Gas Exchange in Animals

An animal’s survival in their environment largely depends on their gas exchange system. For it to be effective, their gas exchange surfaces need to be thin so that the barrier they present to diffusion is minimized. The surfaces also need to be damp and moist. This is because oxygen and carbon dioxide will diffuse faster when dissolved in water. Finally, there needs to be a large surface area for efficient gas exchange. This is because the greater the surface area of the gas exchange surface, the faster the exchange of gases. The gas exchange systems in mammals, insects and fish all have these traits and have adapted to their environment to be able to survive.

Gas exchange is more difficult for fish than for mammals and insects because the concentration of dissolved oxygen in water is less than 1%, compared to 20% in

air. Because of their aquatic environments fish have developed specialized gas-exchange organs called gills, which are composed of thousands of filaments. The filaments in turn are covered in feathery lamellae, which are only a few cells thick and contain blood capillaries. This structure gives a large surface area and a short distance for gas exchange. Water flows over the filaments and lamellae, and oxygen can diffuse down a concentration gradient the short distance between water and blood, whilst CO2 diffuses in the opposite direction, also down its concentration gradient. Each gill is covered by a muscular flap (the operculum) on the side of a fish's head. The gills are so thin that they cannot support themselves without water, so if a fish is taken out of water after a while the gills will collapse, the surface area to volume ratio falls, and the fish suffocates. Fish ventilate their gills to maintain the gas concentration gradient. The fish continuously pump water over their gills by moving their mouth and opercula, sucking in water from in front of the fish, passing it over the gills and then expelling the ‘stale’ water behind. The opercula valve ensures the one-way flow that the high density of water requires. The gill lamellae are arranged as a series of flat plates sprouting from the gill arch. On their upper and lower surfaces there are many thin vertical flaps, which contain blood capillaries. The blood flows through these capillaries in the opposite direction to the flow of water over the gills. This is called a counter-current flow system and gives a highly efficient diffusion pathway since as the blood flows along and picks up oxygen it meets water which always has a greater oxygen content than itself and the diffusion of oxygen into the blood will be maintained. By having internal gills, it means they are protected although when the gills are not supported by water they collapse, meaning the fish gas exchange system is restricted