Choanae is the scientific term for the nasal passages, which connect the nose to the throat. Only the tetrapoda, a classification which includes humans, reptiles, and mammals, have them. If these nasal passages are blocked through birth defects, they can cause many health problems in both humans and animals.
The insides of the nasal passages are lined with a mucus membrane, hairs, and grooves called conchae. These grooves help trap moisture from the breath, keeping the inside of the nasal passages moist. There are also receptors, nerves which, when irritated by a foreign object, will trigger a sneeze.
Even though choanae are a familiar feature of the human body, many people do not have a good idea of the actual anatomical layout. Though they appear to go straight up, nasal passages actually go back in a relatively straight line under the eyes, terminating in two holes right above the back of the throat. The "human blockhead" carnival trick takes advantage of this anatomical ignorance to give the illusion that a performer is hammering a nail into his skull when he is simply inserting it into his choanae.
There is much speculation as to why certain geneses of animals developed choanae while others did not. It is believed that nasal passages evolved in ocean-dwelling creatures so that they could take in more oxygen without having to surface and open their jaws. This allowed them to become better adapted to their new environment. Experts say more fossil evidence is needed to fully understand how the nasal passages evolved.
A condition called choanal atresia, or deformed nasal passages, can cause many health problems. The blockage is caused most of the time by there being solid bone in place of the nasal passages and less seldom by there being an undissolved membrane. In half of the cases of choanal atresia, only one nasal passage is blocked.
In humans, choanal atresia is usually diagnosed shortly after birth when a child has trouble nursing. Most newborns tend to breathe through their noses rather than their mouths shortly after birth, but those with blocked nasal passages will choke when they nurse and will only be relieved when they switch to mouth-breathing when crying. The problem can be cured through surgery, where doctors will use a diamond-tipped drill to bore new nasal passages through bone.
Other animals can be born with choanal atresia. It is a particular problem in animal husbandry, where livestock may suffer from lack of oxygen, lose weight, and have trouble eating. The problem is most common in alpacas, but the wounds caused by surgery are so difficult to keep clean it is not always worth the trouble of having the surgery done.