Exchange Surfaces

• surface area: volume ratio can be calculated by surface area divided by the volume
• The surface area to volume ratio decreases as cells get larger.
• Larger organisms have too small of a surface area to volume ratio so they cannot exchange gases over an outer membrane. Therefore, they have evolved to have specialised exchange systems.
• Insects can be highly active and therefore have a high oxygen demand, especially in flight.
• In insects, the gas exchange system has evolved to provide oxygen directly to cells.
• Spiracles allow gases such as carbon dioxide and oxygen to diffuse into the body of the insect.
• Spiracles lead into a network of tubes called tracheae.
• Tracheae are relatively wide tubes with a diameter of around 1 mm.
• Tracheae extend down and along an insect's body.
• The walls of tracheae are reinforced with spirals of chitin. This chitin prevents the tracheae from collapsing, for example, when an insect moves.
• Extending from the tracheae are very fine tubes called tracheoles. Tracheoles have a diameter of around 1 micrometer or less.
• Each tracheole is a single cell that has extended to form a hollow tube. A huge number of tracheoles extend down in between the cells of the insect's body.
• Tracheoles are not supported by chitin.
• Because tracheoles have such a narrow diameter and are extremely close to cells there is a very short diffusion distance for gases moving between the cells and the tracheoles. This allows for the rapid diffusion of gases during aerobic respiration.
• The huge number of tracheoles provides a very large surface area for gas exchange. This allows insects to maintain a very rapid rate of aerobic respiration, for example, during flight.
• The ends of tracheoles are filled with fluid which is called tracheal fluid.
• During intense activity, cells around the tracheoles undergo anaerobic respiration.
• Anaerobic respiration produces lactic acid which lowers the water potential of the cells. This causes water in the tracheal fluid to move into the cells. This reduces the volume of tracheal fluid, drawing air down into the tracheoles. It also means that more tracheole surface is available for the diffusion of oxygen and carbon dioxide.
• For many insects, gas exchange is essentially a passive process. Oxygen diffuses down its concentration gradient from the high concentration in the external air into the tracheoles where the concentration is lower. Also, carbon dioxide diffuses down its concentration gradient from the relatively high concentration in the tracheoles out to the external air.
• The walls of the tracheoles are moist and the ends of the tracheoles contain tracheal fluid. This means that water vapour can diffuse out of an insect via the spiracles.
• Each spiracle is surrounded by a muscular sphincter. This means that insects can reduce water loss by closing their spiracles e.g. when an insect's oxygen requirement is relatively low.
• Insects have three main body segments: head, thorax and abdomen.
• Some insects can contract muscles to change the volume of the thorax and the abdomen. This causes pressure changes in the tracheae and tracheoles pushing air in and out. This bulk movement of air is called mass transport.
• In some insects, the tracheae contain expanded sections called air sacs and changes in the volume of the thorax and abdomen can squeeze the air sacs causing air to move from the air sacs into the tracheoles. Insects can also use the oxygen in the air sacs during times when the spiracles have been closed for water conservation.