Question

Respiratory surface of animals must always be moist. Why?

Answer 1

Hint: Exchange of gases occurs in both plants and animals. For such exchange to occur, animals have a respiratory system which is made up of particular organs which performs tasks specific to the process of respiration.

Complete answer:
Gas exchange takes place on the respiratory surface which usually includes the gills, trachea, lungs, etc. Alveoli and alveolar blood capillaries lead to the exchange of gases in the lungs.
Image showing respiratory system

During the process of breathing in animals, which is termed as aerobic respiration, an exchange of gaseous components is conducted where the carbon dioxide leaves the cell and oxygen enters the system. By diffusing gases over their cell membrane, single-celled organisms can diffuse gas directly across their cell membrane.
But by diffusing gas across the outer surface, large animals are unable to sustain their gas exchange. Thin, moist epithelial cells cover the respiratory surface, which allows oxygen and carbon dioxide to get exchanged. Therefore, respiratory surfaces must be wet for these gases to cross cell membranes.

Note:
Humans and other living beings benefit from the respiratory system, which consists of a network of organs and tissues that let them breathe.
A human body's airways, lungs, and blood arteries are part of this system as shown in the image above. The respiratory system also includes the lungs' muscles. These components work together to transport oxygen throughout the body and remove waste gases such as carbon dioxide from the bloodstream.