Question 7

Compare and contrast the systemic circulation with the pulmonary circulation.

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College Answer

This question encompasses a wide area of cardiovascular physiology. As a compare and contrast question this question was well answered by candidates who used a table with relevant headings. Comprehensive answers included: anatomy, blood volume, blood flow, blood pressure, circulatory resistance, circulatory regulation, regional distribution of blood flow, response to hypoxia, gas exchange function, metabolic and synthetic functions, role in acid base homeostasis and filter and reservoir functions. A frequent cause for missing marks was writing about each circulation separately but comparing. For example: many candidates stated 'hypoxic pulmonary vasoconstriction', but did not contrast this to 'hypoxic vasodilation' for the systemic circulation. Frequently functions of the circulations were limited to gas transport / exchange.


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

Thin vessels

Minimal smooth muscle

Vessels are dependent on alveolar pressure (surrounded 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 systemic arterial systolic pressure = 120 mmHg

Normal systemic arterial diastolic pressure = 80 mmHg

Normal mean pulmonary arterial pressure = 90 mmHg

Circulatory resistance

Low resistance;
PVR = 100-200

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

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 blood flow 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

Significant active regulation 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 hepatic vascular beds results in the clearance of metabolic wastes and particles.


There is no specific peer-reviewed article where the two circulatory systems are directly compared. 

Locally, in a dangerously non-peer-reviewed fashion, anatomy and physiology of the pulmonary circulation is discussed in greater detail.