Goblet cells of the respiratory tract produce a physical barrier called (blank)
real answers .

I believe the correct answer is mucus.

In the respiratory tract, there exist hairs called cilia that ensure anything entering the tract is moved out through a wavelike movement. To assist them, mucus is produced in pits of specific cells called goblet cells.

The mucus traps dust and other large particles that might cause physical harm to the soft tissue of the linings of the respiratory tract. The mucus starts all the way from the mouth to the nasal cavities, the trachea, bronchi, bronchioles and the lungs. Goblet cells are just but specialized epithelial cells that are usually found in every hollow surface in the body and are non keratinized. These goblet cells are not only found in the respiratory tract but every surface of the gastrointestinal tract. In the stomach, they are very essential since they protect the stomach from corroding itself because it secretes hydrochloric acid that has a pH of 2.3 which is very acidic. It also lubricates the gastrointestinal tract to ensure the smooth movement of food from the mouth to the anus as it is being digested.

the relative atomic mass of an element is the mass of one atom of that element, relative to 1/12th of the mass of a  1212c atom. to convert to an absolute mass in kg, it is necessary to multiply by  1.661×10−271.661×10−27.

the relative atomic mass is roughly equal to the number of neutrons, plus the number of protons, in the nucleus of the atom.

for elements with more than one stable isotope (different types of nucleus with different numbers of neutrons but the same number of protons), the quoted mass is usually the average mass, weighted by the relative abundance of the different isotopes.