Lung physiology 1

Question 1

respiratory pump

Answer 1

• Requirement to move 5l/minute of inspired gas
• Generation of negative intra-alveolar pressure – inspiration active requirement to generate flow
• Bones, muscles, pleura, peripheral nerves, airways all involved

Question 2

Bony thorax

Answer 2

• Structure is intimately linked to function
• Bony structures support respiratory muscles and protect lungs
• Rib movements – pump handle and water handle

Question 3

muscles of respiration

Answer 3

• Inspiration – largely quiet and due to diaphragm (C3/4/5), external intercostals (nerve root at each level)
• Expiration – passive during quiet breathing

Question 4

pleura

Answer 4

• 2 layers, visceral and parietal

* Potential space only between these, few mls of fluid

Question 5

nerves

Answer 5

• Motor – diaphragm C3, C4. C5, thoraco-lumbar nerve roots
• Sensory – sensory receptors assessing flow, stretch etc, C fibres, afferent via vagus
• Autonomic sympathetic, parasympathetic balance

Question 6

airways

Answer 6

• Complex subdivisions
• Conducting, transitional and respiratory zones
• Airways resistance higher in the main airways – 20% of total airways resistance supplied by airways less than 2mm in diameter
• Dead space where air is supplied but no gas exchange takes place
• Inter alveolar connections; pores of Kohn

Question 7

static lung

Answer 7

• Both chest wall and lungs have elastic properties, and a resting (unstressed) volume
• Changing this volume requires force
• Release of this force leads to a return to the resting volume
• Pleural plays an important role linking chest wall and lungs

Question 8

gas exchanger

Answer 8

• Alveoli and capillaries
• Bulk flow in the airways allows;
– O2 and CO2 movement
– Large surface area required
– With minimal distance for gases to move across
– Adequate perfusion of blood also needed
• Terminal bronchioles lead to;
– Respiratory bronchioles, alveolar ducts and alveoli
– Total combined surface area for gas exchange 40-100m2
– Respiratory bronchiole
• Centre of an acinus
– 300000000 alveoli per lung

Question 9

alveolar ventilation

Answer 9

Alveoli at the bases are preferentially ventilated at rest

Question 10

dead space

Answer 10

Volume of air not contributing to ventilation
Anatomic - Approx 150mls
Alveolar - Approx 25mls
Physiological - (Anatomic+Alveolar) = 175mls

Question 11

alveolar perfusion

Answer 11

Capillaries at the most dependent parts of the lung are preferentially perfused with blood at rest
Perfusion of capillaries also depends on;
• Pulmonary artery pressure
• Pulmonary venous pressure
• Alveolar pressure
Pulmonary vascular resistance; Certain pulmonary arteries have smooth muscle within their walls
Hypoxic pulmonary constriction
[opposite from systemic circulation]
• 1000 capillaries per alveolus
• Each erythrocyte (RBC) may come into contact with multiple alveoli
• RBC thickness itself represents an important component of the distance across which gas has to be moved
• At rest, 25% the way through capillary, Hb is fully saturated

Question 12

ventilation and perfusion

Answer 12

• Matching ventilation and perfusion is important
• Hypoxic pulmonary vasoconstriction
• Pulmonary vessels have high capacity for cardiac output (30% of total capacity at rest)
• Recruiting of alveoli occurs as a consequence of exercise

Question 13

CO2 elimination equation

Answer 13

PaCO2 = kV’CO2/V’A

Question 14

alveolar gas equation

Answer 14

PAO2 = PiO2 - PaCO2/R

Question 15

blood gasses

Answer 15

Sample arteriole blood 
Can test:
•	PaCO2
•	pH
•	HCO3-
•	PaO2

Question 16

acid base control

Answer 16

• Body maintains close control of pH to ensure optimal function (e.g. enzymatic cellular reactions)
• Dissolved CO2/carbonic acid/respiratory system interface crucial to the maintenance of this control
• pH normally 7.40
• H+ concentration 40nmol/l [34-44 nmol/l]
• Blood and tissue buffers important
• Carbonic acid/ bicarbonate buffer in particular
• CO2 under prominent respiratory control (rapid)
• HCO3- under predominant renal control (less rapid)
• The respiratory system is able to compensate for increased carbonic acid production, but; elimination of fixed acids requires a functioning renal system

Question 17

Four main acid-base disorders are:

Answer 17

1. Respiratory acidosis: increased PaCO2, decreased pH, mild increased HCO3
2. Respiratory alkalosis: decreased PaCO2, increased pH, mild decreased HCO3-
3. Metabolic acidosis: reduced bicarbonate and decreased pH
4. Metabolic alkalosis: increased bicarbonate and increased pH