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