Differences between the pulmonary and systemic circulations

This chapter is most relevant to Section F5(iii) from the 2023 CICM Primary Syllabus, which expects the exam candidates to be able to "understand the differences between the pulmonary and systemic circulation". Admittedly, these are pretty different. The college has clearly identified an important area of learning here. At least one past paper SAQ (Question 7 from the first paper of 2017)had asked trainees for a list of these differences. Those without a ready classification system to throw at the answer would surely have perished in the 2-3/10 mark zone. Judging by the 26% pass rate this category encompasses the vast majority of exam candidates; many lost marks not because of poor knowledge but because their approach to structure let them down. 

Fortunately, the college comments for Question 7 yield some insight into what they were actually expecting. On the basis of this, the following summary table was constructed. It incorporates elements from the chapters on pulmonary vascular anatomy and physiology as well as corresponding chapters regarding the systemic circulation. 

A Comparison of the Pulmonary and Systemic Circulations
Category Pulmonary circulation Systemic circulation

Thin vessels

Minimal smooth muscle

Vessels are dependent on alveolar pressure (surrounted by intrathoracic air), and expand by radial traction as the lung expands

Thick vessels

Abundant thick smooth muscle

Vessels are embedded in tissues

Blood volume About 500ml, in a 70kg person About 4500ml, in a 70kg person; of which the majority is in capacitance vessels
Reservoir function Contains about 10% of the total blood volume Contains about 90% of the total blood volume
Blood flow = cardiac output (~ 5 L/min) = cardiac output (~ 5 L/min)
Blood pressure

Normal PA systolic pressure = 18-25 mmHg

Normal PA diastolic pressure = 8-15 mmHg

Normal mean pulmonary arterial pressure = 9-16 mmHg

Normal systolic pressure = 120 mmHg

Normal diastolic pressure = 80 mmHg

Normal mean arterial pressure = 90 mmHg

Circulatory resistance

Low resistance;
PVR = 100-200 dynes.sec.cm-5

Trans-pulmonary intravascular pressure gradient is  around 10 mmHg

Most of the pressure drop occurs between pulmonary arterial and pulmonary venous capillaries

High resistance;
SVR = 900-1200 dynes.sec.cm-5

Trans-systemic intravascular pressure gradient is  around 100 mmHg

Most of the pressure drop is due to resistance in the systemic arterioles

Circulatory regulation Minimal capacity to actively regulate flow, except via hypoxic pulmonary vasoconstriction Regional bloodlflow regulation occurs at the level of arterioles
Regional distribution of blood flow

Blood flow is affected by

  • gravity
  • alveolar recruitment
  • hypoxic vasoconstriction

Little active regulation occurs

Signficant variation of organ-specific regional blood flow, depending on organ demand

Blood flow is less affected by gravity

Response to hypoxia Vasoconstriction Vasodilation
Response tohypercapnia Vasoconstriction Vasodilation
Gas exchange functions Absorption of alveolar oxygen; release of capillary carbon dioxide Release of capillary oxygen; absorption of carbon dioxide by deoxyhaemoglobin
Metabolic functions Metabolism of -hydroxytryptamine, prostaglandins and substrates for angiotensin-converting enzyme (bradykinin and angiotensin I) Delivery of metabolic substrates to organ systems, and removal of metabolic wastes
Synthetic functions Source of thromboplastin and heparin, which act to degrade filtered clots Synthesis of nitric oxide, as well as pro-- and anti-coagulants
Role in acid base homeostasis Facilitates CO2 elimination, thereby adjusting acid-base balance Facilitates washout of lactate and metabolic wastes, thereby adjusting acid-base balance
Filter function Filters emboli larger than 8 μm

Filtration of arterial blood in the renal and heptic vascular beds results in the clearance of metabolic wastes and particles.

Where would you find a reliable peer-reviewed resource for something like this? Nowhere, that's where. Most people with even very junior CICM Part One level nous have a clear understanding that the two sides of the circulation are very different, but it is hard to find official journal publication to explicitly explore this difference. One can find endless articles detailing the properties of each system individually, but none of them actually perform this sort of direct comparison. Only occasionally one happens upon an article like Chamarthy et al (2018) where there is a small comparison table (Table 2).


Chamarthy, Murthy R., Asha Kandathil, and Sanjeeva P. Kalva. "Pulmonary vascular pathophysiology." Cardiovascular diagnosis and therapy 8.3 (2018): 208.