Canonical Question
PVR vs SVR
Master answer
| Pulmonary circulation | Systemic circulation |
|---|---|
| Movement of blood from the heart to the lungs for oxygenation, then back to the heart again | Movement of blood from the heart through the body to provide oxygen and nutrients to the tissues of the body while bringing deoxygenated blood back to the heart |
| Deoxygenated blood (RA) → RV → Pulmonary Artery →→→ capillary beds → Oxygenated blood →→→ Pulmonary Veins → LA | Oxygenated blood (LA) → LV → Aorta →Major Arteries →→→ Capillaries → Gas and nutrient exchange → Deoxygenated blood →→→ Veins → Vena cavae → RA |
| Accept the entirety of cardiac output, with little capacity to regulate flow (hypoxic vasoconstriction being the exception) | Regulate flow to different organs at different times Contains resistance vessels which allow it to allocate cardiac output accordingly. |
| Low pressure – low resistance | High pressure – high resistance |
| BP – 25/8 (mean 15) mmHg | BP – 120/80 mmHg |
| Pressure differences from inlet to outlet of the pulmonary system is about (15 – 5) = 10 mmHg | Pressure differences from inlet to outlet of the systemic system is about (100 – 2) = 98 mm Hg which is ~ 10 times that in pulmonary system |
| the walls of the pulmonary artery and its branches are remarkably thin and contain relatively little smooth muscle (they are easily mistaken for veins). | the arteries generally have thick walls and the arterioles in particular have abundant smooth muscle. |
| The lung is required to accept the whole of the cardiac output at all times. It is rarely concerned with directing blood from one region to another | The systemic circulation regulates the supply of blood to various organs, including those which may be far above the level of the heart (the upstretched arm, for example). |
| At any given time, contains ~10% of the circulating blood volume, or about 500ml. | At any given time, contains ~90% of the circulating blood volume, or about 5400ml. |
| Blood Flow = Cardiac output (in the abscence of shunts / recirculation) | Blood Flow = Cardiac output (in the abscence of shunts / recirculation) |
| highly compliant – the volume of blood is able to change substantially with minimal change in pressure | Compliance varies depending on distance from heart and as per different regional circulations |
| The distribution of pressures along the pulmonary circulation is far more symmetrical. Much of the pressure drop occurs within the capillary bed itself | Most of the pressure drop is just upstream of the capillaries |
| The pulmonary capillaries are virtually surrounded by gas, and receive little support. Hence are liable to collapse or distend, depending on the pressures within them and alveolar pressure. | All capillaries are surrounded and supported by solid tissue |
| PVR ~ 1/10th of SVR ~ 0.25-1.6 mmHg/L/min 20-120 dynes.Sec.Cm-5 | SVR 9-20 mmHg/L/min 900-1200 dynes.sec.cm-5 |
| Minimal sympathetic effect (as relatively little smooth mm.) | SVR highly regulated by Autonomic tone |
| PVR decreases significantly as pulmonary artery pressure increases because of capillary recruitment (mainly) and distension | This inverse relationship not so significant |
| Both the arteries & veins increase their caliber as the lung expands by radial traction. | No such phenomenon in systemic circulation |
| Vasodilate in response to hypoxia and hypercapnoea | Vasoconstrict in response to hypoxia and hypercapnoea |
| gas exchange function: between blood to air | between blood and tissues |
| metabolic and synthetic functions: Metabolism of: 5HT, PGs ACE: convert AT I → AT II) Synthesize thromboplastin, Heparin | Delivers substance to organ systems for metabolism, synthesis, excretion Synthesizes NO Synthesizes pro- and anti- coagulants |
| role in acid base homeostasis: by CO2 exchange by buffering | by metabolism and excretion via kidneys, liver, skin, GI, etc by buffering |
| Filters large emboli | Filtration in Liver and Kidney |
| Some reservoir function in pulmonary vasculature | Large reservoir in organs (mainly Liver) and venous system |
JC 2019
Exam appearances
| Exam | Exact wording | Relationship | Success |
|---|---|---|---|
| 2017A Q07 | Compare and contrast the systemic circulation with the pulmonary circulation. | historical_member | — |