Lung Ventilation and Perfusion

Question 1

alveolar ventilation

Answer 1

Rate at which new air reaches alveoli

Question 2

Alveoli in top of lung

Answer 2

are more expanded than those at the bottom
• Pleural pressure is less (more negative) at apex than base of lung, with inspiration pleural pressure decreases further
• As inspiration begins alveoli in lungs are at different lung volumes
• Underinflated (smaller) alveoli at base of lung are more compliant so receive more of tidal volume
• Overinflated (expanded) alveoli at top have a lower compliance and receive less of tidal volume

Question 3

• Lung has 2 blood supplies

Answer 3

– Pulmonary arteries

– Bronchial arteries

Question 4

Bronchial arteries

Answer 4

branch from aorta and supply oxygenated blood to conducting airways

Question 5

Bronchial veins exist

Answer 5

but majority of blood drains into pulmonary veins - Venous admixture

Question 6

how are alveolar walls ruptured

Answer 6

Alveolar walls extremely thin and alveolar epithelium is weak and can be ruptured by a positive pressure

Question 7

• Why do alveoli not fill with fluid?

Answer 7

– Normally pulmonary capillaries and lymphatics maintain a slight negative pressure in interstitial spaces
– Excess fluid will be sucked back into interstitial space from alveoli

Question 8

• Zone 1

Answer 8

– P>P >P
A PA PV
– Apex of lung under specific conditions
– No blood flow during all portions of the cardiac cycle
– Doesn’t occur normally

Question 9

• Zone 2

Answer 9

PA A PV
– Apex to mid lung
– Intermittent blood flow only during the
pulmonary arterial pressure peaks
• Systolic Ppc > Palv
• Diastolic Ppc < Palv

Question 10

• Zone 3

Answer 10

– P >P >P
PA PV A
– Mid to lower lung
– Continuous blood flow during entire cardiac output
• Ppc » Palv
– Get distension of pulmonary capillaries

Question 11

• Zone 4

Answer 11

– P >P >P
PA PV A
– Extreme base of lung
– Constriction of extra-alveolar vessels
– Peak flow decreases

Question 12

Pulmonary Blood Flow and its Regulation

Answer 12

• Regulated by PO2 and PCO2
– areas of low PO2 (hypoxia) or high PCO2 (hypercapnia)
– Arteries constrict so that blood is diverted to better oxygenated areas
– Mechanism thought to involve inhibition of K channels on smooth muscle cells
• This is a local response as remains even after section of autonomic nerves

Question 13

Bronchial Circulation

Answer 13

• Arteries supply oxygenated blood to smooth muscle of airways, intrapulmonary nerves and interstitial lung tissue
• Venous blood returns to heart from bronchial circulation via
– true bronchial veins
– or drains into bronchopulmonary veins where it mixes with oxygenated blood from alveoli (venous admixture)

Question 14

Ventilation and perfusion are matched when

Answer 14

pulmonary blood flow is proportionally matched to the pulmonary ventilation - greatest efficiency for gas exchange

Question 15

• Ventilation-perfusion ratio (V/Q)

Answer 15

– Single alveolus defined as alveolar
ventilation/capillary blood flow
– Lung defined as total alveolar ventilation/cardiac output

Question 16

• If ventilation exceeds perfusion

Answer 16

(V/Q ratio > 1)

Question 17

• If perfusion exceeds ventilation

Answer 17

(V/Q ratio < 1)

Question 18

Normal V/Q ratio

Answer 18

0.85 (4.2L/min /5L/min)

Question 19

• Arterial hypoxemia

Answer 19

abnormal PaO2 (adult at sea level is PaO2 less than 80 mmHg)

Question 20

• Hypoxia

Answer 20

insufficient O2 to carry out normal metabolic functions (PaO2 less than 60 mmHg)

Question 21

• 4 major causes of hypoxemia

Answer 21

– Anatomical shunt (perfusion that bypasses
lung)
– Physiological shunt (absent ventilation to areas being perfused)
– V/Q mismatching (low ventilation to areas being perfused)
– Hypoventilation (underventilation of lung units)

Question 22

Anatomical shunts

Answer 22

– Alveolar ventilation, distribution of alveolar gas and composition of alveolar gas are normal
– Distribution of CO changed as some blood now bypasses gas exchange unit
– Right-to-left shunt (as blood is deoxygenated)
– Hypoxemia cannot be abolished by giving 100% O2
– Cyanotic congenital heart diseases most common
• Shunt occurs when deoxygenated blood from RA or RV crosses septum to LA or LV

Question 23

• Physiological Shunts

Answer 23

– If airway completely blocked alveoli supplied by that airway will receive no ventilation
– All ventilation goes to other lung units
– Perfusion will be equally distributed to both ventilated and non-ventilated lung units
– Lung unit without ventilation but with perfusion has a V/Q = 0
– Atelectasis most common cause of physiological shunt
• May be due to obstruction by mucous plug, airway oedema, foreign body or tumour

Question 24

• V/Q mismatching (low V/Q)

Answer 24

– Most respiratory diseases produce global changes of varying extent in lungs (e.g. chronic bronchitis, asthma)
– So individual airways will have varying degrees of abnormal ventilation, but perfusion will be normally distributed
– Results in V/Q mismatching or low V/Q (V/Q < 1)
– Alveolar and end capillary gas compositions will vary according to degree of obstruction
– Supplemental O2 will correct hypoxemia as poorly ventilated units will get enriched O2

Question 25

• Hypoventilation

Answer 25

– Underventilation will bring less
fresh gas to alveoli
– O2 levels in alveoli will decrease, CO2 levels will increase
– If ventilation halved, arterial CO2 will double
– Patients with respiratory muscle weakness (e.g. muscular dystrophy or diaphragmatic paralysis) are at risk of hypoventilation
• Results in both hypercapnia and hypoxemia