In the case of oxygen and hydrogen gas, if one reacts them together in one way you will get a liquid which will, in that case, be water. The reason as to why humans cannot breathe liquid water is because oxygen used in making water is bound with two hydrogen atoms. This makes it difficult to breathe the liquid that results in which at this stage the oxygen in this form is said to be useless to our lungs.
The oxygen which fish breathe is not the one in water but the oxygen gas which is dissolved in water. In this case, it means that the fish takes the gas and not the liquid content of oxygen. Several different gases also dissolve in water or liquids an example being carbonated beverages. In these beverages, there is a large quantity of carbon dioxide which is dissolved in water which when the beverage is opened, it rushes out in the form of bubbles.
Fish breathe the oxygen which is dissolved in water by the use of their gills. Extracting the oxygen from the water is not such easy as air has about twenty times more oxygen that the same volume of the water. Also, water thicker and heavier than air, therefore, taking a lot more work and energy to take it around. The major reason as to why gills work well for fish is because they are cold-blooded that reduces their demand for oxygen. Warm-blooded animals such as the whale breathe air just like the people do since it is hard to get enough oxygen by their gills.
Massive aquatic animals, in this case, the fish have developed gills meant for respiration which is adopted for their functions. The gills have a large surface area so as to allow much oxygen to penetrate the gills as more of the oxygen gas contacts the membrane. There is also a good blood supply which maintains the gradient concentration needed. The membrane is also thin so as to reduce the diffusion pathway.
The gills contain four-gill arches on either side of the head two on each side to allow chondrichthyes or seven-gill baskets on the sides in Lampreys. In fish, the bony cover which is the operculum is used in pushing water. Without the use of an operculum ram ventilation is used as in sharks.
Bony fish uses the counter-current flow which maximizes the oxygen intake which diffuses via the gill. This flow occurs as a result of movement of deoxygenated blood through the gill in an opposite direction to that of oxygenated water. This mechanism helps in maintaining the concentration gradient thereby maximizing the efficiency of the process of respiration and also prevents the levels of oxygen from reaching concentration.
Physiological adaptations of fish to low oxygen levels.
Adaptations of fish to low oxygen can not only be behavioural but also physiological. Fishes which live in habitats which are frequently hypoxic may have a higher percentage of hemoglobin in the red blood cells which in turn leads to a higher capacity of blood to take up and transport the oxygen. On other hands, the tissues of their body may contain more myoglobin which is a molecule that brings the oxygen molecules together and also acts as a store of oxygen.
The main adaptation of the fish is anaerobic metabolism which is a collection of pathways which are biochemical in nature which requires less oxygen to yield energy. This kind of metabolism is not efficient and may lead to accumulation of toxic by-products, for instance, lactic acid, therefore, oxygen is available and anaerobic metabolism is neglected for aerobic metabolism.
When oxygen is insufficient, and the metabolic demand is minimal as in the cold-water fish anaerobic metabolism contributes to survival for days. For instance, goldfish can manage to survive for up to nine days at four degrees Celsius. Finally, drought also selects whether a fish should adopt anaerobic or aerobic respiration.
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