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The Mechanics of Human Olfaction Ok, here's how your sense of smell works. Let's say you've cut a lemon in half, releasing the odor of the lemon. Little lemon odor molecules drift up from the lemon and into your nostrils. From the nostrils, these little lemon molecules travel into your nasal cavities, of which there are two, separated by a partition called the septum. Within the cavities are bony plates, called turbinates, which serve to break up the air flow, causing it to swirl around the cavity. This swirling action also serves to warm up the air, releasing more molecules. These molecules continue to drift upward until they reach the top of the nasal cavity, known as the olfactory epithelium. Up in the epithelium is where little hairs, called olfactory cilia, are located. Here is where smell begins for all of us.
Above: The olfactory epithelium. First, there are three factors which determine how quickly our brain can detect an odor:
But before an odor can even be detected by our nose, first its molecules must be released from their source, usually under heat. At room temperature, for example, we may not smell steel because the steel molecules are not volatile (are not released) at room temperature. As a sheet of steel is heated, however, it undergoes molecular changes; its otherwise dormant molecules become excited and volatile. Some molecules are more volatile than others, however, depending upon their weight. Odor molecules with high molecular weights, such as musk, for example, dissolve less easily because they move more slowly through the air. They are less volatile than odors with lighter molecular weights, such as lemon, which are detected more quickly. |
