Gas exchange occurs in the alveoli. Incoming air is taken in by the contraction of the diaphragm with air taken in. The oxygen diffuses across the thin alveolar membrane, which has fused with the capillary membrane so that the gases can exchange across the combined “respiratory membrane”. The alveoli contain surfactant that maintains the opening of the alveoli so that they do not collapse. The alveoli provide a very large surface area for gas exchange in mammals.
IMMUNE SYSTEMS There are two main immune systems in mammals with the innate immune system being older evolutionarily-speaking. Higher animals will have both an innate immune system and an adaptive immune system. Together, their role is to defend against pathogens. Pathogens for higher organisms include microorganisms such as bacteria, protists, and fungi. These can be found in the water, in the air, and on surfaces. The goal of the immune system is to recognize self from non-self and to get rid of those things that are recognized as non-self. The innate immune system functions since birth and is relatively nonspecific when it comes to its activity against pathogens. The adaptive immune system stores information about past infections, mounting pathogen-
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