Flow in "blocked" arteries

Sometimes the Atheroma goes on to completely block the arteries.

If such a blockage happened in young people with no other arterial disease, the limb (usually the leg) would instantly be in trouble and would need emergency surgery - or would be lost.

However, in people who have slowly progressing arterial disease - (ie: people who smoke, people who have high blood pressure or people who have had long term and poorly controlled diabetes) - the very fact that the disease is slow in progression allows the body to open other little vessels around the blockages. These vessels are called "Collaterals".

Blood flow through these collaterals is disordered and this also has a knock on effect on the pressure.

The following explains these changes.

Flow in Blocked Arteries with Collaterals:

The animation on the left shows what happens to flow in an artery with a blockage and a Collateral vessel around it.

As you can see the blood flow is very disordered - similar to in the Critical Stenosis on the last page. The blood flows normally to the blockage and cannot pass. It flows along the line of least resistance and so flows into the collateral vessel. There are usually many collateral vessels. Although they can expand with time, they are never as efficient as the original vessel.

The blood flows through the collaterals and then flows back into the main artery when it opens up again. The collateral vessels are smaller than the vessel and so the blood speed up within them as in the Critical Stenosis. When the collaterals rejoin the open vessel, the fast jets of blood cause significant turbulence in the blood flow.

As with the critical stenosis, this pattern of flow has an effect on the pressure of the blood in this section of artery. 

Pressure Change in a Blocked Artery with Collaterals:

The animation on the right shows the pressure changes in the section of artery that is blocked but has collateral vessels.

Once again, there are massive changes in flow causing huge changes of energy in the blood. The blood cannot get through the vessel and so has to go through the collateral vessel. The collateral vessel is much narrower than the original artery and so the blood has to speed up. As it rejoins the larger original artery, the jet of blood 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 or 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 having to push through the collateral vessel is being used up. Therefore there is less pressure, or energy, left in the blood once it has got passed the occlusion or blockage - ie: the blood has had to do Work to get through the collateral.

Occluded Artery:

An occluded artery with collaterals acts very much like a critical stenosis.