Compare and contrast the systemic circulation with the pulmonary circulation.
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.
| Category | Pulmonary circulation | Systemic circulation |
| Anatomy |
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; Trans-pulmonary intravascular pressure gradient is around 10 mmHg Most of the pressure drop occurs between pulmonary arterial and pulmonary venous capillaries |
High resistance; 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
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.