The Venturi Effect and Blood Flow

Lesson Transcript
Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

Blood flows through the body in a specific way, and in general, we want that to continue happening. So, understanding the Venturi effect is important. We will examine how, where, and why this occurs in the body.

The Venturi Effect

When water flows through a pipe, it exerts a certain amount of pressure outwards. This pressure can either keep the pipe's walls from collapsing or can make them explode. In general, we want to avoid the second option—specially when talking about certain kinds of pipes, like the roughly 100,000 miles of blood vessels circulating throughout your body. Yeah, we want those to stay intact. Just like hydro engineers watch water pressure to make sure pipes don't burst, doctors watch our blood pressure. Only, with blood vessels we're not just worried about them exploding. They could also collapse, which is just as terrifying. Now, if every vein and artery and capillary were the same size, blood pressure would be relatively straight forward, but they're not. Some are larger, some are smaller, some are narrower or angled or constricted. So, we need to understand how pressure changes within a system, and for that, we turn to the Venturi effect, which claims that when fluid is constricted, it decreases in pressure but increases in velocity. Need a closer look? Let's check it out.

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  • 0:01 The Venturi Effect
  • 1:16 Venturi Effect &…
  • 2:26 Venturi Effect & Bloodstreams
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The Venturi Effect and Bernoulli's Principle

So, let's see exactly how this effect works. The blood vessel must maintain an overall constant pressure and velocity, so when constrictions like this occur, the blood has to speed up and decrease pressure to compensate. The Venturi effect was named for 18th century Italian physicist Giovanni Battista Venturi and is basically a specific example of the Bernoulli principle, one of the fundamental laws of fluid dynamics. According to this principle, an increase in fluid velocity always results in a decrease in pressure, but that total pressure along a streamline is constant. So, when we see restrictions like this, we know that the velocity has to increase, and pressure must decrease so the overall pressure in the vessel can remain constant. This can be tested by inserting a tube at a right angle in the stream both before and within the constriction. The fluid pressure fills the tube until pressure is stabilized then stops. See how the amount of fluid is lower in the tube over the constriction? That means the pressure is lower.

The Venturi Effect and Bloodstreams

So, now that we understand the basics of the Venturi effect, let's see where this actually occurs in the body. Your blood travels through blood vessels in a specific way, moving from the heart through arteries then capillaries then veins back to the heart. Now, major veins and arteries are substantially larger than capillaries, which are actually pretty tiny. So, for blood to move from arteries to veins, it has to pass through the constricting capillaries. And there it is, the Verturi effect in action. In order for the overall blood pressure to remain constant, blood must flow faster but at lower pressure through these smaller veins.

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