Blood Flow and Velocity – Blood Vessels and Pressure

00:00 So now we have some items related to what type of flow can occur but the last concept we need to address here is something known as velocity. Velocity is the speed which the blood is traveling and this is measured as unit of distance per time. This needs to be separated from flow because they are not the same thing. Flow has a unit of volume per unit time. Now there is a relationship between velocity and flow but they are not the same thing. If you have a velocity you can calculate a flow by this equation where you have flow equals velocity times the area. Unless you know the area of the blood vessel, you do not know what the velocity means and this is a hard concept to try to grab a hold off so I have a picture for you in the next slide and that is going to be very helpful when trying to look at stenoses such as in valve disease or an atherosclerosis. You might have a change in velocity but not a change in flow. So let's take a look at that picture. So this is a picture of this process in which you have a certain velocity and the velocity is then noted as V. R is the radius or the radial diameter of the blood vessel. So you have a given velocity that's traveling down the tube. As it's traveling down that tube if you come to a spot in where the radial diameter has changed, let's say it's half, so it's half of what it was when it started. The velocity increases. In fact, it increases a factor IV.

02:00 So oddly even though the flow has slowed down in this particular example, the velocity has sped up. You've all experienced this at some time if you ever had a garden hose or maybe even the flow that's going out of your faucet. If you put your finger over that flow, you can get it to squirt out at a high velocity but that doesn't mean the flow is high, it means the velocity is high. Same principle happens here with the blood vessel.