Scientists have found that our lungs have odour receptors, suggesting that nose is not the only organ in the body to have a sense of smell.
Unlike the receptors in the nose, which are located in the membranes of nerve cells, the ones in the lungs are in the membranes of neuroendocrine cells, researchers said.
Instead of sending nerve impulses to the brain that allow it to "perceive" the acrid smell of a burning cigarette in the vicinity, they trigger the flask-shaped neuroendocrine cells to dump hormones that make the airways constrict.
The newly discovered class of cells expressing olfactory receptors in human airways, called pulmonary neuroendocrine cells, or PNECs, were found by a team led by Yehuda Ben-Shahar, assistant professor of biology, in Arts & Sciences, and of medicine at Washington University in St Louis.
"We forget that our body plan is a tube within a tube, so our lungs and our gut are open to the external environment. Although they're inside us, they're actually part of our external layer," said Ben-Shahar.
"So they constantly suffer environmental insults and it makes sense that we evolved mechanisms to protect ourselves," he said.
In other words, the PNECs are sentinels, guards whose job is to exclude irritating or toxic chemicals, researchers said.
The cells might be responsible for the chemical hypersensitivity that characterises respiratory diseases, such as chronic obstructive pulmonary disease (COPD) and asthma.
The odour receptors on the cells might be a therapeutic target, Ben-Shahar suggested. By blocking them, it might be possible to prevent some attacks, allowing people to cut down on the use of steroids or bronchodilators.
Ben-Shahar stressed the differences between chemosensation in the nose and in the lung.
The cells in the nose are neurons, each with a narrowly tuned receptor, and their signals must be woven together in the brain to interpret our odour environment, he said.
The cells in the airways are secretory, not neuronal, cells, and they may carry more than one receptor, so they are broadly tuned. Instead of sending nerve impulses to the brain, they flood local nerves and muscles with serotonin and neuropeptides.
"They are possibly designed to elicit a rapid, physiological response if you inhale something that is bad for you," he said.
The scientists suspect these pulmonary neuroscretory cells contribute to the hypersensitivity of patients with COPD to airborne irritants.
When scientists looked at the airway tissues from patients with COPD, they found they had more of these neurosecretory cells than airway tissues from healthy donors.
The study appeared in the American Journal of Respiratory Cell and Molecular Biology.