The external nares, commonly known as nostrils, are the two openings at the front of the nose. These structures are the visible entry points to the nasal cavity, a crucial component of the upper respiratory system. The external nares serve as the primary gateway for air to enter and exit the body during respiration, initiating the breathing process.
Facilitating Airflow
The external nares function as the direct conduit for air to move into and out of the respiratory system. Their open design allows for unobstructed passage, marking the first step in both inhalation and exhalation. During inhalation, air is drawn through these openings into the nasal cavity, directing airflow towards the pharynx and into the lower airways and lungs.
The physical structure of the external nares, supported by cartilage, maintains their patency, ensuring a continuous pathway for breathing. Airflow mechanics involve a pressure differential, drawing air inward during inhalation and expelling it during exhalation. The shape and size of the nostrils can influence this airflow, optimizing the volume of air reaching the lungs.
The nasal cavity, immediately beyond the nares, contains structures like turbinates that guide and regulate airflow. These bony projections increase the surface area within the nasal passages, ensuring inhaled air makes sufficient contact with the nasal lining. This interaction prepares the air for the delicate tissues of the lungs. The continuous flow facilitated by the external nares underpins the entire respiratory process.
Conditioning the Air
The external nares initiate the conditioning of inhaled air, preparing it for the sensitive respiratory tract. The vestibule, just inside the nostrils, contains coarse hairs known as vibrissae. These hairs act as a preliminary filter, trapping larger airborne particles such as dust and pollen, preventing them from entering deeper into the nasal cavity and lungs.
Beyond the vibrissae, the nasal passages are lined with a mucous membrane that plays a significant role in air conditioning. This lining secretes mucus, a sticky fluid that captures smaller particles, bacteria, and viruses. The mucus also humidifies dry incoming air, preventing the drying of delicate lung tissues. The nasal cavity warms incoming cold air due to its rich blood supply, with numerous capillaries close to the surface. This vascular network transfers heat to the inhaled air, raising its temperature to approximate body temperature by the time it reaches the lungs.
Enabling the Sense of Smell
The external nares are indispensable for the sense of smell, or olfaction, by acting as the entry point for odor molecules. When air carrying these molecules is inhaled, it is directed towards the upper part of the nasal cavity. This region houses the olfactory epithelium, a specialized tissue containing millions of olfactory receptor neurons. These neurons detect various chemical compounds that register as scents.
Odor molecules must first dissolve in the thin layer of mucus covering the olfactory epithelium. Once dissolved, these molecules bind to specific protein receptors located on the cilia of the olfactory receptor cells. This binding generates electrical signals transmitted along the olfactory nerves to the olfactory bulb in the brain. The brain interprets these signals as distinct smells, allowing for the perception of a wide range of odors. Without the external nares, odor molecules cannot reach these receptors, impairing the sense of smell.