Nose

The nose is a vital sensory organ and respiratory structure found in vertebrates. It serves as the primary interface between the external environment and the delicate internal lungs. The nose performs two main roles: it enables the sensory perception of smell and it conditions incoming air. To do this, it must regulate temperature and filter out particles. The structure consists of an external nose on the face and a nasal cavity inside the head.

Air enters the body through the nostrils, also known as nares. These are pairs of tubes that provide airflow for respiration. As the nostrils pass into the nasal cavity, they widen into areas called nasal fossae. Inside, the nose uses several mechanisms to prepare air for the lungs. Tiny hairs in the nostrils act as a first line of defense by filtering dust, smoke, and other obstructions. The nose also produces mucus to add moisture to the respiratory system. Furthermore, capillary structures work to warm and humidify the air. This process is essential because it creates the conditions needed for the alveoli to exchange oxygen for carbon dioxide.

Nasal structures vary significantly across different groups of animals. In amphibians and lungfish, nostrils open into small sacs that connect to the roof of the mouth through choanae. In reptiles, the nasal chamber is generally larger and divided into three distinct parts. These parts are the anterior vestibule, the main olfactory chamber, and the posterior nasopharynx. Crocodilians have exceptionally long nasal chambers, which help them breathe while partially submerged. Birds also have a reptilian-style nose, though their olfactory chambers are often small because they generally have a poor sense of smell. Many birds, such as fowls, even have a horny protective shield covering their nostrils.

In mammals, the nasal cavities are often quite large, sometimes occupying up to half the length of the skull. This enlargement occurred partly because the development of a palate cut off the upper surface of the oral cavity. This turned that surface into part of the nose, leaving the palate as the new roof of the mouth. Mammalian nasal cavities contain complex turbinates, which are scroll-like shapes that help warm the air. The cavity also extends into the skull to form paranasal sinuses. These are dead-end air cavities used for humidification and pressure buffering.

Evolution has shaped the nose to suit specific lifestyles. It is theorized that the protruding mammalian nose evolved from the anterior part of the upper jaw of synapsids, which were reptilian-like ancestors. Some mammals have seen their noses change drastically. In cetaceans, the nose has migrated to the top of the head to become one or two blowholes. This adaptation allows for a streamlined body and breathing while mostly submerged. Conversely, the elephant's nose has evolved into a long, muscular, and manipulative organ called a trunk.

Different species use their noses in surprising ways. Dogs have a wet nose, known as a rhinarium, which helps them perceive the direction of a smell. Sensitive cold receptors in the skin detect where the nose is cooled most, indicating the direction of the scent. In fish, the nose functions very differently. It has no role in respiration and no connection to the mouth. Instead, fish use small pouches located behind the nostrils. Many fish have internal folds that form an elaborate "olfactory rosette" to increase surface area for smelling in water.

Nasal systems are deeply connected to other biological processes. In humans, the nasal cavity connects to the pharynx, which acts as a switch track valve between the respiratory and digestive systems. The vomeronasal organ is another important component found in many species. In reptiles, this organ is well-developed, while in many mammals, it is located in the floor of the nasal cavity. However, this organ is lost in many primates, including humans, and in bats. From the single nostril of the earliest vertebrates to the complex systems of modern mammals, the nose remains a central tool for survival.