Non-respiratory Functions of the Respiratory System
The main function of the respiratory system is to facilitate the exchange of gases between the external environment and the blood, so that these can be transported to and from the peripheral tissues. However, the respiratory system performs some other vital functions:
- Removal of inhaled foreign particles and infectious organisms
- Olfaction (sensation of smell)
- Warming and humidification of air (losing excessive heat)
- Filtration of blood at the pulmonary capillaries
- Acting as a volume reservoir of blood
- Metabolic functions of the pulmonary tissue
1. Removal of Inhaled Foreign Particles and Infectious Organisms
The upper respiratory tract has a moist surface, covered by mucus, so that large particles get adhered and are therefore prevented from reaching the lower respiratory tract. The nasal mucosa is lined by a ciliated epithelium, with the cilia beating towards the pharynx, so that the foreign particles can be swallowed. The nasal cavity also harbors hair, covered with mucus, acting like a filter. The nasal cavity is supplied by sensory nerve endings of the trigeminal nerve, which are sensitive to irritants. If an irritant is inhaled, the sneezing reflex is activated and the particles are eliminated.
The lower respiratory tract, above the level of the respiratory bronchioles, is also lined by columnar ciliated epithelium, with a layer of mucus lying over the luminal surface of the cells. This layer also traps foreign particles and they are expelled by the co-ordinated movement of the cilia in the lower respiratory tract in an upward direction (towards the pharynx). The glosso-pharyngeal and vagal nerve endings in the lower respiratory tract, initiate cough reflex in response to stretching and irritation to expel foreign particles entering the lower respiratory tract.
The alveoli are inhabited by macrophages which are responsible for engulfing the foreign particles and the organisms entering the alveoli. In addition, the mucus covering the nasal, nasopharyngeal and the lower respiratory tract is enriched with IgA (immunoglobulin A) and lactoferrin , preventing organisms from colonizing the respiratory epithelium. The tonsils in the pharynx (an aggregation of lymphoid tissue associated with the musoca) also contribute to the immune function of the respiratory system.
2. Olfaction (Sensation of Smell)
The roof of the nasal cavity has nerve endings which detect different odors. These nerves traverse the ethmoid plate and form the olfactory bulb. The physiology of olfaction will be discussed in another hub.
3. Warming and Humidification of Air
The inhaled air flows across the warm and moist upper airways. Therefore, by the time the air reaches the lower airways, the air is saturated with water vapor (i.e. the air carries the maximum amount of water vapor that it can occupy at the body temperature) and is warmed up to 37 centigrade. This is very important to prevent dehydration of the lower respiratory tract and to prevent reflex broncho-constriction that occurs when the lower respiratory tract is exposed to cold air.
The larynx has two vocal cords lining a central orifice, known as the glottis. The size of the glottis can be altered by contraction of the laryngeal muscles. The vocal cords can be brought to a position, at which, they tend to vibrate with the force of exhaling air. This vibration gives rise to the sound. The pitch of the sound produced can be varied by the altering the size of the glottis (by contraction and relaxation of the laryngeal muscles). The produced sound is then modified by the movements of the oral cavity and the tongue (articulation), forming words.
5. Filtration of Blood at the Pulmonary Capillaries
The venous blood entering the right side of the heart is passed through the pulmonary capillaries, before reaching the left side of the heart to be distributed through-out the body. When the blood passes through the small caliber of the pulmonary capillaries, large particles such as emboli, air bubbles, cell debris and fat globules get trapped in the pulmonary vessels. This prevents such particles entering the systemic circulation and obstructing an end-artery supplying a vital organ such as the brain.
6. Acting as a Reservoir of Blood
The pulmonary vascular bed is a low pressure system, which can occupy a large volume of blood. In the presence of a hypovolaemic state, the pulmonary vessels constrict, releasing the blood into the systemic circulation, in order to increase the effective circulating volume.
7. Metabolic Functions of the Pulmonary Tissue
The lower airways are lined by a large number of neuro-endocrine cells responsible for the secretion and release of chemical mediators such as bradykinin, prostaglandins, serotonin, substance P, heparin and histamine. In addition, the pulmonary tissue is responsible for the conversion of angiotensin I to angiotensin II and the catabolism of bradykinins, adrenaline and noradrenaline. Many waste products and metabolites are excreted via the lungs as volatile gases (e.g. – ethanol, acetone).