3. Non respiratory functions of the lung

Answer 1

1. Metabolic functions
2. Neuroendocrine functions
3. Drug metabolism
4. Pulmonary sequestration
5. Barrier function
6. Filtration:
7. Immune functions
8. Vascular reserve

Question 2

Metabolic functions

Answer 2

the best-known metabolic function of the lung is probably the
enzymatic conversion of angiotensin I to angiotensin II by angiotensin converting
enzyme (ACE). (Angiotensin I is an inert decapeptide from which two residues are
removed to form the active angiotensin II which subsequently passes through the
lung without any further metabolic change.) ACE also catalyzes the degradation of
bradykinin, which is another short peptide (9 residues

Otherwise the lung also metabolizes some
amines: noradrenaline (by monoamine oxidase, MAO and by catechol-O-methyltransferase,
COMT) and 5-hydroxytrytamine (by MAO

Question 3

Neuroendocrine functions

Answer 3

secretes a number of substances (some of which it also degrades). These include
5-hydroxytryptamine, histamine, substance P, heparin, bradykinins and prostaglandins.
Type II alveolar cells produce surfactant, which is essential for healthy
lung function.

Question 4

Drug metabolism

Answer 4

some drugs in the systemic circulation, such as local anaesthetics,
are sequestered in the lung and thereby may protect the systemic circulation from
excessively high concentrations. Prilocaine is the exception in that it is actually
metabolized in the lung (although not exclusively in this site). Many inhaled drugs
and other substances are metabolized by pulmonary cytochrome P450 isoforms.

Amiodarone is one
such drug, which accumulates in several tissues, including lung, where it can give rise
to an acute or subacute pneumonitis.

Question 5

Pulmonary sequestration

Answer 5

the extravascular pH of lung is lower than that of plasma,
and so this can lead to ion trapping and the sequestration of some drugs. This can be
important in the attenuation of potential drug toxicity, such as that which follows
high doses of local anaesthetics.

Question 6

Barrier function

Answer 6

the upper airways provide the first barrier to inhaled noxious
substances and toxins, which strictly speaking is a ‘respiratory function’ albeit not
one that is involved directly in gas exchange. The muco-ciliary escalator consists of a
double-layered system in which a layer of high viscosity mucopolysaccharide is
carried proximally on an underlying layer of low-viscosity serous bronchial secretion.

Question 7

Filtration:

Answer 7

Filtration: the lung is very efficient at filtering embolic matter of all types, including
thrombus, fat and air. The system clearly has a limit to its capacity and can be
overwhelmed, for instance by a massive pulmonary embolus. This will result in at
best a substantial shunt and at worst, circulatory collapse. Microemboli may stimulate
the release of local inflammatory mediators which can precipitate in due course
the clinical features of acute lung injury.

Question 8

Immune functions

Answer 8

inhaled particles that are smaller than 5 μm in diameter pass
into the distal airways. Those between 2 and 5 μm are deposited on the airway walls
with the smaller particles reaching the alveoli. Only around 20% of these stay within
the alveolus; the remainder are exhaled. Particles less than 0.3 μm in diameter remain
as aerosols. Pulmonary macrophages are effective against bacteria, and they also
phagocytose other particles

range of inflammatory mediators at the
site of injury. These include as interleukins, cytokines, tumour necrosis factor and
oxygen radicals. IgA is the most abundant immunoglobulin and is found mainly in
bronchial secretions.

Question 9

Vascular reserve

Answer 9

reserve: the pulmonary vessels essentially accommodate the cardiac output
in a low-pressure, elastic circulation which at rest is not fully perfused but which will

distend as cardiac output increases. The potential increase in blood volume can be as
high as 1,000 ml, but this is not an effective vascular reserve that can be utilized in
hypovolaemic states because under such circumstances cardiac output will increase
to maximize oxygen delivery to tissues.