Gills obtain oxygen through a process of diffusion, where oxygen dissolved in water moves across the thin gill tissues into a fish's bloodstream.
The Mechanics of Gill Respiration
Fish gills are incredibly efficient structures designed for extracting oxygen from water. They are composed of thin, highly vascularized tissues (full of blood vessels). As water flows over these tissues, the difference in oxygen concentration between the water and the blood drives the movement of oxygen. The oxygen, being at a higher concentration in the water, diffuses across the gill membranes into the blood, which has a lower oxygen concentration. This is a passive process requiring no energy expenditure by the fish.
- Water Flow: Fish actively pump water over their gills or use their swimming motion to create water flow. Source: https://www.iowadnr.gov/About-DNR/DNR-News-Releases/ArticleID/1454/How-do-fish-breathe This continuous flow ensures a constant supply of oxygen-rich water.
- Countercurrent Exchange: Many fish species employ a countercurrent exchange system, where water flows over the gills in the opposite direction to the blood flow. This maximizes oxygen uptake efficiency, as the blood is always exposed to water with a higher oxygen concentration. Source: https://oceanconservancy.org/blog/2020/01/17/gills/
- Surface Area: The large surface area of the gills, provided by their many thin filaments and lamellae, significantly enhances oxygen absorption. Source: https://www.nps.gov/articles/blue-in-the-face-breathing-underwater.htm
In summary, the gills' structure and the mechanism of water flow facilitate the efficient passive diffusion of oxygen from the water into the fish's circulatory system. This allows the fish to extract the dissolved oxygen necessary for survival. The process is highly efficient due to the thin gill tissue, the high surface area of the gills, and the countercurrent flow system in many species. Source: https://en.wikipedia.org/wiki/Fish_gill
