The human nose has two distinct openings, or nostrils. While each nostril serves as an entry point for air, they are not entirely independent. Both nostrils ultimately lead to a shared internal space, the nasal cavity, before air continues its journey to the lungs. This structure facilitates breathing and other functions through a complex interplay of division and convergence.
The Nasal Divider
The nasal septum physically separates the two nostrils. This partition runs down the center of the nose, dividing the nasal cavity into two distinct halves. The septum is composed of both bone and cartilage, providing rigidity and flexibility. Its front part is primarily flexible cartilage, while the back consists of bony structures. This central divider ensures that air entering each nostril follows its own initial path.
Airflow Pathways
Air enters the nose through the nostrils and travels through these separated passages. Each side of the nasal cavity contains turbinates, curved shelves of bone that project into the airway. These turbinates help create turbulence, directing air over a large surface area. Despite the initial separation by the septum, the individual pathways from each nostril converge deeper within the nasal cavity into a common space called the nasopharynx. From there, air proceeds to the throat and into the respiratory system, eventually reaching the lungs.
The Nasal Cycle
The nasal cycle influences how airflow is distributed between the nostrils. This unconscious cycle involves the alternating partial congestion and decongestion of the nasal cavities. One nostril becomes more open, allowing greater airflow, while the other becomes more congested due to the swelling of erectile tissue within the turbinates, reducing its airflow. This alternation is controlled by the autonomic nervous system and typically switches dominance every few hours, though the duration can vary. Most individuals are unaware of this cycle unless they have nasal congestion.
Advantages of Dual Nostrils
Having two nostrils offers several functional advantages for both breathing and the sense of smell. The differing airflow rates created by the nasal cycle are beneficial for olfaction, the detection of scents. Some scent molecules are better detected in faster-moving air, while others are more readily absorbed in slower airflow. This allows the nose to detect a wider range of odors than it would with a single, uniformly flowing passage. Additionally, the dual passages contribute to the conditioning of inhaled air, warming and humidifying it more effectively before it reaches the lungs.