Exchange Surfaces and The Lungs

• As an organism grows, its volume (or mass) increases more quickly than its surface area
• A large organism has less surface area per mmCubed of body than a small organism for each mmCubed of body.
• This means there is relatively less space on the body surface for uptake of oxygen and removal of carbon dioxide by diffusion
• So, many organisms have special surfaces for gas exchange e.g gills, lungs

Example: ALVEOLI

• In the lungs, oxygen diffuses from the air in the alveoli into the blood.
• carbon dioxide diffuses in the opposite direction - from blood to alveoli

To make gas exchange as efficient as possible, the alveoli have:

• a large surface area
• a thin layer of cells to provide:
  a) short diffusion distance
  b) good blood supply - many capillaries (to absorb useful substances and to deliver waste substances)
• The air in the alveoli is constantly refreshed so that the concentrations of oxygen and carbon dioxide is such that there is a steep diffusion gradient

Ventilation - the movement of the diaphragm and ribcage to provide the alveoli with fresh supplies of air

Breathe In: INSPIRATION
Breathe Out: EXPIRATION

Inspiration:
1) Diaphragm contracts and moves down
2) External intercostal muscles (muscles in between ribs) contract to move ribs upwards and outwards
3) Volume of thorax ( chest = thoracic cavity ) increases
4) Pressure in lungs decreases to being lower than atmospheric pressure

Remember: contract, contract, increase, decrease

Expiration:
1) Diaphragm relaxes and is pushed up by pressure in the abdomen
2) External intercostal muscles relax so the ribs move down with gravity
3) Volume in the thorax decreases
4) Pressure in the lungs increases to being higher than atmospheric pressure

Remember: relax, relax, decrease, increase

[ Here would be a very good time to use a pneumonic or a sequence or something to help you remember - Diaphragm, Muscles, Volume, Pressure. "Doing Many Vampy Poses", or something a little more riske to help you remember! :P ]

If you're not sure what some of these things are, have a look at my diagram of the thorax and breathing mechanism thingymajig...

• Trachea:
  - Cartilage
  - Goblet cells
  - Ciliated cells
  - Smooth Muscle
  - Elastic Fibres
• Bronchus:
  - Cartilage
  - Goblet cells
  - Ciliated cells
  - Smooth Muscle
  - Elastic Fibres
• Bronchiole
  - Goblet cells (a few)
  - Ciliated cells
  - Smooth Muscle
  - Elastic Fibres
• Alveolus:
  - Squamous epithelium
  - Elastic fibres

• Cartilage
  C-shaped rings of cartilage hold trachea open
  It thins out the lower down you go
• Goblet cells
  Secretes mucus to prevent the effects of cohesive forces due to water on the alveolus
  Also to lubricate exchange surface to prevent rubbing
  Also traps bacteria
• Cilliated cells
  Sweeps mucus and bacteria along tissue surface out of the way - towards mouth and nose for expulsion
• Smooth muscle
  Contracts to narrow airways
• Squamous Epithelium
  Gives a short diffusion pathway for oxygen and carbon dioxide in the alveoli

1) Explain why humans have a specialised gas exchange surface, but protocists (bacteria) do not.

2) Describe the process of Inspiration.

3) Name three qualities an efficient exchange surface has.