The sense of smell
The organ of smell is placed in the both chambers of the nasal cavity. It takes approximately 5 cm2 and it is recognisable by yellow coloration of nasal mucous membrane.
The number of human olfactory receptors differs, on average it is about 25 million. For example, a dog has about 220 million of these receptors. The smell receptors regenerate throughout the lifespan over about 30 days. A human needs different concentration for different substances in inspired air. We detect some even there are in small amounts and some have to be in higher concentration. For example, we are not able to identify by smell poisonous carbon dioxide.
A human is able to differentiate flowery, fruity, musk, putrid, acrid and burnt scents.
How does the sense of smell works?
Smell receptors are chemoreceptors irritated by substances dissolved in mucous secretion which protect receptors cells from damage. The sense of smell is very sensitive, but it is quickly adaptable and thus not able to distinguish a particular odour after a lapse of time.
Women have heightened sence of smell than men and they are more sensitive to odours when ovulating. Smokers have lowered sensitivity of smell receptors.
Smell disorders
Hyposmia is a reduced ability to detect odours and the complete loss of smell is called anosmia.
Smell disorders can be caused by head injuries when nerve fibres transmitting information from receptors to brain are interrupted or corrupted. The loss of smell can be the symptom of other disease as Alzheimer’s or Parkinson’s disease. Diminished sense of smell can also be temporary and the most likely it is caused by rhinitis when nasal mucous membrane inflammation impedes airflow through olfactory receptors.
Source: