time(x)
  • express as dm^3 s-1
• Spirometer trace

  A) tidal volume

  B) vital capacity

  C) residual volume

  D) inspiratory reserve volume

  E) expiratory reserve volume

  
• challenges of gas exchange in bony fish

  • low o2 conc. in water
  • low SA:V
• Fish gills are covered by an operculum

  • in gill filaments there are many lamellae where gas exchange takes place
  • o2 diffuses from the water into the bloodstream and co2 diffuses from the bloodstream to the water

  
• How efficient gas exchange is achieved in fish:

  • Lamellae - increases SA:V ratio
  • Countercurrent flow - maintains steep concentration gradient between lamellae for gas exchange
  • Capillaries - one cell thick to reduce diffusion distance
  • Buccal cavity - changes volume to pump water over the gills
• Counter current flow - flow of water is in opposite direction to flow of blood

  allows maximum amount of o2 to be absorbed from the water
• Ventilation in bony fish: expiration

  > fish shuts mouth and opens its operculum

  > floor of the buccal cavity lifts upwards

  • decreasing volume and increases pressure
  • water flows over gills into operculum cavity

  > sides of opercular cavity squeezes inwards

  • pressure in opercular cavity > pressure outside
  • operculum is forced open and water exits

  A) buccal cavity

  B) Opercular cavity

  C) Operculum

  
• Ventilation in bony fish: inhalation

  > fish opens mouth

  > water flows in buccal cavity

  > floor of buccal cavity drops down

  • pressure inside buccal cavity < pressure outside

  > fish shuts operculum and increases the volume of the opercular cavity, decreasing the pressure (contains the gills)

  A) buccal cavity

  B) opercular cavity

  C) operculum

  
• Challenges of insects - desercation
• Desercation - moisture loss
• Insects are covered with a protective exoskeleton made of chitin

  o2 and co2 cannot diffuse through
• Spiracles on insects exoskeleton enable diffusion of gases into tracheae
• trachea in insects are wide enforced with chitin

  prevents tubes collapsing when insects move