Do you wonder how your body directs the smell of food into the nasal cavity and not in the other direction - into the lungs? The answer lies in a 3D-printed model of the human airway from nostril to trachea.

The model developed by a team of engineers from Pennsylvania State University shows that the shape of the airway preferentially transfers food chemicals to the nasal cavity and allows humans to enjoy the smell of good food.

And for a really good meal, taking time to slow down and breathe smoothly will deliver more smell and flavour, they said.

During quiet breathing, there is no valve that can control the direction of food volatile (chemicals) transport.

"However, something must be controlling the movement of these particles and keeping them out of the lungs," said Rui Ni, assistant professor of mechanical engineering.

In the past, physiologists looked at the nasal passages, but not at the pathway from the back of the mouth to the nose.

In the new study, the researchers used data from CT scans and the help of two radiologists to build a schematic of the human airway from the nostrils to the trachea, including the fine structure.

They then used the schematic to make a 3D model using a 3D printer.

Ni and colleagues then tested airflow into and out of the airway.

Chewed food particles end up in the back of the mouth in a sort of side cavity to the main airflow.

The team found that when air is inhaled through the nose, the air flow forms an air curtain to prevent volatile particles released from the back of the mouth from escaping into the lungs.

However, when air is exhaled, it sweeps into the area with abundant food volatiles moving them into the nasal cavity where they are sensed by olfactory cells.

Movement of the particles is also effected by the speed of breathing.

"Smooth, relatively slow breathing maximises delivery of the particles to the nose. Food smells and tastes better if you take your time," Ni said.

The results were detailed in the journal Proceedings of the National Academy of Sciences.