2005A13 Describe the non-respiratory functions of the lung.

Reservoir

·   For gas exchange during expiration, between breaths, during apnoea (e.g.heavy lifting)

Humidification

·   Countercurrent heat and moisture exchange

·   Purpose

o Prevent heat loss

o Prevent water loss

o Prevent crusting of secretions

o Maintain mucociliary function

o Prevent infection

·   Mechanism

o Cool dry air inhaled

§ ↑Temp by conduction, convection

§ Evaporation-> humidification due to ↓ relative humidity

o Warm, humidified air exhaled

§ ↓Temp by conduction, convection

§ Condensation->drying due to ↑ relative humidity

Filtration

·   Particles e.g. dust, pathogens e.g. viruses, bacteria

·   >3um: stick to airway -> mucociliary escalator -> swallowed

·   1-3um: sediment in alveoli -> phagocytosis by macrophage = dust cell

·   <1um free in air ± deposit anywhere

Immune

·   Innate: mucociliary escalator, lysozyme, macrophage

·   Adaptive: IgA, IgG in alveolar lining fluid

Phonation

·   Exhalation against variably closed glottis

Pump

·   Inspiration: ↓ intrathoracic pressure -> ↑ gradient for systemic venous return

·   Expiration: venous valves prevent flow reversal

·   Cardiac output higher during spontaneous ventilation than during apnoea

Reservoir

·   15% blood volume supine (8% arteries, 3% capillaries, 5% veins)

·   9% blood volume erect

Filtration

·   Of microemboli to prevent stroke

Acid-base

·   CO₂ excretion for homeostasis

o 15,000mmol/day by Krebs cycle in mitochondria during aerobic respiration

o Alveolar ventilation is coupled to metabolic rate via control of PaCO₂

·   CO₂ excretion for compensation during metabolic acidosis

o CO₂ + H₂O <-> H⁺ + HCO₃^-

o ↑Alveolar ventilation -> left shift -> ↑pH

Fluid

·   Endothelial angiotensin-converting enzyme: AT1 (inactive) -> AT2 (active)

·   AT2: Na⁺/H₂O reabsorption, vasoconstriction

Metabolic

·   Inactivate serotonin near 100%, bradykinin 80%, noradrenaline 30%

·   Inactivate PGE₂, PGF_2α, leukotrienes

Route

·   Local: e.g. salbutamol

·   Systemic: e.g. naloxone

Reservoir

·   e.g. first pass uptake of fentanyl up to 75%

Metabolism

·   e.g. ? propofol and CYP450

Excretion

·   e.g. sevoflurane