Flow in Significantly Narrowed Vessels

If the arterial disease, or "Atheroma", gets big enough to narrow the artery significantly, then the flow pattern changes - and this has a knock on effect on the pressure.

The following explains these changes.

Flow in Significantly Narrowed Artery:

The animation on the left shows what happens to flow in an artery with a tight, or significant, narrowing (a Critical Stenosis).

As you can see the blood flow is very disordered. Firstly the flow speeds up in the narrowed segment - the same volume of blood needs to get through a narrowed area in the same time - hence it has to flow faster. This is the same reason as why water in a river speeds up when the river gets shallow - like in rapids.

Secondly, once the blood has flowed through the critical stenosis, it "tumbles" back out to the walls - this is called "Turbulence". Once again you can imagine this as the "white water" or froth of the rapids in a river.

The importance of this pattern of flow is the effect that it has on the pressure of the blood in this section of artery. 

Pressure Change in Significantly Narrowed Artery:

The animation on the right shows the pressure changes in the section of artery that has a critical stenosis.

In this situation the changes in flow causes huge changes of energy in the blood. The blood has to speed up to get through the critical stenosis - then does not flow smoothly (ie: not laminar flow) - but instead has turbulance. Turbulant flow means that some of the blood is flowing outwards or backwards for a time - wasting the energy of the blood flow. This causes vibration of the arterial wall.

If you now check the pressure measures P1 and P2, you can see that the pressure in P2 is lower than in P1. This means that the energy in the blood pushing it forwards is being used to get the blood through the critical stenosis. Therefore there is less pressure, or energy, left in the blood once it has got through this critical stenosis - ie: the blood has had to do Work to get through.

Critical Stenosis:

A narrowing that causes a drop in pressure of the blood flowing through it is called a "Critical Stenosis"- this is the definition of whether a stenosis is significant or not.

If a stenosis doesn't cause a pressure drop then it doesn't affect the blood flow or blood pressure and so can be ignored.

If a stenosis causes turbulant blood flow that results in a pressure drop, it is a Critical Stenosis and it affects the blood supply to the body downstream from the problem.