B14 Respiration and the Circulatory System.notebook 1 December 06, 2012 Some of the smallest and simplest life forms (the singlecelled organisms) respire directly through their cell membranes. Oxygen passes readily from the environment into these cells by simple . And because molecules always move from where they are concentrated to where they are concentrated, new oxygen from the environment constantly moves into the to replace oxygen that is used up. http://www.teachersdomain.org Many multicellular organisms also acquire oxygen through simple diffusion. Earthworms, for example, exclusively through their . However, because these animals have many layers of cells, many of which are far from the surface, they must have systems that pump blood through that are just one cell layer below the surface and then on to the innermost cells. diffusion most least cell respire skin circulatory capillaries Despite its simplicity, a respiratory system that relies on diffusion through the skin is very limiting. For example, diffusion works well only when oxygen is in water, which is why earthworms and frogs are confined to environments. Skin respiration also limits an animal's . A hippopotamus, for example, could not possibly acquire enough through its skin because the ratio of its total volume is quite . This means that most of the hippo's cells are internal rather than on the surface. Skin respiration is simply too inefficient to supply oxygen to all of those internal cells. size dissolved surface area moist Oxygen small Respiration and the Circulatory System http://www.teachersdomain.org/asset/tdc02_vid_circulator/ Gas Exchange in Smaller Organisms