Systemic Vascular Resistance: Definition & Formula

Instructor: Artem Cheprasov
This lesson clearly explains the terms and concepts related to systemic vascular resistance, including alternative terms and formulas you might see in other textbooks. This way, you won't be confused!

Blood Pressure

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.

Terms

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

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).

Formula

Main Formula

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.

Variations

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:

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