exchange surfaces

• exchange surface 


  -specialised area that is adapted to make it easier for molecules to cross from one side to the other
• single celled organisms


  -large SA: Vol

  -short diffusion pathway- exchange surface is the plasma membrane
• multi-cellular organisms

  -outer surface is no longer sufficient to supply cells with a substance

  -diffusion to cells at the centre of the organism takes too long- requires a specialised exchange surface
• general features of an exchange surface

  -large surface area

  -thin barrier

  -steep concentration gradient

  -fresh supply of molecules on one side to keep concentration high

  -removal of molecules on one side to keep concentration low
• alveoli adaptations

  -thin walls

  -good blood supply maintains a high concentration gradient

  -moist surface- gases dissolve easily

  -ventilation helps to maintain a steep concentration gradient for diffusion
• alveolar walls


  -flattened squamous epithelial cells- with capillaries in close contact

  -elastic fibres lay between alveolar walls- provide strength and flexibility

  -enable expansion (inhale) and recoil (exhale)
• smooth muscle


  -walls of trachea, bronchi and bronchioles

  -contracts to constrict airway and make lumen narrower- reduce air flow to/ from alveoli

  -exercise- smooth muscle relaxes and airways become wider for more airflow

  -involuntary
• elastic fibres


  -trachea, bronchi and bronchioles

  -recoils- dilates airway after smooth muscle constricted it

  -found in alveoli walls to aid expulsion of air

  -emphysema- elastic fibres in alveoli destroyed- broken down- can't exhale properly
• ciliated epithelial cells


  -goblet cells- lines trachea. bronchi and larger bronchioles

  -lots of mitochondria

  -microtubules allow cilia to move

  -goblet cells produce mucus- traps pathogens to prevent infection

  -cilia move in synchronised motion to push mucus to top of trachea to be swallowed or coughed- if pathogens or dust particles enter
• cartilage


  -trachea and bronchi

  -provide support and help keep airways open

  -strong but flexible

  -distinctive c-shape allows greater flexibility and space for food to move in the oesophagus.
• lung tissues

  -walls of alveoli made of flattened squamous epithelial cells with capillaries in close contact

  -elastic fibres lay between alveolar walls to provide strength and flexibility
• multi cellular organisms

  -smaller surface area to volume ratio means a larger distance that needs to be crossed

  -substances can't enter as easily as single celled organisms
• trachea

  -wide tube supported by c-shaped cartilage rings to keep air passage open during pressure changes

  -ciliated epithelium cells which move mucus to back of the throat

  -carries air to bronchi
• bronchi

  -supported by rings of cartilage and have ciliated epithelial cells

  -narrower than trachea

  -allow air passage into the bronchioles
• bronchioles

  -narrower than bronchi

  -not kept open by cartilage

  -mostly only have smooth muscle and elastic fibres so they can contract and relax easily during ventilation

  -allow air passage into the alveoli