===Advantages=== *Having an internal gas exchange system (lungs) helps to reduce water loss and maintain a moist gas exchange surface. This allows mammals to inhabit a greater variety of terrestrial habitats. *The highly branched structure of the bronchial tree leads to many tiny alveoli. The large number and small size of the alveoli gives the lungs a large surface area to volume ratio. This allows mammals to grow to much larger sizes without the limitations normally imposed by the associated increase in the size and weight of the gas exchange system. The large surface surface area of the lungs also helps to meet the high metabolic demands of these larger, warm blooded animals. *A transport system (blood vessels) reduces the distance dissolved gasses have to diffuse. This also enables mammals to grow to larger sizes than many other animal groups. ===Disadvantages=== *Does not work under water. Water is denser and more viscous and could not easily be ventilated (forced in and out of the tracheal tubes). *Air flow is '''tidal''' (in / out) and not all of the air can be forced out of the lungs. Some oxygen poor air always remains and mixes with fresh air coming into the lungs, slightly reducing the concentration gradient and making gas exchange less efficient. *The gas exchange surface is warm, moist and continuously ventilated, making it a site of significant water loss (despite being internal). Humans exhale over a cup of water each day. Most larger mammals rely on reservoirs of fresh water such as rivers and lakes which may limit their range. *The gas exchange surface is warm and moist making it prone to infection. Respiratory illnesses such as cold and flu can also be easily transmitted when fine droplets containing bacteria / viruses are coughed or sneezed into the air and then breathed in by others.