Oxygen is needed inside most organisms' cells to extract energy from most forms of food, although oxygen gas is harmful and poisonous to anaerobic organisms.
Plants do not need circulatory systems to carry oxygen because they have air spaces between their cells, and gases are able to diffuse through those gaps.
Animals need oxygen. Animals use all of lungs on land, gills in water, and diffusion systems in small animals to acquire oxygen from their environment.
A fish can get oxygen from water through its gills, which work to provide a large surface area for contact with the water to allow oxygen to diffuse in.
Small animals absorb oxygen by diffusion through the skin, while larger animals use breathing systems to carry oxygen in: lungs and gills and in insects, tubes.
Our lungs fill with air when the diaphragm falls, or the ribs move up and out: as the space expands, air can move in, as it contracts, air is forced out again.