External nares are the visible openings of the nose, commonly known as nostrils. They serve as the initial entry points for air into the respiratory system. These openings are a fundamental part of human anatomy, playing a role in physiological processes.
Anatomy and Appearance
The external nares are two distinct openings located at the base of the nose, leading directly into the nasal cavities. These openings are bounded laterally by the nasal alae, the fleshy, cartilaginous structures forming the wings of the nose. A thin wall of cartilage, known as the nasal septum, separates the two external nares, dividing the nasal cavity into two distinct passages. This cartilaginous framework, along with underlying bone, provides the nose with its overall shape and structure.
The skin covering the external nares is continuous with the lining inside the nasal vestibule, the frontmost part of the nasal cavity. This vestibule contains hair follicles and sebaceous glands. Small hairs, called vibrissae, are present at the entrance of the nostrils. The size and shape of the external nares can vary among individuals, influenced by genetic factors.
Essential Functions
External nares serve as the primary gateway for air to enter the respiratory system during inhalation. This initial intake of air is a fundamental step in respiration. The air then moves into the nasal cavities for further processing before reaching the lungs.
Air filtration is a primary function of the external nares, where nasal hairs and mucus work together to trap airborne particles. Vibrissae act as a first line of defense, capturing larger particles like dust and dirt, preventing them from entering deeper into the respiratory tract. Beyond the hairs, mucous membranes lining the nasal passages also trap finer particles, which can then be swallowed or expelled.
The external nares also contribute to warming and humidifying inhaled air. The nasal lining contains a rich blood supply that helps warm the incoming air to a temperature closer to body temperature. Simultaneously, moisture from the mucous membranes humidifies the air, preparing it for the sensitive tissues of the lungs. This conditioning process ensures the air reaching the lungs is less irritating and more suitable for gas exchange.
Role in the Sense of Smell
While primarily associated with respiration, external nares are also important for the sense of smell, known as olfaction. They serve as the entry point for odor molecules into the nasal cavity. As air is inhaled through the nostrils, it carries various volatile odor compounds.
These odor molecules travel past the external nares and deeper into the nasal cavity, where they reach the olfactory receptors. These specialized nerve cells are located in the olfactory epithelium, a tissue in the upper part of the nasal cavity. The nares allow these molecules to access the sensory cells, initiating smell perception.