The Fish Gas Exchange System
Structure
The gas exchange organs in fish are the gills, which are near the mouth and are also near the the surface of the skin. They are covered by the operculum which is a flap of skin that protects the gills. Each gill is made of four bony gill arches that are lined with gill filaments, which have thin, feathery lamellae attached to them. Lamellae have a large surface area but a short distance which is ideal for effective diffusion. The lamellae also contain blood capillaries that have blood flowing against the direction of the flow water. The fish ventilates the water by opening it's mouth and forcing the water to flow over the gills which will enable oxygen to be extracted from the water. Unidirectional pumping occurs as a result, as fish must maintain a certain concentration of oxygen in the water next to the gill so enough oxygen can be diffused. The fish must prevent the accumulation of carbon dioxide in the water next to the gill or else the fish will suffocate as there won't be enough oxygen present. The fish could use buccal pumping or continuously swimming forward to push the water through it's gills in one way, e.g. sharks. It is imperative for the fish to maintain a constant flow of water over the gills.
What are the Advantages of a Fish's Environment?
For efficient gas exchange, one of the many requirements is to maintain a moist surface for the oxygen to diffuse into the blood, and for the carbon dioxide to diffuse out of the blood. As for fish, they live in an aquatic environment where they basically live in water. The fish will not face the problem of dessication 'drying out' which affect many land-based organisms such as mammals and insects, as the gills are constantly kept moist because of the environment they live in.
What are the Limitations of a Fish's environment?
Water is much more denser than air. This is because the atoms in water are slower than the atoms which are constantly flying around in air. Therefore air is easier to ventilate than water. Organisms which live in water must adapt to their environment to be able to respire, as all living organisms need oxygen to survive.
Dust and dirt travel freely through air and water. One limitation that may affect fish is the water that they live in may be dirty and prevent the gills to ventilate the water efficiently. Food particles may also be lodged in the gills by accident, as fish have both water and food entering through the mouth.
Another limitation is the oxygen concentration in water. Temperature, salinity and pressure may all affect the amount of oxygen present in the fish's environment. There is more dissolved oxygen in cooler water than in warmer water. Saltier water also has less dissolved oxygen than fresh water. Pressure also affects the oxygen concentration. Fish which live near the surface of the water have less oxygen to diffuse through their gills than the fish who live in greater watery depths e.g. angler-fish, whales etc.
As water is more vicious than air, it takes more energy to ventilate and therefore the fish must be able to have an efficient gas exchange system that will work efficaciously.
Dust and dirt travel freely through air and water. One limitation that may affect fish is the water that they live in may be dirty and prevent the gills to ventilate the water efficiently. Food particles may also be lodged in the gills by accident, as fish have both water and food entering through the mouth.
Another limitation is the oxygen concentration in water. Temperature, salinity and pressure may all affect the amount of oxygen present in the fish's environment. There is more dissolved oxygen in cooler water than in warmer water. Saltier water also has less dissolved oxygen than fresh water. Pressure also affects the oxygen concentration. Fish which live near the surface of the water have less oxygen to diffuse through their gills than the fish who live in greater watery depths e.g. angler-fish, whales etc.
As water is more vicious than air, it takes more energy to ventilate and therefore the fish must be able to have an efficient gas exchange system that will work efficaciously.
What are the Advantages of the Fish's gas exchange system and how have they adapted to their environment? (Compared to Mammals/Humans)
1. Fish must cope with the buoyancy of the water they live in. They have swim bladders which helps the fish to conserve energy for efficient gas exchange and help them travel at different altitudes in the water. Since humans live on land, there is hardly any buoyant force, so humans do not have swim bladders.
2. Because water is more vicious than air, it takes more energy to ventilate and swim through it. This causes an increase in demand for oxygen, and therefore fish have a continuous unidirectional flow of water over their gills for maximum diffusion efficiency. However, mammals have tidal breathing which is less efficient at diffusing oxygen. This is because mammals breathe in air, which has a higher oxygen concentration than water, so mammals don't need to have such an efficient gas exchange system such as fish.
3. Since fish ventilate water in only one direction, food particles can be caught in the delicate filaments and lamellae. Fish have evolved to obtain gill rakers which filter the water going through the gills, trapping specific sized food/prey (depending on the fish) and sending them down the oesophagus of the fish. Humans, which breathe tidally, and breathe air instead of water, have cilia both in the nasal cavities and also in the tracheal tubes in the lungs which trap bacteria, dirt, and particles in the inhaled air in mucus. The dirt-trapped mucus is then coughed up by the upwards beating of the cilia in the tracheal tubes and up into the oesophagus of the mammal and then swallowed.
4. The counter-current system in fish maximises the diffusion efficiency of oxygen in water. (How much oxygen is diffused into the blood.) The lamellae on the gill filaments allow the oxygenated water to pass through the gills in a specific way, which is in the opposite direction to that of the flow of the fish's blood. This allows the fish to diffuse oxygen out of water three times the rate that a human extracts oxygen out of air. This adaptation fully allows the fish to maximise the amount of oxygen being extracted from the water at any given time.
2. Because water is more vicious than air, it takes more energy to ventilate and swim through it. This causes an increase in demand for oxygen, and therefore fish have a continuous unidirectional flow of water over their gills for maximum diffusion efficiency. However, mammals have tidal breathing which is less efficient at diffusing oxygen. This is because mammals breathe in air, which has a higher oxygen concentration than water, so mammals don't need to have such an efficient gas exchange system such as fish.
3. Since fish ventilate water in only one direction, food particles can be caught in the delicate filaments and lamellae. Fish have evolved to obtain gill rakers which filter the water going through the gills, trapping specific sized food/prey (depending on the fish) and sending them down the oesophagus of the fish. Humans, which breathe tidally, and breathe air instead of water, have cilia both in the nasal cavities and also in the tracheal tubes in the lungs which trap bacteria, dirt, and particles in the inhaled air in mucus. The dirt-trapped mucus is then coughed up by the upwards beating of the cilia in the tracheal tubes and up into the oesophagus of the mammal and then swallowed.
4. The counter-current system in fish maximises the diffusion efficiency of oxygen in water. (How much oxygen is diffused into the blood.) The lamellae on the gill filaments allow the oxygenated water to pass through the gills in a specific way, which is in the opposite direction to that of the flow of the fish's blood. This allows the fish to diffuse oxygen out of water three times the rate that a human extracts oxygen out of air. This adaptation fully allows the fish to maximise the amount of oxygen being extracted from the water at any given time.
By having the blood flowing in the opposite direction (left diagram), it always flows next to the water that has more oxygen content than the water it has already flown past. This way, every time the water flows pass the blood, there is always a difference in the amount of oxygen present in either water or blood. This maintains a permanent concentration gradient, and the oxygen from the water can efficiently diffuse into the blood cells. If both blood and water flowed in the same direction, they would ultimately meet an equilibrium and then the amount of oxygen diffusing in the blood would only be at 50%.
What are the limitations of a Fish's gas exchange system?
When fish have already adapted to breathe in water, their gas exchange systems (gills) must now rely on the buoyancy of the water to keep the gill filaments apart. Fish can only respire when water is flowing pass their gills. This limits fish to only being able to survive in an aquatic
environment, unless they have special adaptations which allow them to breathe
air, e.g. mud-skippers. If fish were taken out of water, then the gill filaments
will collapse and the fish will suffocate. If left out of the water for a long
period of time, the fish will also face desiccation.