Question 13

Describe the anatomical (20% marks) and physiological (80% marks) features of the pulmonary circulation.

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College Answer

The examiners consider that an understanding of the pulmonary circulation is core area of the syllabus. In general, the anatomy section was better answered than the physiological features. As well as a description of the gross anatomy of the pulmonary circulation tracking it from the pulmonary valve to the left atrium, some mention of the microscopic anatomy was required (e.g., that the pulmonary arteries are thin walled with little smooth muscle).

For the second part of the question, a breadth of knowledge was required. Candidates were expected to address the following physiological features of the pulmonary circulation: volume, pressure, resistance, regulation and regional distribution and function. Marks were apportioned to each section, so it was important to write something on each section. Focussing on one section in detail (e.g., a very detailed description of West’s Zones) usually came at the expense of missing one or more of the other sections, most commonly the functions of the pulmonary circulation. Indeed, candidates that scored well provided information on each section and for the functions of the pulmonary circulation mentioned more than gas exchange.

Discussion

Even trying to jot down some absolute minimum to "write something on each section", one ends up with an answer which is 328 words (2322 characters) long, which would require the exam candidates to scribble faster than the fastest police interviewers, who max out at about 28 words per minute. This supports the first point of the CICM Part II Preparation Guide, and explains why only 25% of the candidates passed this question.

Anatomy of the pulmonary circulation:

  • 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)
      • nonmuscular (the smallest)
  • 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:

  • The pulmonary circulation:
  • Pulmonary blood flow:
    • Pulmonary arterial flow is equal to the cardiac output and consists of mixed venous blood
    • Pulmonary venous flow: oxygenated pulmonary capillary blood + physiological shunt 
  • Pressures in the pulmonary circulation
    • PA systolic pressure = 18-25 mmHg
    • PA diastolic pressure = 8-15 mmHg
    • Mean pulmonary arterial pressure = 9-16 mmHg
    • Pulmonary capillary pressure = 8-10mmHg
    • Pulmonary venous pressure =  6-12 mm Hg
  • Pulmonary vascular resistance:
    • Approximately 1/10th of the systemic (SVR).
    • PVR is 100-200 dynes/sec/cm-5, or  255 - 285 dynes-sec/cm–5/m2 for PRVI 
  • Regulation
    • Autonomic (sympathetic nervous system)
    • Intrinsic (eg. hypoxic pulmonary vasoconstriction)
    • Humoral (eg. circulating catecholamines)
  • Regional distribution
    • Hydrostatic and alveolar pressure (West's Zones describe the distribution of blood flow according to hydrostatic pressure due to gravity and alveolar pressure)
    • Regional distribution of oxygenation 
      • hypoxic pulmonary vasoconstriction redirects blood flow away from poorly ventilated regions
    • Dependent on lung volume (relationship between lung volume and PVR is "U"-shaped)
  • Function
    • ​​​​​​​Gas exchange
    • Reservoir of blood (10% of circulating volume)
    • Filter (particles larger than an RBC are trapped; ~8  μm size barrier)
    • Modulation of body temperature (heat exchange with alveolar gas)

References

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.

West, John B. "Comparative physiology of the pulmonary circulation." Comprehensive Physiology 1.3 (2011): 1525-1539.

Naeije, Robert. "Pulmonary vascular resistance A meaningless variable?." Applied Physiology in Intensive Care Medicine. Springer, Berlin, Heidelberg, 2009. 65-68.

Hamilton, W. F. "The physiology of the pulmonary circulation." Journal of Allergy and Clinical Immunology 22.5 (1951): 397-410.