Lung Physiology

Question 1

Respiratory pump - function

Answer 1

Requirement to move 5 litres / minute of inspired gas [cardiac output 5 litres / min]
Generation of negative intra-alveolar pressure
Inspiration active requirement to generate flow
Bones, muscles, pleura, peripheral nerves, airways all involved
Bony structures support respiratory muscles and protect lungs
Rib movements; pump handle and water handle.

Question 2

Respiratory pump - muscles

Answer 2

Muscles of respiration
Inspiration
- Largely quiet and due to diaphragm (C3/4/5) contraction
- External intercostals (nerve roots at each level)
Expiration
- Passive during quiet breathing

Question 3

Respiratory pump - pleura

Answer 3

2 layers, visceral and parietal
Potential space only between these, few millilitres of fluid

Question 4

Respiratory pump - nerves

Answer 4

Sensory - Sensory receptors assessing flow, stretch etc.., C fibres, Afferent via vagus nerve (10th cranial nerve).
Autonomic sympathetic, parasympathetic balance.

Question 5

Static lungs

Answer 5

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 6

Gas exchange

Answer 6

Alveoli and capillaries
VENTILATION; Bulk flow in the airways allows;
O2 and CO2 movement
Large surface area required, with minimal distance for gases to move across. Total combined surface area for gas exchange 50-100 m2
300,000,000 alveoli per lung
PERFUSION; Adequate pulmonary blood supply also needed.

Question 7

Alveolar ventilation

Answer 7

Dead space
Volume of air not contributing to ventilation.
Anatomic; Approx 150mls
Alveolar; Approx 25mls

Question 8

Circulation; bronchial

Answer 8

Blood supply to the lung; branches of the bronchial arteries.
Paired branches arising laterally to supply bronchial and peri-bronchial tissue and visceral pleura.
Systemic pressures (i.e. LV/aortic pressures).
Venous drainage; bronchial veins draining ultimately into the superior vena cava.

Question 9

Alveolar Perfusion

Answer 9

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

Question 10

Ventilation and Perfusion

Answer 10

Matching ventilation and perfusion 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 11

Nomenclature

Answer 11

PaCO2 - arterial CO2
PACO2 - Alveolar CO2
Same with O2
PiO2 - pressure of inspired oxygen
FiO2 - fraction of inspired oxygen
Va - alveolar ventilation
Vco2 - CO2 production

Question 12

CO2 elimination

Answer 12

PaCO2 = kVco2 / Va

Question 13

Alveolar gas equation

Answer 13

PAO2 = PiO2 - PaCO2/R
R - respiratory quotient

Question 14

O2/Hb dissociation curve

Answer 14

Sigmoid shape - As each O2 molecule binds, it alters the conformation of haemoglobin, making subsequent binding easier (cooperative binding).
Varying influences - 2,3 diphosphoglyceric acid, H+, Temperature, CO2.

Question 15

Acid Base control

Answer 15

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].

Question 16

Acid Base control - blood and buffers

Answer 16

Blood and tissue buffers important.
Carbonic acid / bicarbonate buffer in particular.
CO2 under predominant 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

Carbonic acid equilibrium

Answer 17

CO2 + H2O <-> H2CO3 <-> H+ + HCO3-

Question 18

Henderson-Hasselbalch equation

Answer 18

pH=6.1 + log10[[HCO3-]/[0.03*PCO2]]
To keep pH at 7.4, log of the ratio must equal 1.3.
As PaCO2 rises, HCO3- must also rise.
Respiratory acidosis; increased PaCO2, decreased pH, mild increased HCO3-.
Respiratory alkalosis; decreased PaCO2, increased pH, mild decreased HCO3-.
Metabolic acidosis; reduced bicarbonate and decreased pH.
Metabolic alkalosis; increased bicarbonate and increased pH.