3.1 - exchange surfaces

• Multicellular organisms require specialised gas exchange surfaces because of their smaller surface area to volume ratio
• Surface area to volume ratio is calculated by:

  ratio = surface area / volume
• Three features of an efficient gas exchange surface are:

  1. large surface area
  2. Short distance
  3. steep concentration gradient maintained by good blood supply
• Trachea structure and function:

  wide tube supported by C shaped cartilage to keep air passage open

  lined by ciliated epithelium cells which move mucus

  carries air to the bronchi
• Bronchi structure and function:

  supported by rings of cartilage and lined by ciliated epithelium cells and goblet cells

  narrower than the trachea and there are two of them

  allow passage of air into the bronchioles
• Bronchioles structure and function:

  narrower than the bronchi

  dont need to be kept open by cartilage therefore mostly have smooth muscle and elastic fibres so they can contract and relax during Ventilation

  allow passage of air into the alveoli
• Alveoli structure and function:

  mini air sacs lines with epithelium cells and site of gas exchange

  walls only one cell thick covered with a network of capillaries which facilitates diffusion
• During inspiration:

  external intercostal muscles contract

  pulls the ribs up and out

  diaphragm contracts and flattens

  volume of thorax increases

  air pressure inside lungs decrease

  so air moves into the lungs
• During expiration:

  external intercostal muscles relax

  brings the ribs down and in

  diaphragm relaxes and domes upwards

  volume of thorax decreases

  air pressure inside lungs increases

  so air moves out of the lungs
• Spirometer is used to measure lung volume
• Spirometer works by a person breathing into an airtight chamber which leaves a spirometer trace on a graph showing volume of breaths
• Vital capacity is the maximum volume of air that can be taken in or expelled from the lungs in one breath.

  Calculated from spirometer graph by finding maximum peak.
• Tidal volume is the volume of air we breathe in and out during each breath at rest.

  Can be calculated from spirometer graph by finding amplitude at rest
• Breathing rate is the number of breaths we take per minute.

  Can be calculated from spirometer graph by counting number of peaks in one minute
• The two main features of a fish’s gas transport system:

  gills - located in the body, supported by arches, projections of gill filaments which are stacked in piles

  lamellae - right angles to gill filaments, give an increased surface area. blood and water flow across them in opposite directions
• Gas exchange in fish:

  buccal cavity Floor pulls downwards, volume increased to allow water to flow in

  water is pumped over the lamellae by operculum so oxygen diffuses into bloodstream

  Waste carbon dioxide diffuses into the water and flows back out of gills
• Countercurrent exchange system maximises oxygen absorbed by the fish as it maintains a steep concentration gradient. Water is always next to blood of a lower oxygen concentration.
• The three main features of an insects exchange system are

  spiracles

  tracheae

  tracheoles
• Spiracle are holes on the body’s surface which can be opened or closed by a valve for gas or water exchange
• Tracheae are large tubes extending through all the body tissues, supported by cartilage rings to prevent collapse
• Tracheoles are smaller branches dividing off the tracheae with tracheal fluid and the end of branches
• Gas exchange in insects:

  Gas moves in and out of the tracheae through the spiracles and then through the tracheoles

  a diffusion gradient allows oxygen to diffuse into the body tissue while co2 diffuses out

  contraction of muscles in the tracheae allows movement of air in and out
• Large surface area increases the rate of diffusion
• Alveoli has a thin diffusion pathway
• Efficient exchange systems have good blood supply/ventilation to maintain concentration gradient

  eg, gills in fish or Alveoli in animals
• Membrane surrounding lungs is called pleural membrane
• Pleural membrane has a pleural cavity to prevent lungs rubbing on ribcage during inhalation.
• Muscle underneath lungs is the diaphragm
• Goblet cells secrete mucus which traps dust / bacteria that enters the lungs
• Cilia waft mucus to the back of the throat where it is swallowed or coughed up
• Elastic fibres recoil to their original shape; expel air; and prevent the lungs from bursting
• Smooth muscle contracts in order to constrict the airways so fewer harmful substances can enter
• Cartilage provides strength and support for trachea and bonchi. also prevents collapse during breathing in since chest volume increases so there will be a lower pressure in the trachea
• Bronchioles have smooth muscle, elastic fibres and squamous epithelial cells
• In a spirometer tank there must be pure oxygen
• Spirometers contain sodalime to absorb carbon dioxide breathed out by the person
• Residual volume left in lungs so they do not completely flatten
• What is vital capacity?

  maximum volume of air a person can breathe in and out in one breath
• what is tidal volume?

  volume of air breathed in and out at rest
• What is breathing rate?

  number of breaths per minute