Non-respiratory functions of respiratory tract

• Various non-respiratory functions are also shown by respiratory tract.
• Not only gaseous exchange that is the primary function of it.
• Lungs show defense barrier and metabolic functions.
• It is the main organ of respiration.
• Some important compounds are synthesized by it.
• Non-respiratory functions are:

a) Olfaction

• Provides olfactory sensation.
• Presence of olfactory receptors in mucus membrane of nostril.

b) Vocalization

• Larynx forms the speech apparatus.
• It forms this apparatus along with some other structures.
• Though it can play major role in the process of vocalization alone.
• For this reason, also called sound box.

c) Prevention of dust particles

• Filtration action of the hairs generally blocks the dust particles.
• These particles from air enter the nostrils which can finally reach our lungs.
• But due to presence of hairs in nostrils, most of the particles are filtered.
• Though small particles escape this action which then held by mucus later on.
• Secretion of nasal mucous membrane cannot hold all small particles even.
• Finally, phagocytic action of macrophages plays vital role to prevent escaped dust.
• These macrophages are present in alveoli.
• Some particles even escape these barriers which then removed by cough reflex and sneezing reflex.

d) Defense mechanism

• Lungs provide immunological defense mechanism to our body.
• These functions are shown by various structures present in the lungs.
• Lungs own defenses and various cells in mucus membrane that lines the alveoli of lungs are responsible for this function.
• Various cells include leukocytes, macrophages, mast cells, natural killer cells and dendritic cells.

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i) Lung’s own defenses

• Epithelial cells line the air passage or respiratory tract.
• Defensins and cathelicidins are some immune factors secreted by them.
• They are antimicrobial peptides and provide natural defenses to our lungs.

ii) Defense through leukocyte

• Neutrophils and lymphocytes are types of leukocytes.
• Present in the alveoli of lungs.
• Provide defense mechanism against bacteria and virus.
• By phagocytosis, neutrophils kill bacteria.
• Whereas lymphocytes develop immunity against bacteria.

iii) Defense through macrophages

• Dust particles and pathogens are looked after by macrophages.
• They get engulfed by macrophages.
• Thus, macrophages are also called scavengers of lungs.
• Also develop immunity by acting as antigen presenting cells (APC’s).
• No sooner the foreign organisms enter the body, these APC’s and macrophages kill them.
• Antigens from organisms later on digested to polypeptides.
• Macrophages present these polypeptides to T-lymphocyte and B-lymphocytes.
• Interleukins, tumor necrosis factors (TNF) and chemokines are some secretions of macrophages.
• General immune system is activated by TNF and interleukins.
• Whereas, white blood cells are brought to site of inflammation by chemokines.

iv) Defense through mast cells

• Resemble basophil due to its large size.
• Allergy and anaphylaxis reactions are produced by these cells.
• Also have some secretions like heparin, histamine, serotonin and hydrolytic enzymes.

v) Defense through natural killer cells

• Considered third type of lymphocyte and are large granular cells.
• Presence found more in lungs and some other lymphoid organs.
• Hydrolytic enzymes content is high in its granules.
• Thus, microorganisms can be easily destroyed by it.
• These cells can act against viruses also.
• Thus, are considered the first line defense in specific immunity.
• Viruses, viral infected or damaged cells which can form tumor are destroyed by it.
• Malignant cells destruction also occurs along with the prevention of cancerous tumor development.
• Interferons and tumor necrosis factors (TNF) are its secretion.

vi) Defense through dendritic cells

• Shows important role in immunity though present in the lungs.
• Also act as antigen presenting cells (APC’s) like macrophages or along with it.

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e) Maintenance of water balance

• Helps in water loss mechanism.
• In expiration process, some water content vaporizes.
• This cause loss of water through expired air.
• Thus, maintains water level in our body.

f) Regulation of body temperature

• Not only water but also some heat is lost from the body.
• These both losses occur through exhalation.
• Thus, assists in heat loss mechanism also.

g) Anticoagulant function

• Secretion of mast cells acts as anticoagulant.
• In lungs, these cells are present.
• Thus, through heparin secretion, intravascular clotting is prevented.

h) Secretion of angiotensin-converting enzyme

• Pulmonary capillaries contain many endothelial cells.
• These cells secrete angiotensin-converting enzyme (ACE).
• Angiotensin-I is acted upon by this enzyme thus angiotensin-II is formed.
• It is the active form of angiotensin-I.
• Thus, regulates the ECF volume and blood pressure.

i) Synthesis of hormonal substances

• Some hormones are also secreted by lung tissues.
• Prostaglandins, acetylcholine and serotonin are some of them.
• They have various physiological functions in the body.
• One of them is regulation of blood pressure.

j) Regulation of acid-base balance

• Lungs also help to maintain acid-base balance in our body.
• It is done by lungs by regulating CO₂ content in blood.
• In various metabolic reactions that occur in the tissues of the body, CO₂ is produced.
• Carbonic acid is formed as the CO₂ dissolves in water.
• As carbonic acid is unstable, it splits.
• These results in the formation of hydrogen and bicarbonate ions.

Co₂ + H₂O → H₂CO₃ → H⁺ + HCO₃^–

• This reaction reverses as carbon dioxide is removed from blood into the alveoli of lungs.

H⁺ + HCO₃^– → H₂CO₃ → CO₂ + H₂O

• Through ventilation process, carbon dioxide is removed as it is a volatile gas.
• Increase in metabolic activities increases the carbon dioxide content in tissues.
• This increases the hydrogen ion concentration also.
• Thus, pH is reduced.
• Increased pulmonary ventilation results as hydrogen ion increases.
• It occurs through various mechanisms like chemoreceptors in aortic and carotid bodies and in medulla of the brain.
• As ventilation increases, carbon dioxide is also removed in excess from body fluids.
• Thus, this pH is brought back to normal.

References:

i) https://www.open.edu/openlearn/science-maths-technology/blood-and-the-respiratory-system/content-section-1.4

ii) https://owlcation.com/stem/Non-respiratory-Functions-of-the-Respiratory-System

Non-respiratory functions of respiratory tract