Artem has a doctor of veterinary medicine degree.
Every time you go to the doctor, you probably have your blood pressure taken. But what is blood pressure? Mathematically speaking, we can express blood pressure with a simplified equation:
Blood pressure (mean arterial pressure) = (Cardiac Output x Systemic Vascular Resistance) + Central Venous Pressure
This lesson is going to explain systemic vascular resistance and its relation to all of these terms in this lesson.
Before we can get to the equation and understand it, we must define some terms. In order to better understand everything, imagine a water pump (representing the heart) connected to a hose (like our arteries). Arteries are the blood vessels that carry oxygenated blood away from the heart and to the rest of the body.
Cardiac output (CO) is a term that refers to the volume (amount) of blood the heart pumps out per minute. Where does it pump all this blood out into? The arteries.
Systemic Vascular Resistance
Systemic vascular resistance (SVR) is sometimes known as total peripheral resistance (TPR) or peripheral vascular resistance (PVR). So keep that in mind in case you're cross-referencing things. But don't get this PVR confused with another PVR, pulmonary vascular resistance! They're totally not the same thing. And in case you didn't know, systemic is a word that refers to something 'of the body as a whole'. Vascular refers to blood vessels.
Anyways, the reason we use water pumps is because the pumps overcome some sort of resistance or force in order to pump water forward. Perhaps gravity when pumping uphill or the resistance offered by the hose itself. How so? Fill your mouth (like the pump/heart) with water and try to squirt the water out a wide straw (like the hose/artery) and then a narrow straw. Can you tell how much more resistance the smaller one offers?
Our heart must overcome this resistance as well, with respect to the resistance offered by blood vessels, not straws of course. SVR is simply the resistance the heart must overcome in order to pump blood into the rest of the body. It refers to the resistance offered by all of the body's blood vessels except the ones found in the lungs (the pulmonary vasculature).
Mean Arterial Pressure
Thanks to CO and SVR, blood exerts a force against the walls of your arteries, much like water does to the walls of a hose as it flows through it. This force is known as blood pressure. The average pressure found in the arteries is known as mean arterial pressure, or MAP. Mean refers to an average of something. We can use the systolic (upper number, like 120) and diastolic (lower number, like 80) blood pressures measured at the doctor's office to approximate MAP, but that's neither here nor there for this lesson's purpose.
Central Venous Pressure
Of course, not all the blood vessels in your body are arteries. Some are veins. Veins carry deoxygenated blood back to your heart from one tissue or another. Very large veins, called vena cavae, return deoxygenated blood to the right atrium of the heart.
The right atrium is the first chamber of the heart that receives all the deoxygenated blood from your body. The pressure in the vena cavae right outside the right atrium is known as the as central venous pressure, or CVP. This pressure is essentially (but not always) the same as the blood pressure found in the right atrium, the right atrial pressure (RAP).
You're now ready for the equation for SVR! It is very simple:
SVR = (MAP - CVP) / (CO)
That's about it for the equation then. There are variations of it however.
Remember that important point in the paragraph above? Since CVP and the right atrial pressure are often basically the same, you might find other sources that state the SVR as:
SVR = (MAP - RAP) / (CO), where the RAP in this case is the mean right atrial pressure specifically.
Alternatively, some will use the mean aortic pressure instead of MAP in the two equations above. The aorta is the largest artery of the body. Like with CVP for the vena cavae and right atrium, MAP and mean aortic pressure are very similar (exceptions exist, of course).
Units of the Formula
We'll stick to our original equation, however:
SVR = (MAP - CVP) / (CO)
The units of this equation are as follows:
- MAP and CVP are both measured in mmHg, the unit of pressure. This is mm (millimeters) of mercury (Hg). The 120/80 blood pressure reading at your doctor's office has units of mmHg.
- CO has units of L/min (or mL/min), where L is liters and mL is milliliters. The units vary depending on research or clinical settings.
- SVR is measured in mmHg/L/min, known as a Woods unit, or mmHg/mL/min, known as PRU (peripheral resistance unit).
Alternatively, you might see SVR expressed in units called cgs, or centimeter-gram-second which are written out as dynes/s/cm5. Because 80 dynes/s/cm5 = 1 Woods unit, the equation for SVR might be re-written as:
SVR = ((MAP - CVP)/(CO)) x 80
in order to get your answer in dynes/s/cm5 instead of mmHg/L/min.
Systemic vascular resistance (SVR) is the resistance the heart must overcome in order to pump blood into the rest of the body. SVR has an equation of:
SVR = (MAP - CVP) / (CO)
MAP is the mean arterial pressure, the average pressure found in the arteries.
CVP is the central venous pressure, the pressure in the vena cavae outside the right atrium.
CO is the cardiac output, the amount of blood the heart pumps out in a minute.
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