Exchange and Transport

UNIT 1 AS OCR Biology - Exchange and Transport 

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Gaseous Exchange and Lungs

What is gaseous exchange?

- The movement of gases by diffusion between and organism and its environment across a    barrier
- Such as the alveolus wall 

- Oxygen (air) --> into alveoli - to be transported
- Carbon dioxide (blood) --> into alveoli  - to be removed

How are the lungs adapted for exchange?

Large surface area - More space for molecules to pass through

Barrier permeable to oxygen and carbon dioxide - plasma membrane surrounds cytoplasm forming a barrier for exchange. 

Thin barrier to reduce diffusion distance 

  • Alveolus wall - 1 cell thick
  • Capillary wall - 1 cell thick 
  • Walls consist of squamous cells (flattened)
  • Capillaries - in contact with alveolus wall
  • Capillaries narrow- erythrocytes can be squeezed as they pass along capillary - reduce diffusion distance.
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Maintaining diffusion gradient

Diffusion to be rapid - steep diffusion gradient is needed 

  • High concentration on supply side of exchange surface
  • Low concentration on the demand side of exchange surface

To maintain steep diffusion gradient:

  • Fresh supply of molecules on one side - needed to keep concentration high 
  • Way of removing molecules on other side - needed to keep concentration low 


Blood brings carbion dioxide from tissues to lungs - ensures that concentration is higher than that of in the alveoli 

Oxygen carried away from the lungs - ensures concentration of oxygen in blood is lower than concentration in the air of the alveoli. 

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Inspiration & Expiration


  • Diaphragm contracts - flattens
  • Intercostal muscles contract
  • Rib cage moves upwards and outwards
  • internal volume increases 
  • Pressure in chest cavity drops below atmospheric pressure
  • Air goes into lungs


  • Diaphragm returns to normal dome shape - relaxes
  • Intercostal muscles relax - rib cage moves downwards and inwards 
  • internal volume decreases
  • Pressure in chest cavity increases above atmospheric pressure 
  • Air moves out of lungs 
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Trachea & Bronchi

Tranchea & Bronchi have similar structure

- Wall consist of cartilage - C shaped rings

- Inside surface of cartilage - is glandular tissue, connective tissue, elastic fibres smooth muscle and blood vessels - aka: Loose Tissue 

- Inner lining of epithelium layer - two types of cell 

  • Cilia - cells that have cilia - ciliated epithelium 
  • Goblet cells  
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Roles of Tissues in Bronchi and Tranchea

Role of each tissue:

  • Cartilage- structural role 
    holds trachea and bronchi open - prevents collapse when air pressure low during inhalation
    Cartilage - incomplete ring - flexibility 
  • Smooth Muscle
    - Can contract & constrict airways - makes lumen narrower - restrict flow of air to & from alveoli 
  • Elastic Fibres 
    Airway constricts by contraction of smooth muscle - elastic fibres deform
    Smooth muscle relaxes - elastic tissue recoils and returns to original shape - helps dilate and open the airway. 
  • Goblet Cells and Glandular Tissue 
    - Located under epithelium 
    - Secrete mucus - traps any unwanted particles in the air eg) pollen/bacteria 
    - Trapping bacteria - reduce the risk of infection
  • Ciliated Epithelium
    - Consists of ciliated cells 
    - Cilia move in a synchronized pattern - waft mucus up the air way to back of throat, where it is then swallowed into the stomach.
    - Acidity then kills any bacteria 
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