A large diffusion gradient - breathing ensures that the oxygen concentration in the alveoli is higher than in the capillaries so oxygen moves from the alveoli to the blood. Why do we drown when we breathe water? Humans cannot breathe underwater because our lungs do not have enough surface area to absorb enough oxygen from water, and the lining in our lungs is adapted to handle air rather than water. However, there have been experiments with humans breathing other liquids, like fluorocarbons.
How many gills does a fish have? Why can't humans breathe underwater like fish? Fish "breathe" the dissolved oxygen out of the water using their gills. Are there fish without gills? Breathing without gills Although most fish respire primarily using gills, some fishes can at least partially respire using mechanisms that do not require gills. Many such fish can breathe air via a variety of mechanisms.
The skin of anguillid eels may absorb oxygen directly. How do gills demonstrate high surface area? The gills are composed of comb-like filaments, the gill lamellae, which help increase their surface area for oxygen exchange.
Then it draws the sides of its throat together, forcing the water through the gill openings, so it passes over the gills to the outside. Why do lungs work better than gills in air? Much less water is lost via evaporation from lungs than would be from gills suspended in air.
Breathing is initiated by neurons in the medulla oblongata. Inhalation occurs as a result of nervous stimulation of the external intercostal muscles and the diaphragm. Why are the lungs moist? A respiratory surface is covered with thin, moist epithelial cells that allow oxygen and carbon dioxide to exchange.
Those gases can only cross cell membranes when they are dissolved in water or an aqueous solution, thus respiratory surfaces must be moist. Likewise, the high-oxygen blood, which has nearly passed the entire length of the gill, meets fresh, high-oxygen water, and oxygen continues to flow from water to blood. Oxygen would quickly pass from the water into the blood, until the oxygen levels of the blood and water rapidly became the same, and oxygen diffusion into the blood would stop.
The maximum amount of oxygen that the blood could pick up would be only half of the total amount of oxygen in the water. In contrast, countercurrent oxygen exchange allows the blood to pick up 90 percent of the oxygen in the water. This impressive ability to acquire oxygen allows fish to thrive in water that has oxygen levels only a tenth of those at the top of Mount Everest, where most people cannot survive without supplemental oxygen. It just shows that fish are truly masters at breathing.
This system maximises the amount of oxygen diffusing into the blood by having the most oxygenated blood meet the most oxygenated water, and the least oxygenated blood meet the least oxygenated water. This means that the concentration gradient is maintained the whole way through, allowing the maximum amount of oxygen to diffuse into the blood from the water.
This is important because there isn't much oxygen in the water, and fish need to absorb enough oxygen to survive. In addition to this, the lamellae have a rich blood supply so that a steep concentration gradient can be maintained between the blood in the lamellae and the water through.
Hence, oxygen diffusing into the blood is rapidly removed by the circulating blood supply and more oxygen is able to difuse into the blood.
Another way in which a steep concentration gradient is maintained is by ensuring water flows in one direction only. The fish opens its mouth to let water in, then closes its mouth and forces the water through the gills and out through the operculum gill cover. This allows for more efficient gas exchange than if the water had to go in and out the same way.
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