Pulmonary Circulation

Question 1

There are actually TWO physical circulations in the lung. What are they?

Answer 1

Pulmonary and bronchial

Question 2

What is the bronchial circulation? How does it work?

Answer 2

• The bronchial circulation is part of the systemic circulation; it supplies blood to the trachea and bronchi (conducting airways).
• This circulation carries oxygenated blood to the bronchial tree. The bronchial venous drainage is partly into bronchial veins to the right atrium but also partly into the pulmonary veins going to the left atrium. The pulmonary circulation is the output from the right heart blood into the lungs.
• Small bronchial arteries branch off the early part of the descending aorta and travel along the main bronchi to supply oxygenated blood to the tissues of the bronchi and bronchioles
• There are arterial and venous anastomoses (shunts) between the bronchial and pulmonary circulations. These are important ‘safety valves’ to prevent too high a pressure in the pulmonary circulation.
• The right bronchial veins empty into the azygos vein. The left bronchial veins empty into the hemiazygos & accessory hemiazygos veins.

Question 3

How is the pulmonary circulation different to the systemic?

Answer 3

• Remember that the blood pressure in the pulmonary circulation is much less than in the systemic.
• For example:
  o Pulmonary systolic pressure
  o ~24 mm Hg
  o Pulmonary diastolic pressure
  o ~9 mm Hg
  o Mean pulmonary arterial pressure : 14 mm Hg
  o Mean pulmonary venous pressure: 9 mmHg

Question 4

What is the equation for cardiac output?

Answer 4

Cardiac output (Q) = Pulmonary arterial pressure (P) / Pulmonary vascular resistance (R)

Question 5

What happens if pulmonary vascular resistance rises?

Answer 5

• If the pulmonary vascular resistance does rise due to narrowing of the arteries, this creates PULMONARY ARTERIAL HYPERTENSION (PAH) . This is a life-threatening condition which can cause the right heart to enlarge and eventually fail.

Question 6

What are the special features of pulmonary arteries?

Answer 6

• The pulmonary arteries are thin walled. They have far less smooth muscle than systemic arteries.
• They have a larger diameter than systemic arteries.
• The vessels are highly distensible and compressible
• Because of high compliance, pulmonary arteries stretch more than aorta during systole: this smooths the blood flow through the lungs

Question 7

Which nerves innervate the lungs (sensory, sympathetic, parasympathetic)?

Answer 7

• There is a somatic nerve supply to the lungs, this carries pain and touch sensation from the lungs to the spinal cord segments T2-T6
• There is a sympathetic nerve supply to the lungs from spinal nerves T2 to T4-6. The postganglionic nerves from the paravertebral sympathetic ganglia pass into the lungs in plexi around pulmonary arteries and arterioles. The sympathetic fibres innervate smooth muscle within the walls of bronchi and small pulmonary vessels. Activation causes bronchodilation via beta 2 receptors. Bronchial muscle relaxation due to sympathetic nerves is greatly augmented by circulating adrenaline
• There is a parasympathetic supply to the lungs from the vagus. This contains both afferent and efferent fibres. Some afferent fibres detect irritants in airways and stretch of lungs during inspiration; efferent fibres produce bronchoconstriction and stimulate secretion of mucus in the bronchi.
• J receptors are parasympathetic afferents. They are situated in the alveolar walls next to the pulmonary capillaries are stimulated by engorgement of pulmonary capillaries and pulmonary oedema; stimulation of J receptors leads to bradycardia, hypotension and hyperpnea.

Question 8

What is most important to remember about the autonomic stimulation of the lungs?

Answer 8

• Remember:
  o Sympathetic stimulation opens airways and relaxes bronchial smooth muscle
  o Parasympathetic innervation has both afferent (triggering cough reflexes) and efferent (narrowing bronchi, stimulating mucus secretion) components

Question 9

How are plexi formed in blood vessels in the lungs?

Answer 9

• Sympathetic (bronchodilator) nerves form plexi along blood vessels, parasympathetic (bronchoconstrictor) nerves run as separate nerves, ending on local postganglionic nerve cells.

Question 10

What is important to remember about preganglionic neurons?

Answer 10

• Remember that all preganglionic neurons release acetylcholine that acts on nicotinic receptors, whereas postganglionic parasympathetic fibres also release acetylcholine but the receptors on the target tissues are muscarinic.
• In the case of airway smooth muscle and mucus glands the receptors are M3 type. (M2 receptors are autoreceptors)

Question 11

How have humans evolved in terms of pulmonary circulation?

Answer 11

Humans evolved as animals that walked on all fours, so that middle of lungs were at level of the heart
By adopting an upright bipedal gait, we have gained the advantage of being able to see and hear distance objects, but apices of lungs are now above the heart so their physiology have suffered

Question 12

What effect does gravity have on blood pressure? What model does this lead to?

Answer 12

• Because the pulmonary pressures are low, gravity has a significant effect on pulmonary blood pressure; when you are standing upright blood pressure is much lower at the lung apex than the base
• In a standing position the bases of the lungs are well perfused as gravity helps the blood circulate there; conversely the apices are poorly perfused as these region are above the heart. This has led to the ‘three zone’ model of lung perfusion in the upright state:
  o The apices (zone 1) have intermittent flow; flow occurs during systole only
  o The centres (zone 2) have pulsatile flow; flow greater in systole than diastole.
  o The bases (zone 3) have continuous flow of blood.

Question 13

How can distribution of blood flow in the upright human lung be measured?

Answer 13

• The distribution of blood flow in the upright human lung can be measured using radioactive xenon.
• The xenon is injected into venous blood and evolves into alveolar gas from the pulmonary capillaries.
• Radiation counters measure the amount of xenon passing though each level of the lungs

Question 14

How does the original three zone model differ to our understanding of blood pressure in the lungs now?

Answer 14

• In the original three zone model as shown below there was NO blood flow in the apical 1/3 (zone 1) of the upright lung, as alveolar gas pressure (PA) was always greater than capillary blood pressure (Pa) and so the alveolar pressure squashed the capillaries flat and blocked them.
• However, it is likely some flow at the apices does occur, particularly during inspiration when alveolar pressure is less than atmospheric.
• Zone 1 upper 1/3 lung: Intermittent flow (during systole and inspiration)
• Zone 2 is the 1/3 part of the lungs from about 3 cm above the heart up to the lower limit of zone 1. In this zone flow is pulsatile, flow fluctuates during inspiration and expiration due to changes in alveolar pressure (PA). Average flow increases the nearer the base as Pa increases
• Zone 3, is the lowest 1/3 of the lungs, flow occurs continuously, as pulmonary arterial & venous pressures always exceed alveolar pressure.
• Arterial pressure > venous pressure > alveolar pressure

Question 15

What is total lung compliance and what is its equation?

Answer 15

Total Lung Compliance (C) is a measure of ‘stretchability’ or distensibility’of the whole lung. It is the change in volume per unit pressure change
C = dV/dP

Question 16

When is the work of breathing least? Where does this occur?

Answer 16

• The work of breathing is least when a given change in pressure gives the largest change in volume. This occurs in the region of highest compliance as indicated by the blue arrow, ie in a region between full inflation and full deflation. Tidal volume occurs in this high compliance region.

Question 17

Why is it bad to have lower than normal lung compliance?

Answer 17

• It indicates a ‘stiff’ lung This means extra work is required to bring in a normal volume of air. This occurs if the lungs become fibrotic, lose their distensibility and become stiffer.

Question 18

Why is it bad to have higher than normal lung compliance?

Answer 18

• The lung is highly compliant in cases of emphysema where there is loss of elastic tissue in the lung. Patients with emphysema have a very high lung compliance due to poor elastic recoil of the lung tissue. They have extreme difficulty exhaling air. In addition, patients often have difficulties inhaling air as well. This is due to the presence of many collapsed alveoli in emphysematous lungs.

Question 19

Where is complex in the lungs higher? How does this relate to perfusion?

Answer 19

• Compliance is higher in the base than in the apex of the lung; this means bases of lungs are better ventilated (per unit lung volume) than apices. The higher ventilation matches the higher perfusion of the bases of the lungs

Question 20

How do basal alveoli compare to apical alveoli?

Answer 20

Basal alveoli are more ventilated than apical alveoli, as they have higher compliance and thus a bigger volume change per unit pressure change

Question 21

Which, ventilation or blood flow, shows a steeper decline between base and apex of lungs?

Answer 21

Blood flow

Question 22

How does ratio of ventilation (VA) to perfusion (Q) change from base to apex?

Answer 22

The ratio of ventilation to perfusion is a curve that increases steeply at the apex
This is the ventilation perfusion ratio VA/Q: for maximum efficiency this would be unity

Question 23

How does V/Q change in conditions of airway obstruction?

Answer 23

• In conditions of airway obstruction V/Q ratio is LOWER than normal.
  o If the airways are completely blocked then ventilation is zero. Then V/Q is zero, whatever the blood flow. (0/x =0)
  o There is no gas exchange in a lung that is perfused but not ventilated. Thus the PO2 and PCO2 of pulmonary venous blood from the affected lung will be that of mixed venous blood.

Question 24

How does V/Q change in conditions of blood flow obstruction?

Answer 24

• In conditions of blood flow obstruction (eg from an embolus) V/Q ratio is HIGHER than normal.
  o If blood flow to a lung is completely blocked (e.g., by an embolism occluding a pulmonary artery), then blood flow to that lung is zero. If ventilation is normal, then V/Q is infinite.
  o There is no gas exchange in a lung that is ventilated but not perfused. Thus the PO2 and PCO2 of alveolar gas will approach that of inspired air.

Question 25

How does hypoxia affect the local blood vessels of the lungs? What is this effect called?

Answer 25

• Hypoxia in a region of the lung causes vasoconstriction of the local blood vessels
• This response is the opposite of that in the systemic circulation where hypoxia in a tissue (eg muscle) causes vasodilation
• Physiologically, this effect is important because local vasoconstriction diverts blood away from poorly ventilated, hypoxic regions of the lung (eg. following bronchial obstruction) and toward better-ventilated regions.
• Local perfusion in the lung is regulated by local ventilation
• This is called V/Q matching

Question 26

How is pulmonary circulation affected during exercise?

Answer 26

• All of the cardiac output passes through the lungs. So the increase in pulmonary blood flow during exercise must be the same as the increase in cardiac output.
• Suppose during exercise the C.O increases from 5 l/min to 15 l/min, a three fold increase, so the pulmonary blood flow must also increase three fold.
• Remember that the pulmonary artery operates at a very low pressure(~15 mm Hg). Does the pressure in the right heart increase three fold to get this increased flow??
• NO!! The pulmonary arterial pressure increases only slightly during exercise.
• However this is matched by the pulmonary arterial resistance greatly decreasing during exercise

Question 27

What is the mechanism of pulmonary pressure changes during exercise?

Answer 27

• A small rise in pulmonary arterial pressure (say 5 mm Hg) is sufficient to greatly increase blood flow through the apices of the lung and convert intermittent flow (zone 1) to pulsatile flow (zone 2)
• The pulmonary arterioles in zone 1 become distended (stretched) when the pressure in them rises: This stretching generates a reflex extra relaxation of the arterial smooth muscle and so the vessels enlarge , reducing the vascular resistance and increasing the flow through zone 1.
• Blood flow through zone 1 can increase seven to eight fold during exercise
• A similar reflex stretch induced relaxation occurs in zone 2 and 3, where blood flow can increase 2 to three times
• A small pressure increase in pulmonary arterial pressure produces a disproportionately large increase in blood flow

Question 28

What is the mechanism of pulmonary vascular changes during exercise with relation to shunts?

Answer 28

1. Some alveoli in the lungs (mainly in the base of the lung) are relatively poorly ventilated during quiet breathing. Thus the associated capillaries are constricted.(see diagram) The increased ventilation that occurs at the start of exercise increases PO2 in these alveoli. This dilates the associated capillaries and thus reduces total pulmonary vascular resistance
2. Arterio-venous shunts open in the lungs during exercise, allowing blood to go directly into the pulmonary veins. (this is more of a ‘safety valve’ mechanism, as shunted blood will not be oxygenated). Shunts can also open between the pulmonary and bronchial circulations.

(there will be a dip in blood flow as the very bottom of the lung due to vasoconstriction at rest due to poor circulation)