The systemic vascular resistance is the resistance that blood “sees” as it travels throughout the circulatory system of the body.
It is controlled by three different factors: length of the blood vessel (l), radius of the blood vessel (r), and the viscosity of the blood (η). The equation that relates these three factors to resistance is known as Poiseuilles’ equation:
R ≈ (η x l) / r4
Let’s discuss how each of the factors above influences resistance. The longer the vessel, the more likely blood will sludge and stick up against the walls; this tendency intuitively causes an increase in overall resistance. Blood vessel length does not typically change during adulthood, therefore its contribution to vascular resistance in humans is relatively constant.
When blood is more viscous than usual the resistance also increases. For example, molasses flows very slowly because it is highly viscous; blood is no different. There are a few uncommon instances in human physiology where blood viscosity increases; therefore, like vessel length, its contribution to vascular resistance is also relatively constant.
The most important parameter is the radius of the vessel. When the radius of a vessel shrinks, the resistance increases; when the radius of a vessel dilates, the resistance drops. Imagine trying to pump a set amount of fluid through a small opening versus a larger opening. There is much less resistance encountered through the larger opening. The same is true for large and small blood vessels. Even more importantly, as indicated by Poiseuille’s equation above, when the radius of a vessel is halved, the resistance increases by 16 fold! Therefore, even small changes in blood vessel diameter can have dramatic consequences on the bodies’ vascular resistance.
How the Body Controls Resistance
How do you think the body controls resistance most effectively? You guessed it! By controlling the radius of blood vessels. When there is an abnormal drop in vascular resistance, the body compensates by pumping out hormones (ie: epinephrine and norepinephrine); these hormones cause muscle cells surrounding blood vessels to constrict. Constriction leads to a decreased radius of the vessel, which leads to increased resistance.
Clinical Measurements
How do you measure systemic vascular resistance? Well, first off, systemic vascular resistance is not measured, but is calculated from other cardiovascular vital signs such as the mean arterial pressure (MAP), cardiac output (CO), and central venous pressures (CVP). The equation that relates these three values to resistance is:
SVR = [(MAP – CVP) / CO] x 80
Mean arterial pressure is a sort-of average between the systolic and diastolic blood pressure readings. It can be calculated using blood pressures obtained from an arterial catheter or a blood pressure cuff.
Central venous pressures are normally low, and do not contribute much to vascular resistance, but when very accurate measurements are needed they can be obtained from a central venous and/or a pulmonary artery catheter.
Cardiac output can be obtained from a pulmonary artery catheter if accurate values are needed; it can also be roughly estimated from a cardiac echocardiogram (ie: an ultrasound of the heart).
Role in Disease
So what’s the big deal? Why all this talk about vascular resistance? Vascular resistance is important because it is one determinant of blood pressure, and therefore organ perfusion.
The mean arterial pressure is calculated by multiplying the cardiac output by the systemic vascular resistance, and then adding the result to the central venous pressure (MAP = CO x SVR + CVP, a reorganization of the equation above).
Since the mean arterial pressure is the driving force behind the delivery of blood and oxygen to the organs, it is of vital importance that it stays above a certain threshold. Otherwise organs will not receive enough oxygen and will eventually die. One way of ensuring that the mean arterial pressure stays within a set range is for the body to constrict and/or dilate the vessels as needed.
On the flip side, if systemic vascular resistance is too "clamped" down then hypertension occurs. If severe enough, the heart may not be able to generate enough force to eject an adequate amount of blood into the tightened arterial system, which over time could result in systolic heart failure.
Overview
Systemic vascular resistance is the resistance blood sees as it travels throughout the bodies blood vessels. It is influenced by the length and radius of the blood vessel(s), as well as the viscosity of blood. It is calculated from the mean arterial pressure, central venous pressure, and cardiac output. Systemic vascular resistance plays a vital role in maintaining blood pressures within set ranges so that organ perfusion is maximized.
References and Resources
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