Question 15

College Answer

Many candidates described the anatomical pattern of right ventricle to arteries to smaller arteries to arterioles to capillaries to venules to veins to the left atrium. To obtain full credit one needed to describe relevant aspects of anatomy including main pulmonary artery ~5cm in length divides into L and R pulmonary arteries. Arteries are relatively thin walled with little smooth muscle, capillaries form an extensive sheet of blood flow over the alveolar wall, and the pulmonary circulation drains into four
pulmonary veins that empty into the left atrium. A structured approach would then follow regarding physiology. There is ~500 ml of blood in the pulmonary circulation with ~10% in the capillaries and half the remainder in each of arteries and veins. The system has a high capacitance and is very distensible. The volume can halve or double to adjust for posture, respiratory effort and changes to the systemic circulation. Values for normal pulmonary artery pressure were expected and an explanation that this is
just adequate to reach the apices of the lung and that if the pulmonary pressure was higher there would be a risk of compromised perfusion and flow. Comparisons with the systemic pressures or detail such as capillary values gained extra credit. For example, highlighting that regional distribution and regulation is relatively passive compared to the systemic circulation and thus gravity and posture have significant
effects. Many candidates gave the units incorrectly. Of the candidates who described the West Zones, most seemed aware of the influence of alveolar pressure, but few seemed aware of the importance of low pulmonary pressure relative to the effect of gravity, such that the pulmonary arterial pressure was just adequate to reach the apex of the lung. Most candidates were aware of the important role of hypoxic pulmonary vasoconstriction to optimise VQ matching. The autonomic system has relatively little effect upon regulation especially compared to the systemic circulation. Many candidates wrote in generalities about the physiology of circulatory systems without discussing special features of the pulmonary circulation. The detail supplied was often less than the expected level.

Discussion

Anatomical features of the pulmonary circulation:

• A low pressure, highly elastic system, with vessel walls which are much thinner and less muscular than the systemic circuit
• Pulmonary trunk (~ 30mm diameter) divides into pulmonary arteries
• Pulmonary arteries can be divided into elastic (large), muscular (small) and nonmuscular (the smallest), though further subdivisions are histologically apparent
• Pulmonary arteries and veins travel with bronchi, nerves and lymphatics in bronchovascular bundles, which are extensions of the visceral pleura
• The clinical relevance of these structures is the tendency of oedema fluid to accumulate in them, creating "peribronchial cuffing"
• Pulmonary capillaries start from terminal bronchioles, and form a vascular sheet, interrupted by intercapillary posts
• Pulmonary veins drain into the left atrium and are continuous with it, up to the point where they contain some cardiac myocytes (which can be sources of atrial fibrillation)

Physiological features of the pulmonary circulation

• Volume:
  • At any given time, contains ~10% of the circulating blood volume, or about 500ml.
  • Pulmonary blood volume varies by around 50ml over the course of a single cardiac cycle
• Flow:
  • Pulmonary arterial flow is equal to the cardiac output and consists of mixed venous blood
  • Pulmonary venous flow consists of oxygenated pulonary capillary blood as well as physiological shunt (blood from thebesian veins and bronchial veins)
  • Red blood  cells spend about 4-5 seconds in the pulmonary circulation
• Pressure:
  • ​​​​​​​Normal PA systolic pressure = 18-25 mmHg
  • Normal PA diastolic pressure = 8-15 mmHg
  • Normal mean pulmonary arterial pressure = 9-16 mmHg
  • The pulmonary arterial waveform resembles the systemic arterial waveform, and has similar features
• Resistance:
  • ​​​​​​​The resistance in the pulmonary circulation can be calculated as follows:
    • PVR = 80 × (mPAP- PAOP)/CO
  • This varies with:
    • Lung volume ("U"-shaped relationship lowest at FRC)
    • Hypoxic pulmonary vasoconstriction
    • Metabolic factors (hypercapnia, acidosis)
    • Autonomic nervous system (α₁ and β₂ receptors

West, J. B., and C. T. Dollery. "Distribution of blood flow and the pressure-flow relations of the whole lung." Journal of Applied Physiology 20.2 (1965): 175-183.

Fishman, Alfred P. "The volume of blood in the lungs." Circulation 33.6 (1966): 835-838.

Fishman, Alfred P. "The pulmonary circulation." JAMA 239.13 (1978): 1299-1301.

Aviado, Domingo M. The Lung Circulation: Physiology and Pharmacology. Elsevier, 2013 (except the book itself says "1965"...)

Naeije, Robert. "Physiology of the pulmonary circulation and the right heart." Current hypertension reports 15.6 (2013): 623-631.

Waaler, B. A. "Physiology of the pulmonary circulation." Journal of Vascular Research 8.3-5 (1971): 266-284.