Physiology of paranasal sinuses have undergone rapid advancement since the days of Galen. It was Galen who described the anatomy of paranasal sinuses as "Porosity of skull" in the second century AD. It was Leonardo da vinci who made a detailed anatomical illustration and description of the paranasal sinuses. His illustrations contained description of frontal sinuses and maxillary sinus ostium.
The physiological role played by paranasal sinuses have always been conjectural. Several hypothesis have been made. For easier understanding these hypothesis have been grouped under three main heads:
1. Structural theories
2. Evolutionary theories
3. Functional theories
Fallopio's theory: Fallopio in 1600 hypothesized that these para nasal sinuses are present to make the skull bone lighter thereby reducing the load on neck musculature which supports the head.
Another theory belonging to this group suggests that these sinuses contribute to the maintenance of equilibrium and the positioning of the head by lightening the anterior portion of the cranium.
Proetz theory: suggests that the paranasal sinuses play a role in remodelling of facial bones.
This theory is very innovative. It considers the paranasal sinuses as the evolutionary response of anthropomorphic monkeys to shift from terrestrial environment to the aquatic one. This theory was proposed by Hardy. According to him the African monkeys were forced to take to water when the whole of Africa was surrounded by sea about 6.5 million years ago. The necessity to cross large stretches of water enabled them to develop these air filled sinus cavities which helped them to keep afloat for long hours in water. This occurred due to an evolutionary process known as Natural selection.
Bartholini's theory: Bartholin considered these cavities as organs of resonance which added quality and resonance to the voice.
Cloquet's theory: Cloquet hypothesized that paranasal sinuses contained olfactory epithelium aiding in the function of smell. This theory has been disproved as olfactory epithelium has not been demonstrated in the sinus cavity.
The most acceptable theory is that these sinuses improves nasal function. The fact that these paranasal sinuses embryologically originates as invaginations from the nose and its histological continuity with the nasal mucosa leads credence to this theory. These sinuses have been shown to strengthen the defense function through additional secretion of lytic enzymes and immunoglobulins.
Ventilatory function: Since the mucosal lining of the paranasal sinuses are in continuity with that of nasal mucosa they play an important role in ventilatory function. Gaseous exchange are known to occur between the inspired air and blood supply of the sinus mucosal lining. Gaseous exchanges are determined by active and passive phenomena. During respiration the nasal valve transforms the inspired air into a laminar flow. When this laminar flow reaches the middle meatus area turbulence starts to occur. This turbulence causes uniform mixing of air. There is a pressure gradient between the nasal cavity and para nasal sinuses causing airflow in to the sinus cavities. Air enters the sinuses at the end of every inspiration and at the beginning of the following expiration. This is because only during these times a positive pressure exists in the nasal cavities driving air into the sinuses.
Figure showing air entering the maxillary sinus cavity
Figure showing air exiting out of the sinus cavity
It has been estimated that only 1/1000 of the air volume inside the sinuses are exchanged through a single respiratory act. The patency of the ostium is verify important for the gaseous exchange to take place.
The nose and paranasal sinuses form the first line of defense for the lower airways. They protect the lower airways from noxious substances and microbes. They play a vital role in specific and non specific modes of defense. Among the nonspecific protective mechanisms, mucociliary clearance play an important role. This mechanism depends on the integrity of the mucociliary system which includes the ciliated cells, goblet cells and the quality of the mucous secreted. Goblet cells are more numerous in maxillary sinus than other sinuses. The mucociliary transport mechanism needs continuous supply of oxygen. This mechanism clears the nasal and sinus cavity. The microbes and particulate matter if any gets entrapped in the secretions and are transported out.
This defense mechanism can be divided into nonspecific (phylogenetically older) and specific (phylogenetically newer) mechanisms. The older non specific mechanism is also known as natural immunity. This is brought about by factors like:
1. Lysozyme - which destroys the bacterial cell wall
Specific immunity is provided by macrophages and immunoglobin secreting lymphocytes. This immunity is highly specific against microbes invading the nasal cavity. Nose associated local immune tissue (NALT) has been attributed with this function.