Gas Exchange in the Lungs

The heart pumps blood to the lungs to collect oxygen. Lungs have adaptations that make them efficient at gas exchange. Alveoli in the lungs are the site of gas exchange, meaning that they are 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 have a network of blood vessels which are 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.

The capillary is the smallest blood vessel and it carries 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 into the blood at the alveoli
  • 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 Fish

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.

Fishes have the following adaptations which make 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 means that oxygen with the highest amount of water will be in contact with the blood with the highest amount of oxygen.