Gas Exchange in Fish
Environment
Fish live in water, which has a low concentration of oxygen. This means lots of water has to flow over the gas exchange surface for the fish to take in enough oxygen. Water is also harder to ventilate than air because it is more viscous, so fish have to use lots of energy for ventilation. There is no risk of the gas exchange surface drying out in the water, so it is close to the surface of the body rather than deep inside.
Structures and Adaptations
The gills are a fish's gas exchange system. 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 means the water flows through the gills in one direction, allowing for more efficient gas exchange than if the water had to go in and out the same way. This is important for fish becaus of the low oxygen concentration in water. Because water enters the gas exchange surface the same way as food enters the fish's body, unwanted food particles might be in the water. Gill rakers filter out these food particles before the water passes through the gills to keep them clean. Within the gills, there are four gill arches which are each lined with thin filaments to increase the surface area. On the surface of the filaments are lamellae, which contain blood vessels. The filaments and lamellae give gills a large surface area:volume ratio, increasing the efficiency of diffusion of gases between the water and the blood by allowing more particles to diffuse at any one time. Fish have a circulatory system, so their size is not limited. The blood absorbs the oxygen, and then carries it around the body to every cell where it is needed for respiration. The blood flows through the blood vessels in the opposite direction to the water flowing through the lamellae. This is called the counter-current system. This system maximises the amount of oxygen diffused into the blood by having the most oxygenated blood meet the most oxygenated water, and the least oxygenated blood meet the least oxygenated water to maintain the concentration gradient the whole way through. With both blood and water flowing in the same direction, when the blood reaches the same oxygen concentration as the water, diffusion of oxygen into the blood will stop, so less oxygen can be absorbed by the blood. The counter current system allows the maximum amount of oxygen to diffuse into the blood from the water available. This is important because there isn't much oxygen in the water, and fish need to absorb enough oxygen to survive.
Advantages and Limitations
The counter-current system is an advantage of the gas exchange system in fish. It allows maximum oxygen to be absorbed by the blood from the water, by maintaining the concentration gradient the whole way through the gills. This means fish can get enough oxygen without having to push large amounts of water through their gills. Because water is viscous, this would require a lot of energy, so the counter-current system is a more efficient way of increasing the amount of oxygen diffusing into the blood.
The unidirectional flow of water is another advantage. Once oxygen has been absorbed from the water, the water leaves the body through the operculum, and all water that comes in the mouth will pass through the gills. All of the water passing over the gills has oxygen in it, and no water that comes in the mouth will leave without passing through the gills. This helps the efficiency of gas exchange in fish by allowing oxygen to be absorbed from all of the water that comes into the fish. If the water had to go out the same way, some water wouldn't make it to the gas exchange surface for oxygen to diffuse into the blood from it.
A limitation of this gas exchange system is that fish can only live in water. They need water to support the filaments and hold the lamellae apart to keep the surface area large. In air, the filaments and lamellae would stick together, greatly reducing the surface area: volume ratio, and therefore decreasing the efficiency of diffusion of gases. The gills would also dry out without water keeping them moist, so gases would no longer be able to dissolve in order to diffuse into the blood.
Fish live in water, which has a low concentration of oxygen. This means lots of water has to flow over the gas exchange surface for the fish to take in enough oxygen. Water is also harder to ventilate than air because it is more viscous, so fish have to use lots of energy for ventilation. There is no risk of the gas exchange surface drying out in the water, so it is close to the surface of the body rather than deep inside.
Structures and Adaptations
The gills are a fish's gas exchange system. 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 means the water flows through the gills in one direction, allowing for more efficient gas exchange than if the water had to go in and out the same way. This is important for fish becaus of the low oxygen concentration in water. Because water enters the gas exchange surface the same way as food enters the fish's body, unwanted food particles might be in the water. Gill rakers filter out these food particles before the water passes through the gills to keep them clean. Within the gills, there are four gill arches which are each lined with thin filaments to increase the surface area. On the surface of the filaments are lamellae, which contain blood vessels. The filaments and lamellae give gills a large surface area:volume ratio, increasing the efficiency of diffusion of gases between the water and the blood by allowing more particles to diffuse at any one time. Fish have a circulatory system, so their size is not limited. The blood absorbs the oxygen, and then carries it around the body to every cell where it is needed for respiration. The blood flows through the blood vessels in the opposite direction to the water flowing through the lamellae. This is called the counter-current system. This system maximises the amount of oxygen diffused into the blood by having the most oxygenated blood meet the most oxygenated water, and the least oxygenated blood meet the least oxygenated water to maintain the concentration gradient the whole way through. With both blood and water flowing in the same direction, when the blood reaches the same oxygen concentration as the water, diffusion of oxygen into the blood will stop, so less oxygen can be absorbed by the blood. The counter current system allows the maximum amount of oxygen to diffuse into the blood from the water available. This is important because there isn't much oxygen in the water, and fish need to absorb enough oxygen to survive.
Advantages and Limitations
The counter-current system is an advantage of the gas exchange system in fish. It allows maximum oxygen to be absorbed by the blood from the water, by maintaining the concentration gradient the whole way through the gills. This means fish can get enough oxygen without having to push large amounts of water through their gills. Because water is viscous, this would require a lot of energy, so the counter-current system is a more efficient way of increasing the amount of oxygen diffusing into the blood.
The unidirectional flow of water is another advantage. Once oxygen has been absorbed from the water, the water leaves the body through the operculum, and all water that comes in the mouth will pass through the gills. All of the water passing over the gills has oxygen in it, and no water that comes in the mouth will leave without passing through the gills. This helps the efficiency of gas exchange in fish by allowing oxygen to be absorbed from all of the water that comes into the fish. If the water had to go out the same way, some water wouldn't make it to the gas exchange surface for oxygen to diffuse into the blood from it.
A limitation of this gas exchange system is that fish can only live in water. They need water to support the filaments and hold the lamellae apart to keep the surface area large. In air, the filaments and lamellae would stick together, greatly reducing the surface area: volume ratio, and therefore decreasing the efficiency of diffusion of gases. The gills would also dry out without water keeping them moist, so gases would no longer be able to dissolve in order to diffuse into the blood.
Header image from http://tracycochran.org/2012/06/a-fish-in-water/