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Tube (inner)
Inside of most modern bicycle tires you will find an inner tube, which is basically a rubber balloon with a valve that fits through the rim so that you can inflate it.
The purpose of the tube is to make the tire airtight. The tire itself may have small holes and pores without creating a problem, as long as the tube doesn't leak. If the tire has a large hole or cut, the stretchy rubber tube can bulge through and burst. The tube does not, in itself, have the ability to withstand the high pressures used in bicycle tires without the support of a tire around it.
Andy tube will slowly lose pressure as air seeps out through the rubber. Most tubes are made of synthetic (butyl) rubber. The can usually go a week or more without significant loss of pressure. Some tubes have been made of natural latex rubber, for light weight and more flexibility -- but they need to be pumped up at least once per day.
