How do you give mouth to mouth underwater oxygen?

Fish have developed gills, on which they rely for the oxygen necessary for a fish's limited metabolism. Many animals have gills at some stage of their life (even humans have them at an early stage of their development in the womb), but fish retained these gills and they are still a functional part of their anatomy. Fish use their gills to extract oxygen from their watery environment.

The process starts with the fish's mouth, which is how the fish takes in water. When a fish opens and closes its mouth, it is actually pumping water back through the gills and is thus breathing. Most fish have an effective pumping system that involves the mouth and the outer cover of the gills, called the operculum.

When the fish's mouth opens, the operculum closes, drawing water into the fish's mouth. When the fish closes its mouth, the operculum opens, allowing fresh water to cross the gills. Other fish have a less effective pumping system, requiring them to swim constantly to keep fresh, oxygenated water flowing over the gills.

These types of fish, such as tuna, generally swim with their mouths partly open. Incidentally, while many fish have nostrils, the nostrils are used only for a sense of smell, and play no part in respiration. Once through the mouth, the water continues past structures called gill rakers.

The gill rakers are essentially a filter system for the gills, straining the water to sift out floating food particles or foreign material. After passing through the gill rakers, the water continues through the gill arches and actually passes over the gills, which are suspended between the mouth cavity and the operculum. Each gill is made of two rows of gill filaments, which are extremely thin membranes sticking out into the water flow.

Each of the gill filaments is composed of rows upon rows of lamellae, which are thin, disc-like membranes loaded with a capillary network. The water flows across the lamellae, and oxygen and carbon dioxide are exchanged directly across the capillary membrane. The capillaries are situated to take best advantage of the water flow; fish can actually extract up to 85% of available oxygen out of the water.

Since water contains only 2-5% of the available oxygen that air at sea level does, such a high efficiency is extremely important. From the gills, the deoxygenated water passes out the operculum, and the oxygenated blood joins the circulatory system. Despite the efficiency, some fish require more oxygen than others.

This helps to explain why some fish thrive in specific habitats. For example, trout prefer northern streams because the cool water of the streams tends to retain dissolved oxygen, and the active trout need the extra oxygen. Carp, on the other hand, are sluggish and do not need as much oxygen, which is why carp can thrive in warm, relatively stagnant ponds, such as ornamental ponds.

Goldfish, unlike most fish found in home aquariums, can survive in a non-aerated fish bowl because goldfish spend the majority of their time at the surface, where the oxygen content is highest due to the contact of the water with the atmosphere. Despite the obvious advantages of having such an efficient surface for air exchange, the gill method of breathing was replaced in land animals with the lung. There are two reasons for this.