Other Lung Functions

Circulatory functions

• The pulmonary circulation acts as a reservoir for blood

• The lung can increase its blood volume significantly with small rises in pulmonary artery/venous pressures, eg. when a subject lies down after standing

• Filters blood, removing small blood thrombi from the circulation before they can reach the brain/other organs

• The size of particles which can pass through the lungs is variable due to the different routes they can potentially take
• The pulmonary microcirculation is well adapted to maintaining alveolar perfusion even with significant embolisation present
• Thrombi are cleared more rapidly from the lungs than from other organs, as the lung has well developed proteolytic/antithrombotic systems

Defence Against Inhaled Substances

• Cilia push mucous cephalad at a rate of 4mm/min
• Beat in a low viscosity periciliary layer of fluid, pushing a mucous layer sitting above them
• This airway lining acts as a heat and moisture exchanger
• Inhaled particles are deposited in various locations depending on their size:

• Large particles >3μm are usually rapidly inertial impacted, while >8μm rarely reach further than the pharynx
• Particles 1-3μm sediment in the smaller airways and are ingested by macrophages
• Particles <1μm diffuse in and out with breaths, rarely contacting the airway walls

Defence Against Inhaled Pathogens

• Pathogens are directly removed by cilia
• Airway lining fluid also contains bactericidal agents including surfactant, lysozyme and defensins
• Protease enzymes are also present in airway lining fluid, and must be kept away from the lung tissue via this tissue layer and α1 antitrypsin or they will damage lung parenchyma
• Humoral immunity is also important via IgA in the upper airway & bronchi and IgG in the deeper lung areas

Metabolic functions

• As the lung is the only organ receiving the whole circulation, it is uniquely suited to modifying bloodborne substances
• Especially important in vasoactive substance metabolism
• Activates the inactive angiotensin I to potent angiotensin II, catalysed by ACE in capillary endothelial cells
• Major site of synthesis, uptake and metabolism of arachidonic acid metabolites including prostaglandins, prostacyclines, and leukotrienes
• Also involved in deactivation/sequestation of bradykinin, amines (serotonin and norepinephrine), prostaglandins E₂/F_2a and ADH
• Synthesizes dipalmitoyl phosphatidylcholine - a component of pulmonary surfactant