Comparisons
Fish, insects and mammals have very different gas exchange systems. They each increase the surface area:volume ratio in a different way. Fish have filaments and lamellae in the gills. These are very long and thin, maximising the surface area. In mammals, the tissue of the gas exchange surface folds back on itself. These folds of the alveoli increase the surface area. In insects, oxygen diffuses straight from the tracheoles into the cells, so the tracheoles reach every cell, but there is no need to have a large surface area:volume ratio at each place where gas exchange occurs.
The three groups of animals also have different ways of keeping the gas exchange surface moist. Fish live in water, and get their oxygen from water, so this keeps their gills moist. Mammals have mucus in the trachea keeping it moist, and the whole gas exchange system is internal to keep this moisture in. Insects have water at the ends of their tracheoles for oxgen to dissolve in to diffuse. The gas exchange system is internal to reduce water loss, and the spiracles can open and close to control it, depending on environmental conditions.
Both fish and mammals have a circulatory system. Oxygen is absorbed into the blood at the gas exchange surface to be carried around the body to every cell. Because the oxygen is transported by the blood, the sizes of fish and mammals are unlimited. Insects on the other hand don't have a circulatory system. Their tracheoles reach every cell, and they rely on diffusion for oxygen to reach each cell. The bigger the insect, the harder it is for oxygen to get to every cell by diffusion. This means the size of insects is limited, and they don't get very big. The circulatory system makes the gas exchange system in fish and mammals more efficient than the gas exchange system in insects.
Mammals and insects both get their oxygen from air, while fish get oxygen from water. Water has a much lower oxygen concentration than air, and is harder to ventilate because it is more viscous. This means fish need to have a more efficient gas exchange system to get enough oxygen from the water. This is why fish have a unidirectional flow of water through the gills while mammals and insects have tidal ventilation. The unidirectional flow is achieved by taking water in through their mouths, then pushing it through the gills and out the operculum. The water flows through the lamellae in the gills in the opposite direction to the blood. This is called the counter-current system and allows for the most efficient diffusion of gases between the water and the blood. The concentration gradient is maintained the whole way through the gills, because the most oxygenated water meets the most oxygenated blood, and the least oxygenated water meets the least oxygenated blood. If the water and blood flowed in the same direction, the oxygen concentration of blood and water would equalise at 50% half way through the gills, so not as much oxygen would be absorbed by the blood. Mammals and insects have tidal ventilation, which means the air goes in and out the same way. This means with each breath, not all of the old air makes it out of the body, so not all of the air coming in is new. Because of this, mammals and insects don't get the maximum amount of oxygen out of the air. Fish have a much more efficient way of ventilating the gas exchange surface than insects and mammals. The counter current system also makes fish more efficient at getting oxygen into their bloodstream than mammals.
Insects have air sacs which can store extra air. This means insects can close their spiracles and only use oxygen from the air in their air sacs for a period of time. They do this when the air is dry and they are losing too much moisture from the tracheoles through the spiracles. The air sacs also allow them to take in more air when they have a higher energy demand. For example, when they are flying. Mammals and fish don't have anywhere to store an extra oxygen supply, so can't stop breathing for long. When they have a higher energy demand, fish must pass more water through their gills and mammals must pass more air through their lungs. This structure is an advantage for insects as it can reduce water loss through the spiracles.
The three groups of animals also have different ways of keeping the gas exchange surface moist. Fish live in water, and get their oxygen from water, so this keeps their gills moist. Mammals have mucus in the trachea keeping it moist, and the whole gas exchange system is internal to keep this moisture in. Insects have water at the ends of their tracheoles for oxgen to dissolve in to diffuse. The gas exchange system is internal to reduce water loss, and the spiracles can open and close to control it, depending on environmental conditions.
Both fish and mammals have a circulatory system. Oxygen is absorbed into the blood at the gas exchange surface to be carried around the body to every cell. Because the oxygen is transported by the blood, the sizes of fish and mammals are unlimited. Insects on the other hand don't have a circulatory system. Their tracheoles reach every cell, and they rely on diffusion for oxygen to reach each cell. The bigger the insect, the harder it is for oxygen to get to every cell by diffusion. This means the size of insects is limited, and they don't get very big. The circulatory system makes the gas exchange system in fish and mammals more efficient than the gas exchange system in insects.
Mammals and insects both get their oxygen from air, while fish get oxygen from water. Water has a much lower oxygen concentration than air, and is harder to ventilate because it is more viscous. This means fish need to have a more efficient gas exchange system to get enough oxygen from the water. This is why fish have a unidirectional flow of water through the gills while mammals and insects have tidal ventilation. The unidirectional flow is achieved by taking water in through their mouths, then pushing it through the gills and out the operculum. The water flows through the lamellae in the gills in the opposite direction to the blood. This is called the counter-current system and allows for the most efficient diffusion of gases between the water and the blood. The concentration gradient is maintained the whole way through the gills, because the most oxygenated water meets the most oxygenated blood, and the least oxygenated water meets the least oxygenated blood. If the water and blood flowed in the same direction, the oxygen concentration of blood and water would equalise at 50% half way through the gills, so not as much oxygen would be absorbed by the blood. Mammals and insects have tidal ventilation, which means the air goes in and out the same way. This means with each breath, not all of the old air makes it out of the body, so not all of the air coming in is new. Because of this, mammals and insects don't get the maximum amount of oxygen out of the air. Fish have a much more efficient way of ventilating the gas exchange surface than insects and mammals. The counter current system also makes fish more efficient at getting oxygen into their bloodstream than mammals.
Insects have air sacs which can store extra air. This means insects can close their spiracles and only use oxygen from the air in their air sacs for a period of time. They do this when the air is dry and they are losing too much moisture from the tracheoles through the spiracles. The air sacs also allow them to take in more air when they have a higher energy demand. For example, when they are flying. Mammals and fish don't have anywhere to store an extra oxygen supply, so can't stop breathing for long. When they have a higher energy demand, fish must pass more water through their gills and mammals must pass more air through their lungs. This structure is an advantage for insects as it can reduce water loss through the spiracles.