Gas Exchange


This is the transfer of respiratory gases between an organism and the environment.

Efficient Gas Exchange

Specification Reference

"Understand how insects, fish and mammals are adapted for gas exchange."
Larger, more complex organisms require special adaptations to ensure adequate gas exchange. Examples of these adaptations include:
  • Root hair cells
  • Alveoli
  • Thin capillary walls
Exchange structures are needed so that metabolic needs can be met and waste products can be removed.

Gas Exchange in Insects

In insects, gas exchange occurs directly between cells and the atmosphere via diffusion. To increase efficiency, insects have developed a number of adaptations which enable them to exchange gases effectively:
  • Spiracles - are found along the thorax. They are small holes that allow gases to enter and leave the insect.
  • Sphincters - open and close the spiracles allowing gas exchange to be regulated.
  • Trachae - are long elongated tubes which carry gases. They are strengthened by chitin which also makes the tubes impermeable to gases.
  • Tracheoles - smaller tubes that branch from tracheae, they have no chitin and carry blood directly to body cells.
Sphincters tend to keep spiracles closed for as long as possible to reduce water loss.

Active Insects
Some insects have greater metabolic demands. These demands can be met through use of the following mechanisms:
  • Mechanical ventilation - when muscular movements in the thorax pump air into and out of the body.
  • Air sacs - act as reservoirs allowing more oxygen to be collected.
  • Flight - increases ventilation maintaining concentration gradients.
  • At rest water fills the end of tacheoles, during movement thus water is removed increasing he surface area for exchange.

Gas Exchange in Fish

Water contains little oxygen compared to the air. This means that fish have developed surfaces known as gills to exchange gases more efficiently.
  • Gills - fillamentous structures with a large surface area and lots of lamelle which have very thin walls increasing the rate of diffusion.
  • Gill filaments - these overlap ensuring that the water is slowed down allowing more time for diffusion to take place.
  • These gills are supported by the water.
  • During exchange the oral valve opens allowing water to enter and pass over the gills in the opposite direction to the blood.
  • This forms a countercurrent system ensuring that a concentration gradient is maintained across the full width of the lamella.
  • Finally, the operculum will then open and the oral valve will close allowing water to move out.
This opening and closing of the oral valve and the operculum ensures that the fish is sufficiently ventilated, even when it is stationary.

A countercurrent flow also allows the fish to achieve oxygen extraction rates of up to 80%. Parallel current flow would not achieve this since concentrations would quickly equalise.

These factors along with thin capillary walls, a large surface area and a rich blood supply allow fish to survive in seemingly sub-optimal conditions.

Gas Exchange in Mammals

The first step of gas exchange in mammals is breathing:

Inhalation
  1. External intercostal muscles contract
  2. Diagphram moves down
  3. Thoratic volume increases
  4. Pressure decreases
  5. Air is drawn in
Exhalation
  1. External intercostal muscles relax
  2. Elastic lung tissue recoils
  3. Thoratic volume decreases
  4. Pressure increases
  5. Air moves outwards
During forced breathing internal intercostal muscles will contract reducing thoratic volume further. This forces air out of the lungs more quickly. Once air has entered it moves through the:
  • Mouth
  • Trachea
  • Bronchi
  • Bronchiole
  • Alveoli
  • Capillaries
Alveoli are tiny air sacs that increase the surface area for gas exchange. The capillary and alveoli walls are only one cell thick thus shortening the diffusion pathway. This increases the rate of diffusion.

When explaining gas exchange make sure to comment on the following things:
  • Surface area : volume ratio
  • Thickness of the exchange surface
  • Maintenance of a concentration gradient

Gas Exchange in Plants

"Understand gas exchange in flowering plants, including the role of stomata, gas exchange surfaces in the leaf and lenticels."
Gases enter and leave plants through small pores known as stomata. These stomata can open and close depending on certain conditions.
  • Each stomata is surrounded by two guard cells which open and close the small pore.
  • K+ ions move into these guard cells via active transport lowering the water potential.
  • This causes the guard cells to swell and become turgor, opening the pore therefore allowing gas exchange to take place.
  • The opposite occurs to close the stoma.
Plants also have a number of gas exchange adaptations
  • Spongy mesophyll layer provides a large surface are for the diffusion of gases.
  • Waxy cuticles which are impermeable to gas prevent significant water loss to the atmosphere.
  • Lenticels are areas of loosely arranged cells on the stems of plants which allow gases to enter and leave.
Plants are constantly having to compromise between minimising water loss but also ensuring enough evaporation takes place to draw nutrients up the stem.