Gas Exchange in the Lungs

The heart pumps deoxygenated blood to the lungs to collect oxygen. Our lungs have specific adaptations that make them efficient for gas exchange. The alveoli in the lungs are the sites of gas exchange, meaning that this is where gases diffuse in and out of the bloodstream.

  • Oxygen in the air diffuses into the bloodstream and is then transported to the body’s cells
  • Carbon dioxide diffuses out of the bloodstream into the alveoli and is then expelled out of the lungs

The alveoli are surrounded by a network of blood vessels called capillaries. If we look at a cross-section diagram of an alveolus (singular of alveoli), we can see that the capillary is alongside the wall of the alveoli.

Capillaries are the smallest blood vessels, and they carry red blood cells.

Our body needs oxygen for aerobic respiration. When we breathe in, we take in oxygen at high concentrations. The oxygen diffuses across the alveolar wall to get into the capillary. Carbon dioxide (CO2) is produced by the body as waste, which diffuses from the capillary to the alveoli and is then expelled when we breathe out.

So remember:

  • When we breathe in – Air enters our lungs, and oxygen diffuses from the alveoli into the blood.
  • When we breathe out – Carbon dioxide is diffusing out of the blood and into the alveoli

Features of Alveoli

The alveoli are adapted to be efficient and effective at gaseous exchange due to these features:

  • Thin walls (just one cell thick) which shorten the diffusion distance
  • Millions of alveoli in the lungs give the lungs a large surface area, which means we can diffuse many gases at once
  • A good blood supply helps to maintain a steep concentration gradient, as the alveoli are continuously supplied with blood that is high in carbon dioxide and low in oxygen
  • Moist surface helps gases dissolve for diffusion

Gas Exchange in Fishes

Water can only hold low concentrations of oxygen, so fish have a unique exchange system.

Oxygen-rich water flows into the mouth of the fish, then over the gills, which is where gas exchange takes place. After this, the water flows out of the operculum. The gills contain filaments which have a large surface area.

Deoxygenated blood flows through the filament, so oxygen from the water diffuses into the bloodstream. Then the oxygenated blood returns to the body.

Fish have specific adaptations that make their gas exchange efficient:

  • Filaments give the gills a large surface area
  • Each gill filament has a thin membrane, so there is a short diffusion distance
  • The filaments have a good blood supply because the water and blood flow in opposite directions – This countercurrent flow ensures that water with the highest oxygen concentration is in contact with blood having the lowest oxygen concentration.

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