Question

Why should we breathe in through the nostrils?

Answer 1

Nostrils, or napes, are the two holes in the nose. They lead to two nasal chambers divided by a cartilage wall known as the septum. Sinus cavities are a complex system of canals and air pockets that run across the face. Sinus cavities go from the back of the skull to just above the mouth cavity, through the cheekbones, and between the eyes and brows. All of these regions are involved for breathing, smelling, taste, and immune system protection, at least in part.

Complete answer:
Breathing through nostrils is important for healthy respiration for a variety of reasons. Few of the major reasons are listed as follows:
Fine hair and mucus coat the inside of the nose canal. The air we breathe in passes through the nasal channel when we breathe through our nostrils. This permits any tiny dust particles or microorganisms present in the air to be trapped by the hair and mucus and filtered before reaching the lungs.
In addition, breathing in via the nose maintains the blood's oxygen and carbon dioxide equilibrium. When breathing by the mouth, a higher volume of air is breathed, making it impossible to maintain this equilibrium.
Nasal breathing also adds moisture to the air we breathe in.
Nasal breathing also warms the air we breathe in.

Note:
Breathing is the process of moving air out and into the lungs in order to enable gas exchange with the internal environment, primarily to remove carbon dioxide and bring in oxygen. During expiration by nasal breathing, air resistance causes some air to flow backwards, allowing additional time for gaseous exchange in the lung alveoli. This backflow of air also accounts for residual volume.